Piston Pickup / Vibes

Had problem of excessive engine vibration ( bike would progress backwards when reved up on stand).
Other symptoms (may not be related) Smokey LH exhaust on start up
and more lately vibration on over-run / light throttle.

Problem thought initially to be mains. Engine had been rebuilt by PO with most parts new (inc mains rods, block, mains, pistons etc) only a few miles put on since.(200ish)

I've started the stripdown & found pickup on LHS piston (front & back, just below the oil ring & about 1/2 in sq)

At this stage I suppose I'm debating wether the pickup could have caused the vibrations. Suspect I'm really just putting off the decision to take out & split the crankcase!

Would welcome the benefit of experience from those who've already seen this sort of thing!

Most post '70 Commando's will vibrate backwards on the centerstand when revved. It is a combination of the balance factor and the fact that the centerstand is mounted to the engine plates rather than the frame.

As far as "pick-up" on the LH piston, if this means dark carbon, then along with your smoking on that side, it probably is the rings. If the rebuild was recent, look at the cylinder. You should still see a slight cross-hatch pattern. Check the rings to make sure they aren't broken or sticking in the grooves. It could be the rings never bedded properly and the dark carbon is blowby.

Did you do a compression check before you tore the engine down?

If the crank turns freely and the endplay is normal, I would not be inclined to separate the cases just yet.

Pick up is aluminium - a few flakes in the head & the score marks on the piston. nothing much to see on the bores, a bit of a streak but no ridges, I'd guess a rub with 600 grade would eliminate that. Compression was good before ( tested but don't remember the figure). Rebuild was all recent. Rings look OK.

I'll probably try to smarten up the piston, re-assamble & re-try ?

Sorry to hear about your problems. 750 yes? Sounds like the piston has picked up and the reasons are the obvious ones.....lack of oil, too tight clearance, weak mixture, carbs unbalanced etc. It could show up as vibration if that cylinder is not pulling as well as the other. The oil smoke on startup seems logical but I'd expect to see damaged rings and bore. how do they look? It could also be a bad inlet seal, not seating or broken (they can be sheared if the valve compressor is clamped down too hard and the retainer squeezes the seal against the guide). A sump full of oil can also manifest the same and it's often only the drive side pot that smokes. Mine did the same.
Are the pistons oversize or std?. Since you say the PO put on new block (barrels) I guess std. Suggest you measure the pistons at 90deg to each other and compare. They might be slightly oval near the pin to allow for expansion but not sure which way. Get the bores measured as well and you should be aiming for 003"-004" clearance at the bottom of skirt. Production barrels used to be graded but we're only talking .0005" difference.
The piston should clean up with careful sanding and provided it's close to specs at 2.8713/2.8703" I'd then get the bores honed to that 004" cold clearance specced in the manual at 2.8746/2.8750" or whatever the pistons are + 004".
If you're worried about big ends going south, with the barrels off, it should easy to determine if there is any vertical play in the rods.

Did I pass you last Sunday week at Abinger Hammer, parked outside a Restaurant? Yellow 750?
Don't forget I'm just down the road if you need a second opinion or any help. Let me know

Thanks for that - the clearance looks to be about .006"across the pin & a tight .003 front to back, where the pickup was. Standard bore, new pistons & block, so could just have been tight.

Must have been another commando out & about - I was out, but only briefly to heat the oil through for draining!

I was out in the area too, but mine is an all black 850. We should organise a club outing!

Please educate this ignorant Yank on what the term "pick-up"means. :?:
Over here a "pick-up" is a small lorry,or part of an electric guitar! :lol:

Cheers!
Bruce

The piston seized, the surface of the piston will have alloy smeared on the face of the piston where the alloy was picked up and dragged.

10-4. Roger that!