Caterpillarwheel tractor-scraperwheel scraper

Caterpillar 631B

Maintenance schedule, common problems & OEM parts breakdown

The Caterpillar 631B is a wheel tractor-scraper (motor scraper) built for bulk earthmoving and haul-road cut-and-fill work. It runs a Cat D343T six-cylinder turbocharged, aftercooled diesel of roughly 14.6 L (893 cu in) displacement, rated in the 335-400 hp range depending on the production-year update, paired with a 9-speed planetary Cat power-shift transmission. Operating weight empty runs roughly 31.6-31.8 t (about 69,700-70,000 lb), varying with bowl configuration, and bowl capacity is rated near 16 m3 struck / 23 m3 heaped (21/30 cu yd). Caterpillar built the 631B from 1962 to 1969, as the first fully hydraulic scraper in this line, succeeding the cable-operated 631A (1960-1962) and giving way to the 631C in 1969. Over the run Cat offered a standard bowl configuration plus a reinforced-bowl "Special Application" variant for abrasive digging conditions.

Across its production run the 631B represented a generational break from the 631A: cable-controlled bowl, apron, and ejector functions were replaced by full hydraulic actuation, cutting rigging complexity and improving control. Within the 631B's own run, Caterpillar raised engine output to 400 hp at the flywheel in 1967 and introduced the Cushion Hitch, a pivoting gooseneck/drawbar arrangement that cut structural stress at the hitch and reduced the ride "loping" common to two-axle overhung scrapers; that update also brought squared-off fender styling. More than 4,000 units were built before the 631C took over in 1969. In today's used and parts market the 631B sits firmly in classic/collector Cat iron: production ended over five decades ago, so replacement parts come from used take-offs, rebuilt exchange components, and machine-shop remanufacture rather than new-production Cat stock. It remains relevant to collectors restoring early hydraulic-generation Cat scrapers and to owners of surviving units doing niche earthmoving or reclamation work where an older, mechanically simple machine is easier to keep running than electronically managed modern equipment.

Below: full specifications, fluids & capacities, the factory service schedule, common service parts, verified fault codes, what owners discuss, attachment guidance, the complete assembly directory, and a serial-number reference. Complete parts lists with full OEM part numbers, exploded diagrams, quantities, and fitment data are available free in Heavy Parts AI.

Caterpillar 631B specifications

Engine

Engine model	Caterpillar D343T, inline 6-cylinder diesel, turbocharged.
Power (gross, at flywheel)	268 kW (360 hp) at introduction in 1962, raised to 298 kW (400 hp) at the flywheel with a 1967 running change. Some records show 335 hp for the base engine; treat exact figure as varying by configuration/series.
Displacement	14.6 L (893 cu in)
Cylinders / bore x stroke	6 cylinders, 137 mm x 165 mm (5.4 in x 6.5 in)
Emissions tier	Pre-emissions mechanical diesel. Built 1962-1969, before EPA/EU nonroad emissions standards existed — no tier rating applies.

Weights

Operating weight (empty)	About 31.6-31.8 t (69,700-70,000 lb), varies by configuration.
Shipping weight	Not separately documented in available sources.
Ground pressure	Not published in available sources.

Dimensions

Overall length (transport)	About 13.1 m (42 ft 11 in).
Overall width	About 3.81 m (12 ft 6 in).
Overall height	About 3.48 m (11 ft 5 in).
Wheelbase / ground clearance	Not documented in available sources for this vintage model.
Tire size	37.25-35, per a tire-fitment chart covering the 631 through 631G scraper family (bias-ply earthmover tire).

Performance

Travel speed (top, loaded/unloaded)	About 51-55 km/h (32-34 mph); sources vary slightly on the exact top-gear figure.
Transmission	9-speed planetary powershift.
Bowl capacity (struck)	16.1 m3 (21 cu yd)
Bowl capacity (heaped)	22.9 m3 (30 cu yd)
Gradeability	Not published in available sources.

