Big Twin Harleys were equipped with four-speed transmissions from 1936-86, 50 years in all.

The greatest source of trouble for four-speed transmissions are oil leaks. The idea of a bike marking its spot is bullshit. Oil is meant to be kept on the inside of the gearbox, not dripping off the bottom of the transmission, onto the frame and splattering all over the rear tire. A low oil level in the transmission will lead to hard shifting and early (read expensive) failure of the transmission’s internal components. The key to keeping the oil on the inside is careful assembly of the components and paying attention to the small details. The photos that follow, illustrate some of the details to watch when servicing your four-speed transmission.

Four-speed transmissions were produced by H-D from 1936-86. Changes to the cases were made as different features were added to the motorcycle. Pictured is an early 1977 ratchet top transmission, with an aftermarket mainshaft bearing support (arrow). The ratchet top for the 4-speed transmission was first introduced in the early 50s as an option to foot shift operation.

In mid 1979 the factory changed the transmission case and top to a newer style of shifting mechanism, shown is a 1983 4-speed transmission. This style of 4-speed was produced from mid 1979- 1986. The shifting mechanism changed from a rotating drum in the ratchet top to a rotating plate in the later models. This style of transmission has been called by different names such as the “cow pie top”, “pancake top” or the “washing machine top”.

Four-speed transmission must breathe. The rotating gears and shafts inside the trans churn up the 60 weight oil, creating moderate pressure. Ratchet top transmissions use a vent screw (A) to accomplish relief. The vent screw is located on top of the right side of the transmission’s case, in the threaded hole nearest the dowel pin (B). The vent screw is a two way street, it allows air to escape from the gearbox, but can also let moisture in (like when you wash your bike). A small amount of water in the transmission oil won’t hurt, as long as it doesn’t sit there for a long time. The moisture in the oil will turn to steam vapor and escape back through the vent screw once the oil in the gearbox reaches operating temperature, but still be aware of the vent screw when washing your bike. CCI offers chrome vent screws (PN 09326).

Four-speed transmissions are easiest to work on when they are removed from the motorcycle and placed on the bench or clamped in a bench-mounted vise. Before any disassembly takes place, the outside of the transmission should be cleaned, especially the top cover screws behind the ratchet assembly. Dirt and mung accumulates in these deep recesses filling the screw, sockets and making removal difficult

Here is an example of what can accumulate in the recesses of the trans top (arrow). Depending on how long the transmission has been in service you may need loosen and dig out the crud with a screwdriver, then blow it out with compressed air. Once you can see the head of the screw in the recess, check the socket of the screw head to see if it is clean, a sharp scribe and compressed air will remove any packed dirt from the screw socket.

If you haven’t drained the oil from the gearbox while the transmission was in the frame, now would be a good time. The drain bolt (arrow) is located on the bottom of the case, just inboard of the fifth frame mount and should have a shouldered copper-sealing washer. Drain the oil into a clean container, such as the bottom of a plastic 2 liter coke bottle, this way you can clearly see any metal or other debris. CCI offers a chrome replacement drain bolt and copper-sealing washer (PN36191) or replacement sealing washers (PN54223).

Looking from the front of the gearbox this is hopefully what you will see once you have removed the ratchet top. The main shaft gears (A) shifter fork shaft (B) and the countershaft gears (C). Here we’ve locked the gearbox in two gears at the same time, (second and third), so that we can remove the sprocket nut from fourth gear.

The fourth gear main seal is the greatest source of oil leaking from any 4-speed transmission. To replace the seal you need to remove the transmission sprocket. To accomplish this, you first need to disengage the locking tabs (A) that hold the nut (B) in place, by taping them back against the sprocket with a small punch and hammer.

A good investment is a JIMS Machine sprocket nut socket (CCI PN 20691), that will work on any 4-speed transmission from 1936-86. The JlMS tool comes with a collar (A) that threads on the end of the mainshaft and a socket (B) that slides over the collar and fits over the sprocket nut and has a 1/2-inch square drive opening in the other end. Both the main shaft and the sprocket nut have left-hand threads, which means to remove the nut you will have to turn the socket clockwise. It’s tough to hold the sprocket while removing the sprocket nut, that’s why we locked the transmission in two gears at the same time, to prevent the sprocket from turning.

With the sprocket removed from fourth gear, we pried the old seal out of the case and are ready to install the new seal. The seal ring (A) must be installed inside the seal (B) before the seal is pressed into the transmission case. Here we’re coating the outside of the new seal with Permatex aviation sealer, before pressing the seal back into the case.

On early model 4-speed transmissions, the seal ring is locked in place to fourth gear by this L-key (arrow). The long part of the key slides in one of the grooves of the fourth gear and locks the seal ring with the short side of the key in to the notch of the seal ring. The key is held in place by the sprocket. If this key is not in place, it can allow the fourth gear to rotate inside the seal ring, which in turn will cause fourth gear or the seal ring to disintegrate and leak oil.

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