R100rt 1981 117k miles original but oil leak

[Triluki1]

Evening all! Seeking wisdom! My boxer has covered 117k miles, starts on the first turn hot or cold, lovely smooth tickover, no smoke....but oil leak from a push rodĺ/ block rubber seal. I don't want to do unnecessary work, but do want to do whats necessary. Heads have never been off. I did a compression test in an attempt to find out piston condition, but confusingly got 150 psi on both cylinders. I was expecting far less on this mileage. I need to pull the barrels...I spose...to replace push rod seals, but do I leave rings alone? Or little ends? Or big end bearing caps (sorry I'm also a classic car lover so maybe wrong term! ) bottom line, original 117k miles no work other than servicing greasing and new clutch, runs great, oil leak between push rod and block. What work should I tackle when heads are off to replace the rubber seals?

[Kurt in S.A.]

Welcome to the forum! The 1981 came with Nikasil-lined cylinders so you might be OK in terms of cylinder wear. Typically one replaces the rings only at some interval with a light hone of the cylinder and go on from there. But I think the bigger issue is your heads. They were of the rather poor design in the 1981-1984 era. It's surprising that your valve clearances haven't shown some issues in all those miles. The 1000cc RT version of this era of engines was even worse because of the higher heat experience especially with the fairing.

If it were me, I'd probably get the heads redone with the proper valve seats, valves, and guides and also consider new rings. Certainly removing the jugs completely allows to at least measure the end gaps.

Kurt in S.A.

[Triluki1]

Thanks Kurt...guess I'm gonna have to go that way...there's a good classic bike engineers only a few miles from me, never used them, but seems worthwhile giving them the heads and asking their opinion. Would be good also not to have to keep adding lead substitute to fuel!

[Rob Frankham]

I think it's a matter of degree really... you are going to have to pull the pots to change the push rod tube seals so it's actually very little work to examine everything else. Personally, I wouldn't pull the big end caps unless the there is evidence of wear (i.e. radial play and/or excessive axial play) and the same for the mains.As has been said, the cylinders are unlikely to need anything more than a visual check and a check of the ring end gap. The heads are worth examination as has been mentioned and replacement exhaust valve seats are always worth considering if they haven't been replaced before. Think 'preventative maintenance' rather than 'rebuild' 117k miles isn't an excessive mileage for an airhead engine.

A couple of tips and trick if you haven't dismantled this sort of engine before...

1. The easiest way to remove the cylinder/piston is to remove the stud nuts, withdraw the cylinder so theat the skirt of the piston is outside the cylinder spigot then remove the small end circlips so the you can pull off the cylinder with the piston still inside. When replacing, it is much easier to set the piston inside the cylinder before you start and reverse the process. Apart from anything else, it allows you to use a proper ring compressor to introduce the piston from the top rather than fiddle about with the chamfer in the skirt.
   
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2. When the connecting rod comes free of the cylinder, support it so that it can't fall against the edge of the crankcase aperture. If you don't, it will damage the sealing face and lead to base oil weeps. A length of dowel through the small end will support the rod on the lower cylinder studs
   
   
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3. When replacing the cylinders, avoid using too much sealant on the cylinder/crankcase joint. The joint is designed to be made without sealant, the seal being effected by the thin 'O' ring around the cylinder spigot. A small smear of sealant isn't a bad idea but be very careful around the upper stud holes. The oil supply to the rocker gear passes through those holes and along the stud 'tunnels' (hence the 'O' rings) the oil is introduced into that area by small ports in the case outboard of the thread. Over enthusiastic use of sealant can (and will) block those ports resulting in zero lubrication to the rocker gear. It is a good idea, once the engine is back together to turn the engine over (or even start it briefly) without the rocker covers on to ensure that you get a steady stream of oil from each rocker.
   
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4. Exhaust nuts... If they haven't been disturbed before, there is every chance that the exhaust nuts will have seized to the heads. If the nuts won't move with 'reasonable' force or if they move a short distance and stick do not persist cut the nuts off... I can't emphasise this enough... forcing the nuts WILL destroy the threads on the head and the nut. New exhaust nuts are availabe and they aren't particularly cheap but they are a d*mn sight cheaper than a repair to the head. If the threads are slightly damaged (and they can be, even if you cut off the nuts), use a thread file to clear them. What you want is a nut that will thread onto the stub by hand without sticking. On a more general note, it is a very good idea, at each service, to remove the nuts and coat the threads with anti seize.

I realise that you are experienced with engines but these points are 'airhead' specific and it's good to know the 'gotchas' for a specific engine before you start, If I'm teaching Grandma, I apologise in advance.

Rob