Service capacities (summary)

Fuel tank	Not documented in available sources for this model — verify with dealer or OEM shop manual.
Hydraulic system	Not documented in available sources for this model — verify with dealer or OEM shop manual.
Engine oil	Not documented in available sources for this model — verify with dealer or OEM shop manual.
Cooling system	Not documented in available sources for this model — verify with dealer or OEM shop manual.
Water tank (water-wagon field conversion)	About 30,280 L (8,000 US gal), rear-mounted tank on a water-wagon conversion built on the 631B chassis. This is an aftermarket/field configuration, not a standard scraper-bowl spec.

Values vary by configuration, region, and serial range — confirm against your machine before planning transport or lifts.

631B fluids & capacities

System	Capacity	Recommended fluid
Engine crankcase (with filter) - Cat D343T diesel	Not documented in accessible public sources for this serial range (11G, 28F, 51F prefixes, 1962-1969 build) - verify against the factory OMM or your dealer	Factory-era Caterpillar heavy-duty diesel engine oil selected by ambient-temperature viscosity grade, the predecessor lineage to today's Cat DEO diesel engine oil. Exact grade table by climate not confirmed for this serial range - confirm with dealer or original OMM lubrication chart.
Cooling system	Not documented in accessible public sources for this serial range - verify against the factory OMM or your dealer	Caterpillar-branded ethylene glycol engine antifreeze/coolant of the era (predecessor to today's Cat DEAC/ELC coolant lines). Specific glycol ratio and freeze-protection notes not confirmed for this serial range.
Fuel tank	Not documented in accessible public sources for this serial range - varies by configuration; verify against the factory OMM or your dealer	Diesel fuel per Caterpillar's period fuel specification for the D343T. Exact cetane/sulfur guidance not confirmed for this serial range.
Transmission (9-speed planetary powershift)	Not documented in accessible public sources for this serial range - verify against the factory OMM or your dealer	Caterpillar powershift transmission oil of the era (predecessor lineage to today's Cat TDTO multipurpose tractor/drivetrain oil). Exact grade not confirmed for this serial range.
Differential	Not documented in accessible public sources for this serial range - verify against the factory OMM or your dealer	Caterpillar-specified gear lubricant for differential/final drive service of the era. Exact grade and climate notes not confirmed for this serial range.
Final drives (each)	Not documented in accessible public sources for this serial range - verify against the factory OMM or your dealer	Same gear lubricant family as differential service per factory lubrication chart of the era. Exact grade not confirmed for this serial range.
Hydraulic system (scraper implement circuit)	Not documented in accessible public sources for this serial range - verify against the factory OMM or your dealer	Caterpillar hydraulic/tractor oil of the era used for the implement hydraulic circuit. Many Cat scrapers of this generation shared a common hydraulic/transmission oil family; exact spec not confirmed for this serial range.
Chassis and pin grease	Spec only - no fill volume applicable	Caterpillar multipurpose chassis grease, standard NLGI consistency grade for pins, bushings, and linkage points. Exact NLGI grade and climate variant not confirmed for this serial range.

Capacities are refill values from factory literature — always fill to the dipstick/sight gauge, not the number.

