Big Twin Harleys were equipped with four-speed transmissions from 1936-86, 50 yearsin 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 ventscrew (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 thetransmission 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. TheJlMS 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.
Before we install the sprocket back on fourth gear, we like to take a dab of silicone and push it into each of the grooves of fourth gear (arrows) effectively sealing the seal ring to fourth gear. A word of caution here, you don’t want to fill the entire groove, just push enough silicone in to fill the gap between the ring and the gear. The silicone will prevent any oil from seeping past the sprocket. Now you can reinstall the sprocket, locking tab and the sprocket nut, remembering the left hand threads on the end of fourth gear. When the nut is tight, bend one of the locking tabs solidly against one of the flats of the sprocket nut.
If you took the time to replace the main seal in your 4-speed transmission, it would be a good idea to replace the main shaft seal in theend of fourth gear while it’s easy to get to. These seals can be a real pain in the ass to install, you have to have them dead straight to install them correctly and they must be sunk into the end of fourth gear a few thousandths. After years of cussing and ruining a couple of these seals per try, we purchased a JIMS seal tool (PN 2256) that makes easy work of installing the main shaft seal. The JIMS tool is comprised of two parts, a sleeve (arrow) that fits over the mainshaft and a driver (next photo) that pushes the seal place. Here we have the sleeve in place and the seal fit onto the sleeve, again we coated the outside of the seal with some Permatex.
To install the seal, slide the driver (arrow) over the sleeve andtap new seal into place in the end of fourth gear. The sleeve and driver ensures that the new seal goes in straight and to the proper depth every time.14: (5240) We have come across this problem only a couple of times – whereNthe steel bushing for the clutch release arm has come loose and allowed therelease arm to move up and down, causing misalignment with the throw-out bearing on the clutch pushrod. The simple fix is to mark the correct height on the bushing, then knurl the lower section of the bushing, cleanthoroughly, apply some green stud and bearing mount Loctite and press the bushing back into the kicker cover to the original mark on the bushing.
Threaded holes, that hold the transmission top and side cover tothe transmission case, can also be a source of potential oil leaks. When youtighten the mounting hardware to hold the ratchet top to the case, it’spossible for the screw to pull the threads in the case and raise a slight burr at the gasket surface. This burr, no matter how slight, will prevent the gasket from being compressed thus allowing oil to pass by the gasket. Using a handheld counter sink, give it a couple twists in each of the threaded holes.
Here is an example of our handheld countersink that we’ve had in the toolbox for the last 15 years or so. The handle is from a 50-cent screwdriver we picked up at a swap meet, with the tip cut off and a $4.99 countersink brazed to the shank.
Part of the preparation of the transmission case is to clean each and every threaded stud. Most often a wire brush will remove any dirt or mung, if there is any dried Loctite left in the threads, a thread die can be screwed on the stud to remove the Loctite.
For gaskets to work properly, they must be squeezed equally between two perfectly flat surfaces. Any dings or burrs in the gasket surface will prevent the gasket from sealing. To remove any offending burrs or dings, use a fine cut file or, even better, a small sharpening stone to smooth the gasket surface. A light touch is called for here (don’t get carried away with the file or stone), just use enough pressure to remove the ding and flatten the surface.
Another source of potential oil leakage is the transmission mounting studs that fasten the case to the transmission mounting plate. The stud mounting holes are drilled through into the inside of the case (arrow); the factory originally used an interference fit style of threads that sealed themselves as they were installed. Depending on the previous owners of the transmission and how many times it has been in-and-out of the bike it’s possible that the studs loosened up and may leak. Once you have checked that the mounting studs are secure, clean the inside of the case and cover the stud-mounting hole with a dab of gasket sealer
Four-speed transmissions shift much easier when the shifter forks have been aligned with their top. This is easy to accomplish with an alignment tool from JIMS, offered by CCI. Although they appear similar there are two different tools; one for 4-speed transmissions with ratchet tops (A) up to 1979 (PN20001) and one for the late model tops (B) for transmissions 1979-86, (PN20108). The gauge is set to the shifter mechanism of the top, then place onto the transmission case, over the shifter forks, and the placement of the shifter forks between their respective gears are checked against the tolerances given in the service manual for your model year.
Thanks Frank, (he has over 35 years of H-D Tech writing experience). Terrific tech. If readers have any 4-speed tranny questions, send us a “Your Shot” at http://www.bikernet.com/shot/ and we’ll forward it to the Wizard, Frank Kaisler.
