Replacement of primary side crank oil seal

Jack, the short answer is yes. I have not had to do this job myself but I remember spending quite a while talking to a man with a Mk3 Commando on Douglas Promenade (Manx GP about 1992) with the drive side all neatly laid out on a large sheet of plastic.He was all ready to fit a new seal and just waiting for his friend to find a new oil seal.

The reason for the surgery was that his primary drive was gaining engine oil, causing clutch slip. (One advantage of ATF is that you can see when the primary is contaminated) The only thing to watch out for when replacing the seal in situe is to make sure it is seated properly. That was the other advice from this chap who knew what he was doing. Maybe you should buy two spare seals?

Yes, I have just done it on a Mk2. You do not need to remove inner chaincase.Drilled 2 small (shallow!) holes in the old one and worked it out using a pointed tool; people often use 2 self-tapping screws.

As suggested above, buy 2 seals for peace of mind because it can be tricky to keep the seal going in evenly and it can get damaged. I bought 2 but only used 1....just.

It is a fairly straightforward task to replace this item but if you have an endless chain....which most sensible Commando owners have, Then you will have to take off both sprockets at the same time to get at the seal. So you are looking at removing the alternator rotor,front sprocket, clutch drum and the chain.

You will need a sprocket puller and clutch spring compressor. You might also need one of those fancy 2 in 1 clutch locking gadgets. They help when tightening up the crankshaft and gearbox nuts.

Usually behind the clutch drum are some Phosphor Bronze shims to help align the chain. Don't lose these or the two Woodruff keys that hold the rotor and front sprockett in place.

As people have mentioned above there are several ways of removing the old seal. It is possible to lever it out with a screwdriver but care must be taken not to scratch the crankshaft surface or it will quickly chew up the replacement.

Make sure that the replacement seal is a quality item. There are some cheapo versions around which fail as soon as heat and oil hit them. Buy from a known dealer. Be generous with grease when putting the new seal in place. Coat the crankshaft, the seal recess and the seal itself. I use a long piece of tube to drift my seals in place and a small rubber mallet if they go cock-eyed. Remember that the open side of the seal faces in towards the bearings.

It is a fiddlier job on the Mk3 but quite possible. It differs from the earlier models in that Nortons machined a circlip groove outside of the oil seal, to prevent it being popped out. The circlip is, contrary to the parts book illustration, simply a curved wire and not a Seeger ring. It's a bit of a pig to get out of the groove.

Unfortunately, in machining the cases to accept the Mk3's more widely spaced main bearings, the shoulder behind the seal was lost. This means that the oil seal cannot be drifted fully home as it will then be considerably behind the circlip and tight up against the main bearing.

The seals are quite shallow and don't need much encouragement to move put of square on assembly.

The only system that works for me is to use seals with a steel outer, thouroughly degrease the housing with a cotton bud and solvent and then apply bearing fit to the seal. I then gently push the seal until it is just in the housing, then enter the circlip behind it. It's then best to very carefully ease the circlip further until it just springs into the groove. At this point, if your luck has held, the seal will be sitting square and tight against the circlip. If it's any deeper then the circlip isn't providing any positive location.

Very useful information Richard. I always remembered seeing this MK3 with the primary side laid out on a plastic sheet. All parts clean while the owner was carefully cleaning the bearing housing with a small screw driver. Incidentally, he did carry a spare but it didn't seat properly and so was wasted, hence his mate looking for another. For that reason I recommended having two, just in case. (I have them in my spares) This chap knew what he was doing, he even transposed the gearshiftwith the rear brake pedal. Perhaps someone knows this bike. I know the owner was from the same locality as Mick Hemmings' shop. All these years on and I've never had to replace the seal, says something for RGM who rebuilt the engine in 1989. But that's how I came to know the seal could be replaced without splitting the cases. Thanks Richard for the extra tips.(+ others and thanks for the question jack, brought a few memories back)

Appears to be not an uncommon problem, but why did the seal fail?

Either excessive crankcase pressure and/or excessive oil in crankcase are two possibilities.

Crankcase pressure is dependent on the efficiency of the breather system and cylinder to piston ring leakage. Excessive volume of oil could be due to wet sumping, pressure relief valve opening too soon - oil from pump dumped straight into crankcases or inefficient oil pump i.e worn out and struggling to return oil fast enough.

Simon, I think that they're only NBR seals. A combination of age, heat cycles and contamination will see them off eventually. If they have a rubber outer, they can lose their grip and then spin, which leads to housing damage...

Excessive crank end float can see the lip coming perilously close to the sprocket keyway (which is much longer than it needs to be).

I can't imagine a situation where crankcase pressure can turn the lip out but if the seal isn't tight to the retaining circlip then it could push it outwards, possibly not square to the housing.