A Different Perspective On Transmission Oil

[CCI]

I just took the top off a 4-speed Shovelhead transmission that I built in 1992. I have no idea how many miles are on the gear set because the bike didn't have a speedometer for about 15 of those years, but I know the bike got three top end overhauls during that time, was flat worn out at the third one, and is now due for a well -earned fourth top end as part of this tear-down. So the transmission has a few miles on it.

The transmission looks new inside. There is no damage, no obvious wear patterns, and even upon the closest inspection no discernible wear that would be a cause for concern or influence the function of the transmission in any way. The gear teeth look great, the shift clutches look great, there is nothing to repair. It's getting a new shifter top gasket and going back together.

So what was the oil? Leftovers from engine oil changes mostly, nothing special. Straight-weight SAE50 in cool weather, SAE60 in hot weather, a shot of 20w-50 if it was cold out, just whatever premium motorcycle oil there was extra of.

I'll defer to the oil experts here, but my guess is the transmission looks new because it leaks as bad as most four-speeds and it was getting a steady supply of clean, new oil. For all the HD riders here who keep saying it doesn't really matter what you use as long as it's good quality and you change it at the right interval, well, it looks like this old bike says you're right.

 

[BigCahuna]

For the most part, Harley transmissions are bullet proof. The oldest bike I've ever owned was a "48 Panhead. I had issues with everything on the bike but the transmission, never a problem. It got straight 60 weight oil. If you noticed the engine oil would get dark with age, my clutch was the dry type , so no oil was used there. But the trans oil always came out clean like you put it in yesterday. Being it wasn't run thru the engine, there was no contaminants in there to make it dirty. It's just a splash lube system, so oil starvation isn't a worry.The most problems I've ever heard about any year Harley trans, is seal leaks. I would guess that's from dirt grinding into the seal after a few years. As long as you kept oil in it, it did it's thing. My newer bikes always got 70/90 wt gear oil. Again always like new when changed. They say a stock Harley trans will handle between 100-150 HP, before something lets go. Or you have size 15 feet and shift like your trying to kill it. I never had a built engine with that much umph. General maintenance goes a long way.,,

 

[repairman54]

I've been running Redline heavy shock proof in my '01 5 speed tranny. Shifts fine and even damped down the clunk a little. I'm not a fan of motor oil in a trans. like HD recommends.

 

[CCI]

For the most part, Harley transmissions are bullet proof. The oldest bike I've ever owned was a "48 Panhead. I had issues with everything on the bike but the transmission, never a problem. It got straight 60 weight oil. If you noticed the engine oil would get dark with age, my clutch was the dry type , so no oil was used there. But the trans oil always came out clean like you put it in yesterday. Being it wasn't run thru the engine, there was no contaminants in there to make it dirty. It's just a splash lube system, so oil starvation isn't a worry.The most problems I've ever heard about any year Harley trans, is seal leaks. I would guess that's from dirt grinding into the seal after a few years. As long as you kept oil in it, it did it's thing. My newer bikes always got 70/90 wt gear oil. Again always like new when changed. They say a stock Harley trans will handle between 100-150 HP, before something lets go. Or you have size 15 feet and shift like your trying to kill it. I never had a built engine with that much umph. General maintenance goes a long way.,,

"They say a stock Harley trans will handle between 100-150 HP, before something lets go."

Easily. I built a Shovelhead stroker that would launch in second gear, I never put it on a dyno but the horsepower was such that keeping spokes in the back wheel was a chore. The only thing on that bike that didn't suffer was the transmission.

The four-speed Big Twin transmissions are almost bulletproof. The Sportsters, not so much, they were plenty strong if set up properly but they didn't handle abuse quite as well. I've rebuilt more than I can remember, and you can tell what happened by the wear patterns or type of damage.

The trick, which I suspect you already know but I am passing along for the general benefit here, is to get the gear lash (slack) out of the driveline before you apply a real lot of power. Where I see the most damage to HDs, and especially the older ones, is (to the engine) revving the bike from a cold start and (to the engine and transmission) launching hard from a stop without taking up the slack.

Parenthetically, this is why the pressure plate on the old 4-speed clutches wobbled the way it did -- it was designed that way.

If a rider is in the habit of winding the bike up and dumping the clutch that shock load is really tough on everything, from bent/twisted connecting rods to shattered gear faces and shift dogs. If the rider simply loads the driveline just a little bit before applying power so that the lash is taken up and any excess is turning into heat, smoke, and noise at the back tire, the transmissions hold up to tremendous load -- works fine/lasts a long time.