T120 1972Bonneville

[triman65]

I have had the barrel rebored and fitted new pistons and rings. a lot of oil seemed to be getting to the exhaust. So I stripped it down again and found the pistons had been rubbing against the barrel front and rear just below the oil scraper rings. Ring gaps set at 10 thou. There seems to be no play in either Big or little ends. Anyone with a suggestion to solve this problem.

[twolitre]

The piston skirts below the gudgeon pin are the bearing surface which guides the pistons up and down the barrel. Looking at the crankshaft from the clockwise rotating end - the right hand face of each piston is the thrust and carries most load on the power stroke but the left also carries thrust on the compression stroke. So they will "rub". The question is:- is the "rub" excessive or are the pistons just developing a normal polished, bedded in finish?
 Three things to consider besides that. Are the sraper rings (oil rings) upside down? And have you considered valve stem/guide wear which is often a major cause of oil burning? Finally worn big end bearings letting excessive oil pass can throw oil with a catherine wheel effect up the bore to swamp the oil control rings.

[triman65]

Hello Twolitre

The rubbing looks a bit groovy than a polish look, you can feel it with your finger nail. It has had new valves and guides fitted and only done about 20miles. As I said there seems no play in either big end or little end. The pistons fitted were EMGO and the rings were not marked top nor where they tapered. Will try and upload photos later as camera batteries are flat.

I have purchased a set of Hepolite rings and will use these when I sort out the rubbing on pistons.

[triman65]

Update I had a word with Len Craig and he said it should be 2-3 thou clearance back and front between piston and barrel. On checking I found it was only 2 thou in total between clean part of skirt and barrel  ( 1 thou back and 1 thou front)

[Revband]

New pistons in  rebored barrels will need at least 2 to 3 hundred miles to bed in up until this happens smoking is not unusual, just give it a chance.

[triman65]

contacted Supreme Motorcycles who did the rebore and they say possible wrong clearance between bore and piston which has slightly caused piston to lightly seize causing scoring on pistons.