Caterpillar 631B maintenance schedule

Service interval	Tasks
Every 10 h	Check engine oil level on the D343T before startup and top off with the correct diesel engine oil grade. Drain water and sediment from the fuel tank sump and check the primary fuel filter/water separator. Grease gooseneck, drawbar, and bowl-apron-ejector pivot pins (cushion hitch pivots on 1967-up units). Check coolant level and inspect the radiator core and hoses for leaks or debris packing. Walk around and check bowl, apron, and ejector hydraulic rams for weeping seals or external leaks. Inspect tires or wheel-tire assemblies for cuts, low pressure, and wheel hub seal weeping.
Every 250 h	Change engine oil and filter on the D343T using the rated diesel engine oil class. Service the air cleaner: clean or replace the primary element and check turbocharger inlet ducting for leaks. Check power-shift transmission oil level and top off with a transmission/drive-train-rated oil. Inspect and adjust the fan belt and charging-system belt tension. Grease steering-clutch and brake linkage points and the remaining bowl-apron-ejector linkage. Check final drive and differential oil levels front and rear.
Every 500 h	Change power-shift transmission oil and clean the transmission suction screen. Change the hydraulic system oil filter serving the bowl, apron, ejector, and steering circuits. Check and adjust engine valve lash on the D343T per the service manual clearances. Inspect the turbocharger for shaft end-play, oil leakage at the seals, and boost line integrity. Inspect or repack front wheel bearings and check kingpin and steering-arm wear. Load-test batteries and clean terminals; check the starting and charging circuit.
Every 1,000 h	Change final drive and differential oil, front and rear. Change hydraulic system oil in the bowl/apron/ejector/steering reservoir and clean the suction strainer. Inspect the fuel injection pump and injectors for spray pattern and pop pressure. Check and adjust steering-clutch and brake band linkage travel. Inspect gooseneck and cushion hitch bushings and pins for wear and replace as needed. Pressure-test the cooling system and inspect the water pump and hoses for leakage.
Every 2,000 h	Change power-shift transmission oil and clean magnetic drain plugs throughout the drivetrain. Inspect steering clutches and brakes; replace friction discs or bands showing excess wear. Remove and inspect the turbocharger cartridge; rebuild or exchange if compressor or turbine wheel play exceeds limits. Check the frame, gooseneck, and drawbar welds for cracking, especially on pre-1967 units without cushion hitch reinforcement. Inspect the bowl cutting edge, side walls, and apron for wear; rotate or replace the cutting edge. Check and recalibrate fuel injection timing and governor settings on the D343T.
Every 4,000 h	Pull and inspect the D343T long block: cylinder liners, pistons, rings, and main/rod bearings against wear limits. Rebuild or exchange power-shift transmission clutch packs and planetary gearsets. Reseal and rebuild bowl, apron, and ejector hydraulic cylinders; check rod straightness and wear. Overhaul front axle steering knuckles and rear tandem final drives. Recondition or replace gooseneck/cushion hitch bushings, pins, and structural reinforcement plates.

Servicing the 631B beyond the schedule

Predictive Maintenance & Fluid Analysis

Run regular oil sampling on the D343T to catch turbocharger bearing wear and injection-pump metal early, since a turbo failure on this engine can dump debris downstream fast. Track power-shift transmission oil condition for clutch-pack wear metals. Sample the bowl/apron/ejector/steering hydraulic reservoir for water and particulate contamination, a common issue on machines that sat outdoors for years. Test coolant concentration and condition given the aftercooled engine's tighter core passages.

Corrective & Common Repairs

Expect gooseneck and drawbar cracking on pre-1967 units built before the Cushion Hitch reinforcement; inspect welds closely. Bowl cutting edges, side walls, and apron lips wear fast in abrasive material and need regular buildup or replacement. Steering clutches, brake bands, and D343T injection pump components are frequent wear/failure points on high-hour units. Front wheel bearings and tires take heavy abuse from off-road hauling and should be checked at every service interval.

Overhaul & Rebuild Points

Major rebuild work centers on the D343T long block (liners, pistons, rod and main bearings) and the turbocharger cartridge once compressor or turbine play exceeds limits. The power-shift transmission clutch packs and planetary sets are a common mid-life overhaul item. Bowl, apron, and ejector hydraulic cylinders need reseal and rod inspection after years of cycling. Gooseneck and cushion hitch bushings, pins, and reinforcement plates should be reconditioned as a set to restore hitch geometry.

