Crankshaft oil seal appears loose.

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Jul 24, 2006
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I seem to have a problem with the crankshaft oil seal (drive side) on my MK3 Commando. When I fit it, initially it is an interference fit in the space, but as it is pushed home to where it stops, the interference fit all but dissapears, and it can be moved easily if I was to tap one edge very lightly with a blunt punch, the opposing side of the seal will tip outwards.

Whats puzzling me is there is no shoulder for the seal to butt up to. It stops when it comes to rest against the main bearing. I seem to recall from my previous experience with Commandos that there was a shoulder present to locate this seal. The other unusual thing is that when the seal is fully home, it sits about 5mm beyond the circlip which retains it. This doesn't seem right to me.

So the question is, should there be a shoulder to locate this seal and if not should it sit approx 5 mm beyond this clip? I have a feeling that another repair is looming for the future!

Whether or not there is a fault, I don't want to dismantle it for repair until the end of the year, so could anybody suggest how they would fix this seal in place for the short term. I have considered bearing fix, but have held back as I thought that this may be a bit too severe for the problem, and will probably try silicone RTV.

The reason I have investigated it is because I am running a belt drive, and have left the drain plug out that the MK3's have, and every time I park up on the side stand after a run, there are a couple of oil drips, so I suspect there is some oil getting past the shoulder of the oil seal as I have gummed up the four alternator studs with silicone, and don't think it's from there.
Hello Reggie,
I have experience of this problem. Nortons fitted a wire circlip outside of the oil seal for the first time to prevent the seal popping out. However, they deleted the flange which it previously sat against meaning that it can go in too far if you're changing it in-situ.

This is yet another of those special Mk111 problems. The bike had lots of changes but of course no development whatsoever after introduction!

The first thing to try is to degrease the crankcase really thoroughly. I have than had success with Loctite bearing fit but you will need a seal with an exposed steel outer ring, not the full rubber type. The important thing is to push the seal in square until it is just in the housing, then put the circlip in and very gently ease it further a little at a time all round with a small probe until the circlip just expands into its groove. With a bit of luck the seal will be just retained by the circlip.

This is an easy job with everything dismantled because it can then be done from the inside.

I got away with this repair for a number of years. The ultimate solution is to machine the crankcase to take a steel "top hat" bush. There is enough depth to machine two circlip grooves (one in front, one behind) and this is what I now have. Norman White modified my cases.
That's very interesting. I have been thinking about it since I wrote my first post, and I was thinking of doing it as you described, that is push it until it in until it just clears the circlip groove. There is some interference fit in the first part of the crankcase recess, hopefully it will still be there at the point where the seal sits just behind the circlip. As I said, the seal is quite a loose fit when pushed fully through.

I'm pleased also to hear that there isn't anything abnormal with my crankcases as such.

I will have to consider having a top hat bush machined in as you described, but that won't be until it gets stripped down again, hopefully not until the end of the year at the earliest. It's as you say, difficult to locate correctly when the engine is not in pieces
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