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Old 06-21-2015, 03:24 PM   #1
trampred OP
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Gear oil migrating from final drive to driveshaft and leaking out at boot

I recently changed all the gear oils (transmission, driveshaft, final drive) on a /5 I've had for less than a year. After filling, I got a leak almost immediately at the base of the driveshaft boot, which I thought might be because I overfilled the driveshaft level.

After riding it a bit, it's started leaking like crazy from the same boot area... not individual drops but more like small puddles on the ground. I read up on oil transfer paths for gear oil and went to check all the levels, and it turns out that the final drive level is very low (I filled it up to the bottom threads a couple of days ago). The only thing I can think of that is happening is it's moving into the driveshaft and leaking out at the boot. I haven't noticed gear oil leaking anywhere else.

The transmission oil level is slightly higher than when I filled it as well, so I suspect that some of oil may have transferred into the transmission – is that possible?

I'm thinking of taking it to a reputable mechanic here in LA, since I think this is more than my sidewalk "workspace" and and on-bike tools are capable of handling, but I'd like to get some idea of what I'm in for here...

And yes, I should've done all the oils as soon as I got it, so much for just changing the engine oil & filter and wishful thinking...
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Old 06-21-2015, 03:58 PM   #2
disston
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It is a step above sidewalk repair although I get away with this sort of job in my neighborhood. Not for everybody tho I agree.

The final drive has to come off. A large nut on the pinion removed which requires a specil holding fixture. Then a special socket used on the seal holding threaded ring. New seal then installed and all put back together. I forget the the actual torque value needed but you need a torque wrench good up to at least about one hundred lbs/ft.

The special tools needed are made and sold by Cycle Works. If you want to think about it some more check the prices on the final drive tools I mentioned. i think that info should help you find the two tools needed. http://cycleworks.net/
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Old 06-21-2015, 04:09 PM   #3
Bill Harris
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If it's leaking from the boot, you probably have a leaky boot. There is supposed to be 150mL of oil in the drivesahft housing.

Fill/adjust the transmission and final drive oil levels to exactly the spec'd levels. Put 150mL of oil in the driveshaft. After letting it sit overnight, recheck the trans and FD oil levels. Drain the driveshaft housing and measure the volume of oil in the driveshaft. That will tell you if and where oil is going.

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Old 06-21-2015, 04:42 PM   #4
trampred OP
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The boot definitely needs to be replaced as it well-worn, but I don't understand how just leaking at the boot would account for the missing oil in the final drive. I've browsed older threads here and the most common occurrence seems to be oil from the driveshaft making its way into the final drive and overfilling it. Since the final drive is lower than the drive shaft at rest, presumably any oil transfer that may happen when riding wouldn't take place if I just let it sit... unless the final drive oil is going somewhere else, like into the rear drum...?
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Old 06-21-2015, 04:49 PM   #5
CafeDude
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What oil did you use to refill?
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Old 06-21-2015, 04:53 PM   #6
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Bel-Ray 80W90 hypoid gear oil.
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Old 06-21-2015, 06:31 PM   #7
trampred OP
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Just a quick update: I drained the FD and driveshaft to see how much would come out, and got a little over 150ml out of the FD (should be 250 in there) and about as much, about 160ml, out of the driveshaft (should be between 100-150 depending on who you listen to). That said, when I filled the FD back up again, I made the mistake of trying to put 250ml in and it overfilled at about 180ml, which means the level was not as low as it looked from just inspecting the fill hole.

The 160ml that drained from the driveshaft suggests that I either grossly overfilled it, or some of the missing oil from the FD had transferred there.

In any case, I gave up on the whole 2mm screwdriver test and just put 120ml into the driveshaft, and will leave it overnight to see what, if anything, happens.
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Old 06-21-2015, 06:33 PM   #8
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I would suspect that the oil came from the output shaft seal in the transmission. Your final drive may have been low for a long time, having leaked out slowly over time. I've had two cases of the pinion seal going bad, and it always ended up draining the driveshaft oil into the final drive, which then oiled the brake shoes.

I don't think just letting things sit static overnight will tell you accurately what's going on. many leaks will only happen when the machine is in operation.
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Old 06-21-2015, 06:38 PM   #9
trampred OP
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I had the same thought, but if that were the case then the transmission oil level should be getting lower, whereas it's gotten slightly higher after refilling and riding a few miles... so I don't know. It was also high (gushed out after unscrewing the fill plug) when I finally changed the tranny oil after eight months of having the bike, and it wasn't milky (water adding volume), so I think that if anything, oil from elsewhere must be migrating to the transmission. Especially since when I drained the driveshaft the very first time, I barely got a few drops. But I suppose that could've easily gone out the bad boot or slowly into the FD over a long period of time, even though the FD level was low.
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Old 06-21-2015, 06:44 PM   #10
JonnyCash
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Your bike is amazing…. It's making oil!
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Old 06-21-2015, 06:46 PM   #11
trampred OP
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I'd have quite a (rubber) cash cow on my hands then, but no, I don't think so... and if it is producing oil I wish it would stop!
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Old 06-21-2015, 07:40 PM   #12
bmwrench
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'70-'72 /5s had no vent above the gearbox output shaft seal. This allows pressure to build in the swing arm. This causes a sound boot to balloon, and weak boots to leak. The solution is a pain; remove the gearbox and drill a hole above the seal. Try to find an example of one that has been done at the factory in '73, because the hole needs to be precisely located.
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Old 06-21-2015, 09:10 PM   #13
trampred OP
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Mine's a '73 so I believe it has the vent.
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Old 06-22-2015, 10:03 AM   #14
trampred OP
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How much oil should be in the transmission/driveshaft boot on a healthy airhead? I know the purpose is to keep water and dirt from contaminating the driveshaft and transmission. But it also needs to keep the driveshaft oil in? Am I right in assuming that during operation, oil from the driveshaft gets flung around inside the boot, and then some of it (all of it?) drains back into the driveshaft? Or is there any sort of seal or barrier that should be preventing or limiting that oil flow?
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