Seasonal & Environment Servicing

In abrasive or rocky digging conditions, run the Special Application reinforced-bowl configuration where available and shorten cutting-edge and apron wear checks. In cold climates, verify starting aids and coolant concentration ahead of season, since the D343T has no modern cold-start electronics to fall back on. In wet or muddy conditions, check wheel bearing seals and final drive breathers more often for water intrusion. For long idle or storage periods, stabilize fuel and protect exposed hydraulic rod surfaces from corrosion.

631B attachments & work tools

Scraper bowl and cutting edge (primary work tool)

Single open bowl is the machine's only work tool, rated about 16 m³ (21 yd³) struck and 23 m³ (30 yd³) heaped. A bolt-on replaceable cutting edge spans the bowl mouth for the cut/load pass; this is a wear part, not an interchangeable tool.

Tractor-to-scraper coupling (gooseneck/drawbar)

Two-wheel tractor and two-wheel scraper unit are joined at a pivoting gooseneck drawbar/yoke, the standard articulation joint for this overhung two-axle scraper layout. This joint is the mounting point for the cushion-hitch accumulator upgrade.

Apron and ejector controls (hydraulic bowl kit)

631B moved to fully hydraulic control of apron and ejector, replacing the cable-operated system of the earlier 631A. Apron raises/lowers to meter material in and out of the bowl and the ejector pushes the load forward to dump; both circuits run off the tractor's PTO-driven hydraulic pump.

Cushion hitch (gooseneck accumulator)

Nitrogen-over-oil accumulator hitch with a restricting orifice damps shock at the gooseneck, curing fatigue cracking and reducing the 'loping' ride typical of overhung two-axle scrapers. Offered as an option starting in 1967, then made standard equipment from serial number 67M5012 forward.

Special Application heavy-duty bowl package

Reinforced bowl variant adds roughly two extra tons of steel plating around the bowl area for abrasion resistance when handling rock or hard, cutting material. Offered as a factory alternative to the standard-duty bowl for this class.

Push-pull self-loading arrangement

Cat's bail-and-hook coupling lets two scrapers assist each other's loading pass without a pusher tractor. This arrangement is well documented as a standard factory feature on the follow-on 631C-based 637 series from 1970 onward; its availability as a factory-fit option on the 631B itself is not clearly documented and should be verified against the specific unit's records.

Tires

Bias and radial tires around 37.25-35 are catalogued across the shared 631 through 631G tire-compatible family, but exact factory-fit size and single-vs-dual rear setup for the 631B varies by configuration/series and production year; treat as an approximate range rather than a confirmed spec.

Engine/power arrangement

Single Cat D343 diesel (naturally aspirated or turbocharged D343T depending on source), documented at roughly 335-360 flywheel hp; treat as varies by configuration/series. This is a single-engine, two-axle tractor, distinct from twin-engine Cat scrapers of the same era such as the 627.

Operator protection (ROPS/cab)

Factory ROPS canopy is not confirmed as original equipment for the 631B's production run (circa 1962-1969/70), which predates widespread ROPS regulation. ROPS retrofit kits and enclosed-cab conversions appear on later 631C-era resale listings rather than as documented original 631B factory content.

All 631B assemblies by section

Every catalogued assembly group for the Caterpillar 631B. Open an assembly to preview the parts inside — full OEM part numbers are available in Heavy Parts AI.

631c Scraper

7j4938 Apron Group

7J***38

Apron

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Apron (7j4938 N/S)

7J***38

Apron

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5j5451 Apron Cylinder Group--8.25" Bore X 28.60" Stroke

5J***51

Apron Cylinder Group

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6j6228 Apron Group

6J***28

Apron Group

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5j5046 Apron Lift Group

5J***46

Apron Lift Group

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Bowl (6j5546 N/S)--Part 2 Of 2

6J***46

Bowl

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Bowl (6j5546 N/S)--Part 1 Of 2

