Harley-Davidson Transmission Rebuilds

Big Twin 4 speeds :: Big Twin 5 speeds :: Big Twin 6 speeds :: Big Twin conversions :: Sportster 4 speed :: Sportster 5 speed

All of our gearbox rebuilds and conversions are done using the correct recommended procedures and service tools. We only use the highest quality parts from Harley, Andrews, Jims, S&S, and Baker.

We do not supply, fit, or rebuild the low cost transmissions and parts that the Americans call "imported". We have seen, and been made aware of, too many issues with these components.

We prefer to let others deal with these. As with our engine builds, we want the transmissions to last and cope with the power that we put through them. Many of the bearings inside the gearboxes have a different internal clearance to normal bearings, but some of the distributers don't seem to realise this and consequently supply parts that fit, but are likely to cause premature failure.

If an engine lets go, pull the clutch in. If a gearbox lets go, look for a soft landing.

Big Twin 4 speeds

Amazingly, this gearbox was first produced in 1940, and Harley used this right into the start of the Evolution era. The first Softails were 4 speeds, as was the Shovelhead styled FXSB and FXWG.

Although the basic design remained unchanged there were numerous running changes, including mainshafts, countershafts, bearings, threads, and selector mechanisms.

This gearbox design is essentially very strong but there are some wear prone items, particularly if the oil level is allowed to drop. We have experience of all the year groups of these as well as all the relevant tooling, including the main drive gear bearing lapping tool for the '76 and earlier versions.

Shovelhead gearbox rebuild

Shovel gearbox on the left is for a '72 FLH restoration (no kick starter). The one on the right (a '78) is for a custom build.

The kicker shaft bushings, starter gear bushing and main drive gear bushing require accurate resizing after installation, and we hone these to the appropriate clearances on our horizontal honing machine.

Correct end floats, accurate positioning of the selector forks in the sliding clutches, and removing excessive wear in the gear linkage (especially with the '79 up turret top), give the least agricultural feel to the gear change.

The use of a complete Andrews close ratio gear set also gives a much improved, slicker, gear change and gets rid of the annoying jump in ratios between second and third. We also have a good synthetic oil available which helps.

Complete Andrews close ratio gear set

Andrews close ratio gear set gives a better spread of gear ratios, a much improved gear change, and better reliability.

Big Twin 5 speeds

The first of these appeared in 1980 on the newly introduced rubber mounted FLT and had a number of upgrades until it gave way to the 6 speed in 2007.

Failures are rare and are normally limited to replacing the 5th gear due to damaged splines caused by a loose pulley, breakage of the selector return spring, and the mainshaft seal being pushed out by a walking 5th gear needle bearing.

We have all the required tooling to rebuild these, either in or out of the chassis.

Rebuilding Harley Big Twin 5 speed transmission

Rebuilding Harley Big Twin 5 speed transmission into Delkron FXR case

Big Twin 6 speeds

Introduced in '06 on the Dynas, and then on all Big Twins for '07, there's not much to say about these, other than we have all the tooling to rebuild them when required. We also have oil that quietens them down and improves neutral selection.

Big Twin conversions

We supply and install all of the useful upgrades, such as 5 in a 4, 6 speed direct drive in a 5 speed case, and the Baker DD7. We can either install these kits into an existing casing or supply a complete box.

We can also install them in the chassis and, where necessary, recalibrate the later electronic speedometers on the dyno.

Sportster 4 speed

This gearbox was first introduced on the K model Flatheads in 1954 and ran through until 1990. This is, with some justification, perceived as a problem gearbox, invariably made worse when strict attention to clearances and end floats isn't adhered to.

Correct assembly and the use of higher quality aftermarket parts is essential with the Ironhead versions. Pre '85 versions have a mainshaft race that MUST be accurately sized and aligned with the trapdoor using a line lapping service tool. We have this, although, strangely, there is no mention of it's use in the service manuals.

Since the shafts and gears are mounted on a trapdoor all of this work can be done with the engine in the frame, but repairing the '75 and '76 cross over gear linkage does require some engine disassembly. There is no room for corner cutting on the 4 speed, but they can be made reliable, and with a slick gear change.

Bear in mind, though, that the racing XR750 still uses this box, so it can be made to work.

Sportster 5 speed

Introduced in 1991 the 5 speed has proved very reliable, being similar in design to it's Big Twin counterpart (some parts are the same) they are far easier (and therefore cheaper) to work on than the 4 speed, but still require a number of service tools, that we have.

Harley Sportster 5 speed transmission rebuild

Harley Sportster 5 speed transmission rebuild

The Buell XB9, XB12 and all the '04 up rubber mount Sportsters no longer utilise a trapdoor, so work on these requires the engine to be removed from the frame, stripped, and the crankcases split. The gear clusters, still on the shafts, then need to be pressed as a complete assembly from the crankcase.

The correct service tool for this, and reassembly, is essential, and we, of course, have this.


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