scoring on piston skirt

John D · Post by **John D** » Mon Jul 30, 2007 7:48 pm

Hi,

I have a number of old pistons and some of them have score marks on the skirt below the rings and at right angles to the gudgeon pin. As part of an ongoing oiling issue with my model 20 i removed the barrels and there is similar scoring on both pistons, particularly the rear of the drive side piston. The skirt of the piston has rubbed against the barrel, this you can see. These marks were not there when i put the bike together 800 miles ago. One reason for the oiling probs could be to do with a mistake fitting what I thought were rebored barrels and +40 pistons - I know, but I'm not an engineer and the ring clearance up the bore is as per the book. I have had crankcase pressure problems and when sripped the pistons were carbon coated and although not producing blue smoke the exhausts are very oily.
Anyone help explaining the scoring and please add any other comments, at the moment I'm frustrated with my lack of expertise.

thanks,

John

itma · Post by **itma** » Mon Jul 30, 2007 11:02 pm

This may be due to inadequate running in, hard to tell without seeing them; do they have a sort of black appearance? bits of metal shining in the scores?
With modern oils it takes far longer to run the things in, and picking up on the back of the pistons results.
I would not have thought ring clearance comes into this,
other possibilities are too close a clearance on rebore, or dirt in the engine; it shows up on the drive side piston as this is the one farthest from the pump [at least that was the received wisdom I learnt]
you can clean the piston up with an oilstone, and reuse with out ill effect, maybe it`ll rattle a bit more thats all.

John D · Post by **John D** » Tue Jul 31, 2007 8:58 am

John D · Post by **John D** » Tue Jul 31, 2007 9:08 am

Thankyou. I had a set of barrels rebored and oversize pistons 25 yrs ago. Memory then becomes an issue. They were only fitted for a short time before the bitsa was off the road. They were stored with many other bits. When I came to the rebuild I selected the pistons and barrels by a process of elimination. Fitted a couple of new rings as I'd broken them carelessly at some point and put them on the bike. I thought i'd run the bike in carefully. I've had ongoing oiling problems wit this bike which I have asked about before and having tried all sorts ended up returning to crankcase pressure being due to bore wear. This coupled with the oily exhausts and a leak on one head gasket (a failed gasket) led to my checking as above. Your point about the clearance might have something in it, but I'm puzzled about the oily mess! Guess I'll have to find someone locally who can measure everything very accurately and advise. Anyone in Cornwall who fancies a cuppa?