6J***46

Bowl

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7j2395 Bowl Cylinder Group--R.H.--(Change 4)--7.25" Bore X 34.40" Stroke

7J***95

Bowl Cylinder Group

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7j2396 Bowl Cylinder Group--L.H.--(Change 2)--7.25" Bore X 34.40" Stroke

7J***96

Bowl Cylinder Group

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7j2396 Bowl Cylinder Group--L.H.--(Change 4 & 5)--7.25" Bore X 34.40" Stroke

7J***96

Bowl Cylinder Group

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7j2395 Bowl Cylinder Group--R.H.--(Change 2)--7.25" Bore X 34.40" Stroke

7J***95

Bowl Cylinder Group

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Bowl (7j4860 N/S)--Part 1 Of 2

7J***60

Bowl Group

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Bowl (7j4860 N/S)--Part 2 Of 2

7J***60

Bowl Group

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7j4860 Bowl Group--Part 1 Of 2

7J***60

Bowl Group

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7j4860 Bowl Group--Part 2 Of 2

7J***60

Bowl Group

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Brake Control (7j5397 N/S)--As Viewed From Rear Right Hand Side Of Machine

7J***97

Brake Control

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7j5397 Brake Control Group--As Viewed From Rear R.H. Side Of Machine

7J***97

Brake Control

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Brakes (3j9522 N/S)

3J***22

Brakes

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3j9522 Brake Group

3J***22

Brakes

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7j4488 Cutting Edge Group--For Replacement Of End Section

7J***25	Cutting Edge 901.7 mm (35.50 In.) Long 34.9 mm (1.375 In.) Thick	2
7J***88	Cutting Edge Group	1

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8j5931 Apron Cylinder Group--8.25" Bore X 28.64" Stroke

8J***31

Cylinder Group-Apron

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7j2841 Draft Frame Group

7J***41

Draft Frame

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Draft Frame (7j2841 N/S)

7J***41

Draft Frame

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Draft Frame Lines (7j4215 N/S)--As Viewed From Right Hand Side Of Scraper--Part 3 Of 3

7J***15

Draft Frame Lines

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Draft Frame Lines (7j4215 N/S)--As Viewed From Rear Of Scraper--Part 1 Of 3

7J***15

Draft Frame Lines

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Draft Frame Lines (7j4215 N/S)--As Viewed From Rear Of Scraper--Part 2 Of 3

7J***15

Draft Frame Lines

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7j4215 Draft Frame Lines Group--As Viewed From Rear Of Scraper--Part 2 Of 3

7J***15

Draft Frame Lines

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7j4215 Draft Frame Lines Group--As Viewed From Rear Of Scraper--Part 1 Of 3

7J***15

Draft Frame Lines

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7j5215 Draft Frame Lines Group--As Viewed From R.H. Side Of Scraper--Part 3 Of 3

7J***15

Draft Frame Lines

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4j9014 Ejector Cover Group

4J***14

Ejector Cover Group

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6j1419 Ejector Cylinder Group--(Change 3)--8.25" Bore X 74" Stroke

6J***19

Ejector Cylinder Group

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6j1419 Ejector Cylinder Group--(Changes 4, 5, 6, 7)--8.25" Bore X 74" Stroke

6J***19

Ejector Cylinder Group

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6j1419 Ejector Cylinder Group--(Change 2)--8.25" Bore X 74" Stroke

6J***19

Ejector Cylinder Group

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3j8338 Ejector Group

3J***38

Ejector Group

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4j8180 Ejector Group--Type 2

4J***80

Ejector Group

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4j8180 Ejector Group

4J***80

Ejector Group

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4j8180 Ejector Group--Type 1

4J***80

Ejector Group

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Ejector Lines (7j4875 N/S)

7J***75

Ejector Lines

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7j4875 Ejector Lines Group

7J***75

Ejector Lines

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6j4635 Fender Group--Replaced By 8j3035

