Yanmar 4TN78TL, YB401W Parts Catalog – Factory OEM Specs

If you are rebuilding a Yanmar 4TN78TL, YB401W Parts Catalog — the factory one, not a reprint — is the only thing between you and a third teardown. I have watched guys order main bearings off a generic cross-reference and then wonder why the thrust clearance is off. Aftermarket catalogs often combine engine families; this one isolates the 4TN78TL and the YB401W wheel backhoe exactly. Forty-six pages of OEM part numbers, exploded views, and spec codes. No guesswork, no missing seal washers. That is what you get.

46 Pages of Factory Specified Component Lists

Every page comes straight from Yanmar’s own parts system. The catalog breaks down into sections: ENGINE and MAIN, VERTICAL (YB). The ENGINE section starts with the cylinder block assembly, then gear housing, oil pump, and all the hardware. Each item lists the OEM part number, quantity, and any supersession notes. For example, the cylinder head gasket has two variants: one standard (129553-01350) and one marked with a revision code. If you grab the wrong one, you are pulling the head again. That is page 2. The spec code list on page 3 gives you D29500-01100 and similar identifiers to match your engine serial number.

Does This Catalog Include Torque Specs for the Cylinder Head Studs?

It does not spell out torque sequences — that is in the service manual, not the parts catalog. But it does show you the exact studs, bolts, and washers with part numbers and dimensions. The cylinder head stud (129150-01200) appears 18 times per engine. The bearing cap bolts (129150-02020) are 10 per engine. If you need torque values, pair this parts catalog with the Yanmar 4TN78TL Engine Technical Manual for the full repair procedure. The parts catalog tells you what goes where; the tech manual tells you how tight.

Verify Your YB401W Backhoe Parts Against the Factory List

The YB401W shares the 4TN78TL engine, but the MAIN, VERTICAL section covers the backhoe-specific parts like the gear housing flange, hydraulic pump cover gasket, and O-rings. On page 4 you find the housing assembly with oil seal (129795-01800) and the gasket for the hydraulic oil pump (124240-01882). A non-asbestos version is also listed (124240-01883) for later builds. If you are rebuilding a YB401W, do not use a generic Yanmar tractor parts list. Use the Yanmar YB401W Technical Manual – Factory Parts Catalog for Wheel Backhoe to get the correct seals and bolts.

Cylinder Block Assembly – Exploded View and Part Numbers

Figure 1 on page 2 shows the cylinder block with all liner, main bearing, and thrust bearing positions. The catalog lists standard and undersized main bearings (0.25 U.S.) and oversized thrust bearings. The part numbers are exact: 129150-02870 for 0.25 undersize mains, and 129150-02940 for 0.25 oversize thrust. If you have a crank that needs grinding, you can order the correct shell without guessing. The exploded diagram also shows the location of each plug — including the PT1/4 plug (121850-11860) and the camshaft bush (129795-02411).

Gear Housing and Oil Pump: Section Breakdown

The gear housing section (page 4) is where most leaks start. The catalog gives you the O-ring for the fuel injection pump (121850-51960), the seal washer for the housing bolts (43400-500410), and the knock pipe (129795-01950). It also specifies the bolt lengths: M8x12, M8x45, M8x55, and M8x85. Aftermarket kits often substitute a different length bolt and cause interference. If you are replacing the hydraulic pump gasket, note that there are two versions — standard and non-asbestos (N on the revision code). That detail saves you a headache.

Forty-six pages. That is it. But those pages cover every nut, bolt, seal, and gasket for the engine and the YB401W backhoe setup. For related equipment, check the Ammann Yanmar B12 B17 Parts Catalog if you work on compact rollers.

As noted in the Automotive Industry Overview, factory documentation remains the standard for reliability. This parts catalog is no exception — it is the same PDF the dealers use. If you want to avoid ordering wrong parts and wasting time, stop using online forum threads. Automotive Industry Overview

Frequently Asked Questions

Is this the genuine factory parts catalog?

Yes, it is the official Yanmar factory parts catalog.

Does this catalog include torque specifications?

The catalog contains torque specifications for engine assembly such as cylinder head studs and bearing caps. See page 3 for spec codes and page 2 for bolt sizes. However, it does not provide torque sequences — those are in the service manual. Combine this with the technical manual for full rebuild instructions.

Can I use this for the YB401W wheel backhoe?

Absolutely. The catalog covers both the 4TN78TL engine and the YB401W vertical main assembly. It includes all part numbers for the engine block, gear housing, oil pump, and more. If you are rebuilding the backhoe, you need this to order the correct seals, bearings, and gaskets. Aftermarket catalogs often combine models; this one isolates the exact YB401W parts. The catalog lists each part with its unique OEM number, and the exploded diagrams show how everything goes together. For example, on page 4 you will find the gear housing with the oil seal, O-rings, and bolts. Without this, you risk ordering the wrong O-ring for the hydraulic pump cover and causing leaks. That is the difference between a two-hour job and a two-day job.

Is this a PDF download or a physical book?

It is a digital PDF download available immediately after purchase. Print it or keep it on your tablet in the shop.

Stop trusting internet forum part numbers. Get the real thing right now. Download the Factory Manual and keep it on your tablet in the shop.