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Harley Four-speed Transmissions

Harley Magazine Blog Techline - Donny Petersen

Harley Four-speed Transmissions


Locking setups, chain vs belt, and other comparisionsby Donny Petersen

This month we have an excerpt From Chapter IX of Donny’s Unauthorized Technical Guide to Harley-Davidson 1936-Present, Volume III.  (Some content has been altered to fit American Iron Magazine’s style and format.)

The locking methodology for the transmission sprocket or pulley evolved over years, overlapping with the latter years of the four-speed and the beginning years of the five-speed. There really never was an issue with locking transmission sprockets until the factory changed its long-standing, wraparound locking tabs to screws. The new style gives the sprocket/pulley nut some wiggle room for working its way loose. The methodologies described encompass both the four-speed and five-speed transmissions, though the story begins with the four-speed.

After the sprocket hold-down nut is torqued to H-D spec, the 10-24 screw is threaded into the most convenient of the three threaded holes in the sprocket to lock the nut in place and prevent it from loosening.

Locking The Sprocket/Pulley
The 1977-86 Shovelhead and Evolution four-speed transmission’s mainshaft final drive (fourth) gear extension (#350067-77A) supports the final drive transmission chain sprocket or the belt drive pulley, as the case may be. The gear portion sits inside the transmission case. The bearing surface next to the gear sits in, and is supported by, the transmission case, but is separated from it by a needle bearing (#8095) group, while its extension protrudes through the transmission housing. The six-spline extension can support either a mating six-spline transmission rear drive pulley (#33341-83) with a locking tab (#35216-36) and a locknut (#35211-36) that holds the early four-speed transmission pulley. (Both a nut and lock tab have been in use since 1936 but usually in conjunction with lighter and smaller chain sprockets.) The six-spline extension may also support a chain sprocket, which either has the aforementioned lock tab or the simple 10-24 x 3/8″socket head screw (#857) to hold the sprocket nut tight. There are three small, threaded 10-24  screw holes in the face of both late-style transmission sprockets and the later four-speed pulleys. Also, on later-style four-speed pulleys, the same locking methodology is used on the five-speed transmission sprocket (#33336-83). The pulley is the same except that the three 10-24 screw holes were bored into the outside pulley face the same as with the late-style chain sprocket. After the sprocket/pulley holddown nut (#35211-36) is securely tightened, the (#857) screw is threaded into the most convenient of the three 10-24 holes to lock the nut in place, preventing it from turning out and loosening. The belt drive itself, along with the pulleys, is virtually maintenance-free. It was a blessing when compared with a rear chain drive. However, the transmission rear pulley is much larger, wider, and heavier than the narrower chain sprocket (#33338-83).

Horsepower and, more importantly, the twisting force of torque were also on the increase during those years. If the nut loosened, it could move slightly back and forth until the screw prevented further movement in that direction. On the heavier pulley, this small 10-24 screw arrangement could not always cope with the force of movement. Many times, the screw head would shear off, allowing the pulley nut to unthread. The pulley could not fall off the shaft, but it could move laterally along it until its outer face rubbed on the inner primary chain cover. When this occurred two things would happen. First, the splines on the softer metal pulley began to wear due to its movement against the harder mating splines of the final fourth or fifth gear extension (depending on year) that protrudes out of the transmission case to hold the pulley. This mating arrangement transfers the engine’s torque as it comes out of the transmission, through the pulley/sprocket, and back to the rear tire where it contacts the asphalt. This wearing action will necessitate pulley replacement. Secondly, the clue that this is happening is a bad oil leak from the transmission. If the oil leak goes away, all is not well, since this means the transmission is now dry and without that necessary and vital lubricant. There’s a thrust spacer (#33334-85 or #33344-94) that keeps the five-speed pulley properly positioned when it’s tightened down between a properly mounted transmission pulley and the transmission. The oil leak occurs because this spacer also seals against the transmission output shaft main seal (1984-94, #12050 and 1994-2006, #12067B). If the pulley nut loosens, allowing the pulley to move out along the mainshaft, the thrust spacer is now free to do so, too. In this case, there is no sealing action, and the transmission fluid can and will flow out. This is also a safety issue, since splashing lubricant on a rear tire can be dangerous.

The MEGA nut from JIMS is the best option for precisely locking down the pulley nut. The multiple choices for the two locking screws means there’s no wiggle room for the nut.

Harley-Davidson came up with effective and simple solutions that made these problems disappear. First, the H-D engineers made a 360-degree lock plate (#40252-91) that fit over the pulley locknut after it was fully tightened with red Loctite 262 adhesive to the torque specification of 110-120 ft-lbs. Two screws with Loctite are then tightened to 7-9 ft-lbs. or 50-60 in-lbs., depending on manual. These screws secure the lock tab to the pulley face, preventing any lock nut movement. Previous torque specifications for the thinner nut and 10-24 x 3/8″-long screw were 105-120 ft-lbs. and 25-30 in-lbs. respectively. No movement means no momentum to shear the screws. Secondly, the locknut (#35211-91A) increased in width from 1/4″ to 5/16″ in 1991, proportional  to the bigger pulley. Thirdly, the two new 1/4″-20 x 5/8″-long locking screws (#3594) improved in 1992 from the measly 10-24 x 3/8″-long OEM (#857) single screw. Of course, the pulley holes for these threaded lock screws also increased in size to accommodate them. Fourthly, the pulley became lighter in 1985 (#40210-85). Original ones were heavier while the later ones (#40210-85A, #40210-85B, #40210-85C, and #40250-92) became lighter. Last, but not least, the spline arrangement on the fifth gear (#35029-85), as well as the female pulley splines, also improved. Instead of six sloppy splines like the four-speeds and early five-speeds, the number increased to 32 splines, which are more precise.

One of the aftermarket responses from JIMS is a round plate with exterior holes that incorporates a pulley nut. This innovation is called the MEGA nut (JIMS #1708). In my opinion, it is the best option of all. It is even more precise in locking down the pulley nut. The multiple choices for the two locking screws means there is no wiggle room for the nut, even if it were separate. If the rider/mechanic wants even more security, extra holes can be drilled and threaded into the pulley face to accommodate more holddown screws.

To get the rest of this story visit GreaseRag.com and pickup the February 2011 issue of American Iron Magazine.

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  1. kieth daniels September 20, 2014

    Can you tell me who makes and sells a leak proof trans seal for a 4 speed harley trans . I have seen videos on how to make them but i know someone out there must sell them. Im tried of hours of looking please can you HELP

  2. Jim Gabel February 6, 2011

    Great article Donny. I have a 1995 FLHT with 102k miles and I’m still running the original belt. I put it on the lift a few times a year to perform a visual inspection. It still looks good. Going to begin putting a finger on each rib to be sure I don’t have what you depicted in article. I don’t when I should get into the preventive maintenance mode, bite the bullit and change the belt the two pulleys out. I’ve heard it can be pretty nasty when a belt fails while tooling down the road.

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