6J***35

Fender Group

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8j3035 Fender Group

8J***35

Fender Group

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Name Plates And Transfers

1H***68	Plate-Serial Number-Scraper On Left	1
1P***80	Transfer Group	1
1P***78	Film--Cat; Film -Cat	2
1S***93	Plate- Scraper Capacity	1

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7j650 Quick Drop And Check Valve Group

7J***50

Quick Drop And Check Valve Group

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7j6160 Rear Fender Group

7J***60

Rear Fender Group

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4j7973 Rotochamber Assembly

4J***73

Rotochamber Assembly

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6d6355 Brake Shield Group

6D***55

Shield Group-Brake

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7j4294 Shipping Bracket Group

7J***94

Shipping Bracket Group

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5d5755 Slack Adjuster Assembly--R.H.

5D***54

Slack Adjuster Assembly

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4j9134 Three Piece Cutting Edge Group

4F***42	Bolt 7.62 cm(3 In.) Long	21
4J***32	Cutting Edge End Section--35.50" Long--1.375" Thick	2
4J***34	Three Piece Cutting Edge Group	1
7J***26	Cutting Edge (Center)--54.25" Long--1.250" Thick; Cutting Edge- (Center) 54.25" Long--1.250" Thick	1

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7j4885 Three Piece Cutting Edge Group

4J***58	Bolt--Per Change 1 To 7j***85	21
7J***57	Cutting Edge--54.25" Long--1.625" Thick; Cutting Edge- 54.25" Long--1.625" Thick	1
7J***59	Cutting Edge--35.50" Long--1.625" Thick; Cutting Edge- 35.50" Long--1.625" Thick	2
7J***85	Three Piece Cutting Edge Group	1

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7j4490 Three Piece Cutting Edge Group

7J***25	Cutting Edge 901.7 mm (35.50 In.) Long 34.9 mm (1.375 In.) Thick	2
7J***26	Cutting Edge (Center)--54.25" Long--1.250" Thick; Cutting Edge- (Center) 54.25" Long--1.250" Thick	1
7J***90	Three Piece Cutting Edge Group	1

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7j8642 Three Piece Cutting Edge Group

2J***07	Nut-Full; (1-8 X38.1 Thd)	21
4F***42	Bolt 7.62 cm(3 In.) Long	21
4J***58	Bolt--Per Change 1 To 7j***85	21
7J***57	Cutting Edge--54.25" Long--1.625" Thick; Cutting Edge- 54.25" Long--1.625" Thick	1

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5j8538 Three Piece Cutting Edge Group

5J***28	Cutting--Edge 54.25" Long--1.125" Thick; Cutting Edge- 54.25" Long--1.125" Thick	1
5J***38	Three Piece Cutting Edge Group	1
6J***45	Cutting Edge--35.50" Long--1.125" Thick; Cutting Edge- 35.50" Long--1.125" Thick	2

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7j4891 Three Piece Cutting Edge Group

5J***17	Cutting Edge--54.25" Long--.875" Thick; Cutting Edge- 54.25" Long--.875" Thick	1
7J***25	Cutting Edge--35.50" Long--.875" Thick; Cutting Edge- 35.50" Long--.875" Thick	2
7J***91	Three Piece Cutting Edge Group	1

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7j4892 Three Piece Cutting Edge Group

5J***28	Cutting--Edge 54.25" Long--1.125" Thick; Cutting Edge- 54.25" Long--1.125" Thick	1
6J***45	Cutting Edge--35.50" Long--1.125" Thick; Cutting Edge- 35.50" Long--1.125" Thick	2
7J***92	Three Piece Cutting Edge Group	1

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4j2280 Automatic Speed Change Valve Group

4J***80

Valve Group

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8j8840 Quick Drop And Check Valve Group

8J***40

Valve Group-Quick Drop & Check; -Carry, Quick-Drop

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4j9349 Relief Valve Group--Apron Sequence

