Harley-Davidson 4 speed Big Twin gearbox

The venerable 4 speed Big Twin gearbox was used on everything from Knuckleheads right through to the early Evolution era. Whilst the overall design concept remained constant throughout it's life, there were many detail changes. Although strong, there are some wear prone items, so it is false economy to use sub-standard parts. We do not recommend, supply, or fit the "import" parts to these. Like our engine builds, we do not see premature failures when using quality parts and meticulous assembly.

Andrews mainshafts

Andrews mainshafts

This is the industry standard, and there are 4 versions available.

Andrews countershafts

Andrews countershafts

There are 3 versions of countershaft

Main drive gear bearing race and needle rollers

Main drive gear bearing race and needle rollers

These were used 37-E77. The race is also available with various oversize outside diameters to fit cases that require repair.

After installation the race must be line lapped to the bearing at the other end, to remove distortion and accurately size it for correct clearance. We do this here. The rollers (44 used) are also available in oversizes. The same rollers are used with the 37-E77 countershaft.

Main drive gear Torrington bearing

caged Torrington needle bearing

Late 77 saw a change to a conventional caged Torrington needle bearing to reduce assembly time.

It presses directly into the aluminium gearbox case. However, they were rather hit and miss regarding clearance, so this resulted in a steel insert being installed in the casing and a different version of this bearing with a ground outer race being introduced. So there are two year groups of this bearing style. They are not interchangeable.

Countershaft bearings

Countershaft bearings

L77-86 uses these Torrington needle bearings at either end of the gear cluster.

Main drive gear bushing

Main drive gear bushing

The mainshaft runs inside this bushing, which is pressed into the main drive gear.

After installation this bushing must be precisely machined to size to achieve the correct clearance. This is one of the wear prone items, and once worn the mainshaft oil seal and main drive gear oil seal suffer due to excessive movement. This is the major cause of oil leaks from this gearbox, so top quality parts and accurate machining are essential. We do this on our horizontal honing machine and work to .0001".

Mainshaft ball bearing

Mainshaft ball bearing

We use the shielded version of this bearing. Introduced in 1941, this bearing is metric. Strange but true.

Countershaft thrust washers

Countershaft thrust washers

Available in 7 thicknesses, these are used to set the correct end float on the countershaft gear cluster.

Insufficient end float can burn up the thrust washer, case bushing, and the end of the gear cluster. Excessive end float can cause missed shifts and rapid wear of the selector clutches.

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