Mk3 Oil pump check value issue

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May 12, 2008
I have been working on my 75 MK3 oil pump to reduce wet sumping and
have run across a bit of a problem. The plunger for the check valve
is getting stuck in the timing side housing. I had to drill out the
old plunger it was stuck so badly, and the new plunger got hung up
almost immediately while I was checking its movement. I deburred the
new plunger but was afraid to mess with the timing side cover for fear
of making the plunger seat out-of-round. Has anyone else run into this
problem? How loose a fit can the check valve have before oil leaks
past it? Thanks, this board has been a life saver more than once...
You're going to have to have a close look at the hole, sounds like it's tapered or burred. If you haven't the equipment perhaps you could take it to a small engineering firm and they could measure it and advise. A little extra clearance shouldn't cause a problem if the pump is in good nick.

My impression has always been that they tend to stick open because of the small oil bleed hole behind allowing thick oil to cause a suction.

The seal is made between the top of the plunger and the oil pump rubber so I can't see any objection to a slightly loose fit, assuming the plunger stays the right way up. I polished my plunger and this seems to have helped.

The plunger should be free to move with just the slightest finger pressure.
The plunger will work fine with slight finger pressure for several pushes, then gets hung up, usually when I push it all the way to the bottom of the seat. I rigged up a "honing" device (a dowel wrapped with emery paper chucked in my drill) and will try to loosen things up there a bit. I just finished work on the oil pump so maybe that will help with the wet sumping. Thanks again for the advice.
Don't do it that way. If you do for goodness sake be careful you could quite easily remove way too much metal and and put more taper in the bore. I would be inclined to get the bore measured first. The problem might not be the bore, the vent hole could be too far forward??

If it is the bore a lead in on the piston skirt might help, or perhaps a machinist could turn you up a broach. The material wouldn't need to be anything but mild steel or a 8.8 bolt shank.

Never had a wet sumping problem with my Mk3
I checked the plunger (just purchased) and it isn't completely round. Soooo I ordered a few more of them, and just hope one will be correct. If that fails I will take it to the local machine shop and have them make one. One little check valve has kept this bike off the road for a few weeks now, waiting for parts... I should probably start hoarding little bits and pieces so this doesn't happen so often. Thanks again.
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