4J***49

Valve Group-Relief; -Carry, Quick-Drop

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Wheel And Axle (3j9249 N/S)--29.50 X 35 Tubeless Tires

3J***49

Wheel And Axle

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3j9249 Wheel And Axle Group--29.50 X 35 Tubeless Tires

3J***49

Wheel And Axle

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631B serial number reference

On these vintage wheel tractor-scrapers the tractor unit and the scraper (bowl) unit each carry their own serial plate and prefix - a shop or parts manual for this machine may list two different prefixes together (one per unit), so do not assume a second prefix seen alongside 13G belongs to the tractor or to the 631B model. The plate is a riveted metal tag on the main frame, most commonly found near the operator's platform or along the frame rail. Pre-1970s Cat machines use the short-form serial: a short letter-and-number prefix identifying the model/production block, followed by a sequential unit number - there is no full 17-character PIN on machines this old. Match the tractor-unit prefix specifically against dealer or parts-book records to confirm model and production block; do not assume adjacent or paired prefixes belong to the same model.

Prefix	Identifies	Notes
13G	631B wheel tractor-scraper, all-hydraulic bowl controls	Tractor-unit prefix for the 631B, successor to the cable-operated 631A (51F prefix) and predecessor to the 631C. Covers 631B production from introduction (examples on record from 13G1 up past 13G2400). D343T turbocharged inline-six, rated 268 kW (360 hp) at the flywheel at introduction, paired with a 9-speed planetary powershift transmission. A 1967 running change added the Cushion Hitch as standard and raised rated output to 298 kW (400 hp); this late block (1967-1969) may sit on a distinct sub-range within 13G or on a separate prefix - not confirmed in available records, verify late-production (cushion-hitch, 400 hp) machines against the parts book or with your dealer. Whether the full 4,000+ unit run (1962-1969) stayed on this single prefix block through the end of production is also unconfirmed.

Frequently asked questions

What engine does the Caterpillar 631B use?

A Cat D343T: a six-cylinder, turbocharged and aftercooled diesel of about 14.6 L (893 cu in) displacement. Output is documented in the 335-400 hp range depending on production year, with the 400 hp flywheel rating tied to the 1967 update that also introduced the Cushion Hitch.

What is the operating weight of the Caterpillar 631B?

Roughly 31.6-31.8 t (about 69,700-70,000 lb) empty. Exact figures vary by configuration, since the reinforced-bowl Special Application variant adds structural weight over the standard bowl.

What replaced the Caterpillar 631B?

The Cat 631C, which took over the model line in 1969 when 631B production ended. The 631B itself had replaced the cable-operated 631A in 1962.

What 631B owners discuss

Is the D343T engine in the 631B considered reliable, and what maintenance quirks come with it?

The 631B runs Cat's D343T, a turbocharged six-cylinder diesel with twin overhead cams and four valves per cylinder, valve lash set with bucket followers (long screwdriver adjustment, not a wrench). Rated output is reported as roughly 335-360 hp at introduction depending on serial number, then raised to 400 hp at the flywheel starting in 1967 - treat the exact figure as varying by series and go by the tag on your machine. Old-timers who ran these describe them as tough once broken in, capable of running weeks between shop visits, but heavy on oil and coolant upkeep by modern standards. Because the same basic D343 block was used across loaders, graders, compactors, and marine/generator sets of the era, parts commonality is decent, but if you ever swap an engine between machines, match the fuel system to the original application - injection pump and governor calibration are not interchangeable across uses.

What is the gooseneck/hitch cracking problem people bring up with 631B scrapers, and how was it fixed?

