Oil in rear boot: Drive or Tranni case seals?? - K-Bikes.com - Excellence in Motion
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post #1 of 1 (permalink) Old Sep 24th, 2006, 5:55 am Thread Starter
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Oil in rear boot: Drive or Tranni case seals??

Since I'm about to replace the big bearing in my final drive and have some oil in the boot (definitely hypoy) - This question bugs me:

Why? because if I'm dealing with the rear drive only and there is no suspicion of a seal leak there, I can do the work either on the bike or remove the rear drive on the stand and be pretty confident the pinion seal should be ok. Of course, once the drive is opened, if there's a lot of metal residue floating in the oil, that commits to both pinion and large drive seal replacement. Then there's the other worry, you change the seals and whilst the drive is handy on the bench, you want to be confident it's oil tight, before fitting and filling with oil.

Now, if I know the oil comes from the transmission case seal, planning the work is much more involved. I don't want to be removing the rear drive then find the bike has got to be setup on my lifting table with hoist in position and a whole set of tools needed to strip the motor back to the tranny case. Then there's the other thought, if I get this far I might as well dive into the clutch (36 K miles).

So here's my method to answer the title in the post:

I used a cheap balloon/soccer ball/ cycle pump inflator with my gas leak water manometer gauge (0-12" WG). The pump which cost $4 on Ebay and had a non-return air valve, but as expected it wasn't good enough to do the test, which involves leaving the casing under a small pressure for 15 minutes or so. The pressure has to be small (12" WG is just less than 0.5 psi) so as not to deform the seals causing them to seal better. The non return valve is very important. I found that a windshield washer pump valve worked perfectly and by blocking the end of the test tube, the test pressure remained solid for 2 hours! I already had a water manometer, but you could easy make a 'U' tube with some pvc tube. This is a very sensitive test for pressure loss.

Now the test: The inflator came with a tapered adaptor which conveniently fitted the 5mm pvc tube I used, this formed a leak free push fit seal in the top vent. I also had ideas to use grease nipples drilled out and fitted in a bolt - but this wasn't needed for the final drive.

Pump up the drive to 12" WG. If the level drops back rapidly, check first at the ABS sensor fitting, top breather, oil fill and drain plugs, you can use soapy water. Since most of these areas and the seals themselves will be coated in oil, a sudden pressure drop shows a pretty bad seal leak.

What is more likely (in my case) is the rear drive will pressurise and settle at some value. Now you first grab the rear wheel without rotating it and try to rock it in the vertical and horizontal planes. A leak might suggest the big output shaft seal.

Next rotate the wheel whilst looking at the manometer level- this is revealing. The level will drop slightly whilst rotating but should come back to about the same level. If not there's a rear drive seal leak and probably the pinion seal putting oil in the boot.

If you leave the drive pressurised like I did for 15 minutes and there was no significant pressure drop you can assume the final drive seals are probably OK. Unfortunately, you also know you are into the whole transmission cover seal job but can at least get tooled up in advance to pull everything back, including the clutch. I'm also wondering if there's a single vent easily accessible on the transmission and can this be pressure tested as well.

As far as the rear drive is concerned, this is simple to do, non-invasive requiring no tools and can easily give you a good or bad feeling about your rear drive seals, without even looking inside the boot.

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