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.
This is the industry standard, and there are 4 versions available.
- 37-64 11.370" long. Knucklehead and Panhead
- 65-69 11.985" long. Last year of the Panhead and generator Shovelhead. The introduction of the starter motor with the 65 Electraglide required the mainshaft to be extended to accommodate the ring gear on the clutch basket. The sprocket shaft on the crank was also lengthened a similar amount.
- 70-85 12.470" long. The introduction of the alternator on the cone motor Shovels required a longer sprocket shaft on the crank to accommodate it's width, so the gearbox mainshaft had to be extended. This is the most common version over here.
- 84-86 13.725" long. This was for the belt drive 4 speed Evolutions such as the FXSB. This was due to the width of the gearbox pulley, although this did not require the sprocket shaft to be extended.
There are 3 versions of countershaft
- 37-E76 this shaft uses 22 loose needle rollers at each end of the gear cluster.
- L76-79 this shaft uses the caged Torrington needle bearings at each end of the gear cluster.
- 80-86 this shaft uses the Torrington bearings, but the shaft is no longer retained by a nut.
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
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.
- 82-86 ground outer race
L77-86 uses these Torrington needle bearings at either end of the gear cluster.
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
We use the shielded version of this bearing. Introduced in 1941, this bearing is metric. Strange but true.
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.
Powerglides - In The Press