Mid-1960s 631Bs had a documented run of fatigue cracking in the gooseneck (hitch) area, in some cases breaking through completely during operation. Owners also complained the ride was rough enough at higher transmission speeds that they avoided using top gears, which is what drove the stress in the first place. Cat's fix was the Cushion Hitch, introduced in 1967, which cut stress at the gooseneck and killed the 'loping' motion typical of two-axle overhung scrapers. It was later offered across the two-axle scraper line (except the 613) and eventually became standard. On any pre-1967 631B, or one you can't confirm was retrofitted, inspect the gooseneck closely for weld repairs, doubler plates, or crack indicators before putting it back to work - have your dealer or a qualified vintage-Cat mechanic verify the hitch structure is sound before operating under load.

What do mechanics check on the transmission and hydraulic seat when troubleshooting a 631B?

The 9-speed Cat powershift has screens and magnets at the rear of the case, under covers on each side, plus a separate screen at the bottom of the flywheel housing - all worth pulling and cleaning during a rebuild or when chasing a pressure complaint. There's a gauge port at the top of the filter housing for checking trans pressure directly. A quirk specific to B-series machines with the optional hydraulic seat: seat lift oil is tapped off the transmission filter housing, so if the seat won't raise, or only comes up partway, that's usually a sign of low transmission pressure rather than a bad seat cylinder - check the trans before chasing seat parts.

How does the bowl/apron/ejector hydraulic system behave, and what wears out first?

The 631B was fully hydraulic for bowl, apron, and ejector, a step up from the cable-operated 631A it replaced. Typical cycle times cited for the 631 family run around 3.5 seconds each way for the bowl, roughly 4 seconds up and 3.8 seconds down for the apron, and about 8.5 seconds each way for the ejector - exact numbers vary by series and pump condition, so use them as a rough baseline rather than a spec to chase. System capacity is on the order of 142 liters (37.5 US gal). Expect apron seals, the cutting edge, and the ejector/bowl wear plates to be the first things needing attention on a working machine; sluggish or uneven cycle times usually point to worn pump output or a control valve linkage needing adjustment rather than a cylinder problem.

Why does the 631B always seem to need a dozer pushing it while loading?

It's normal for the class, not a fault. The 631B is a single-engine, 16-23 m3 (21-30 cu yd) struck/heaped scraper, and operators consistently note that a single-engine 631 'always needed a push to load enough to get anything done,' unlike twin-engine machines like the 637 that could push-pull or self-load without help. Budget for a pusher tractor or dozer in the loading cycle when planning production on one of these; it is inherent to the two-axle, single-engine layout, not a sign of a weak engine or hydraulics.

What wear patterns show up on the tires and running gear since there's no undercarriage to inspect?

Being a wheel tractor-scraper, there are no tracks to worry about - wear concentrates on the tires (631B commonly ran 37.25x35), wheel bearings, and the draft arm pins/bushings at the gooseneck connection. Given the model's documented history of gooseneck fatigue cracking, pay particular attention to the draft arm and hitch pin area for elongation, cracking, or prior weld repair, not just tire tread depth. Tire replacement cost is a real budget item on a machine this size, so factor tire condition heavily into any valuation.

What should I check before buying a used 631B?

Confirm the serial number falls in the 631B production range (1962-1969) and match it against known Cushion Hitch cutoff dates - ask whether the gooseneck is original or retrofitted, and inspect that area for crack repair or reinforcement plates regardless of what you're told. Pull the transmission filter housing screens and magnets, check trans pressure at the gauge port, and if the machine has the optional hydraulic seat, use its lift behavior as a quick field check on trans pressure. Look for smoke, oil consumption, and boost response on the D343T, and get an oil sample if the seller allows it. Check tire condition and confirm the fuel system components match the engine's original application rather than being mismatched from a parts-machine swap. Cycle the bowl, apron, and ejector through a full stroke and compare timing side to side for a worn pump or valve. Because this is a 1960s machine with a documented structural failure mode at the hitch, have your dealer or a qualified vintage-Cat mechanic verify the gooseneck and hitch pin integrity before it goes back into service.

Compiled from owner and technician discussions across the industry — experiences vary by serial range and machine history.

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