- Thu Jul 03, 2014 10:22 am
#37156
The piston does not seal the bore, it's the rings, top two are compression rings and the lowest is the oil ring. The damage to your piston will not be the cause of the loss of compression, light scores on the piston are not a big deal. I would examine the rings for damage if the bore looks OK.
If the fault is a sudden onset then there is likely to be some sort of failure that has caused it opposed to general wear and tear, such as broken rings or a partial seizure through lack of oil and / or overheating.
Before you spend too much you need to find the cause or you may damage your new parts. Low oil, timing way out, or fuel starvation and a weak mixture.
Being a tight arse I would just chuck in a new set of rings and see how things go. If you are a perfectionist you can measure the amount of wear in the bore by placing a piston ring at the top and then the bottom of the bore and measure the difference between the gap in each location.
I have an old Suzuki SP400 singe where cheap pistons are not available, I have chucked together the best combination of worn pistons and scored bores with a set of new rings and achieved an acceptable result.
Whatever you do, don't try and clean the piston up with emery paper as apparently the abrasive grit sticks to the soft piston and causes even bigger problems.
If you have loads of money burning a hole in your pocket then you could treat yourself to an alloy barrel rather than pay for a re-bore.
As for the side play then I would agree with the other posts that up and down movement is the important consideration, the con-rod wont move from side to side very much when the piston is inside the barrel, don't forget that the smallest amount of sideways movement at the big end is magnified by the time it reaches the little end. There has to be a small amount of movement at the big end or there wouldn't be room for the oil to get to the bearing surfaces.
If the fault is a sudden onset then there is likely to be some sort of failure that has caused it opposed to general wear and tear, such as broken rings or a partial seizure through lack of oil and / or overheating.
Before you spend too much you need to find the cause or you may damage your new parts. Low oil, timing way out, or fuel starvation and a weak mixture.
Being a tight arse I would just chuck in a new set of rings and see how things go. If you are a perfectionist you can measure the amount of wear in the bore by placing a piston ring at the top and then the bottom of the bore and measure the difference between the gap in each location.
I have an old Suzuki SP400 singe where cheap pistons are not available, I have chucked together the best combination of worn pistons and scored bores with a set of new rings and achieved an acceptable result.
Whatever you do, don't try and clean the piston up with emery paper as apparently the abrasive grit sticks to the soft piston and causes even bigger problems.
If you have loads of money burning a hole in your pocket then you could treat yourself to an alloy barrel rather than pay for a re-bore.
As for the side play then I would agree with the other posts that up and down movement is the important consideration, the con-rod wont move from side to side very much when the piston is inside the barrel, don't forget that the smallest amount of sideways movement at the big end is magnified by the time it reaches the little end. There has to be a small amount of movement at the big end or there wouldn't be room for the oil to get to the bearing surfaces.