MK3 850 engine problem

My 850 died the other day, lost power and seem to be firing on 1 cylinder.

Stripped the top end and found RHS Barrel and Piston badly damaged,. The piston has picked up on the front and rear face.

Barrel has gone for rebore, my problem is now what caused the seizure. If an oil starvation fault why only one side? I noticed the underneath of the damaged piston is blackened suggesting that the rings may have failed allowing blowback. Another suggestion is a bent rod, although before the problem the engine was fine - no unusual noise or vibration.

Do I need to undertake a complete rebuild or just do the rebore with new pistons.

Any suggestions appreciated

How does the connectin rod "feel". There can be slight side-to-side movement, but no "rocking" on the crankpin. If you are satisfied the rod big end is O.K., I don't think you need to split the cases. Oil starvation will normally manifest itself with rod big ends first. Oil is supplied thru the crank to the big ends and then squirts to the underside of the piston. Therefore you would expect a rod big end failure before a piston or piston pin seizure.
Did the engine seem to overheat before the seizure? Perhaps the original bore and pistons had insufficient piston-to-wall clearance. Had it been recently bored? Could the oil level have been low? The oil carries a lot of heat away from the underside of the piston.
Personally if I could not find a reason for the seizure I would do a complete engine strip and inspection so I could ride the bike with some peace of mind. But that's just me.

Ron L

This sounds like a classic case of the one side running too lean for some reason. This assumes that the bike is still running twin carbs. You didn't mention that there was any detonation damage to that side so we can assume the timing didn't change. Without further information my guess is this happened as a result of something plugging the main jet on that side and causing an extremely lean condition on that cylinder. Believe it or not the incoming fuel charge also cools the piston and valves (remember the old hot rodders mantra "gas is cheaper than valves" or at least that used to be true). As for a toal rebuild; you need to do a judgement call based upon several factors 1. has the engine ever been rebuilt 2. how many miles does the engine have on it 3. how reliable do you need the bike to be 4. how much money do you want to spend 5. how lucky do you feel.
Hope this helps some.

more info

Before the problem the engine felt fine, there was no sign of detonation on the piston. The exhaust valve on the damaged side was not carboned up, but a sort of rusty colour.

My gut feeling is that the engine overheated, it was a hot day, stuck in slow moving traffic for 20 minutes. When I toook the barrel to Norvil they said that the barrel had been plasticoted ( similar to powder coating ) this would stop proper cooling. The underneath of the piston is blackened suggesting that the rings failed. This fits in with the sudden loss of power, although the engine continued to run there was no compression.

The oil level was ok I am running silkolene sae 50 classic oil. The botom end is fine no paly on the big end and all feels free.

I think I may rebuild top end, fit an oil cooler and hope for the best