Rebore Advice Please

[bikerbob]

Hi there, I have a 1963 model A65 which I believe needs a rebore, it still has the original BSA pistons in from new I did fit +20 thou oversize rings in a few years ago because at that time I could not afford a rebore new standard rings had a gap of about 20 thou. I have got the barrel off and pistons and measured both the pistons and the bores. The pistons measure 74.33mm this size was taken just above the gudgeon pin, the bores measure 75mm this leaves a difference of .67mm which equates to about 26 thou. The pistons were measured using a micrometer, the bores were measured using a spring loaded bore measuring tool then locked removed and then measured with a vernier gauge. Looking at the workshop manual it gives the standard bore size as 73.983mm to 75.0062mm, but it does not give the actual size of the pistons it just gives clearance sizes for the top and bottomof the skirt. Given the existing differences between the original pistons and the present bore size do I go to + 20 thou pistons or +40 thou pistons. Thankyou.

[Cyclops]

Hi bikerbob
From an engineering point of view, i would take the barrel to a reputable engineering company. They would be able to accurately measure the bore at a number of points and determine whether it would take a rebore to available oversized pistons or if resleeving back to standard was the better option. Fitting 0.0020" O/S rings to worn pistons is not sound engineering practice.

[MALCOLM.JOHNSON]

The standard bore size you quote I think should read 74. 983 not 73.983mm. The standard bore size for your model is 75mm +/- a very small tolerance. You should get a confirmation of this figure by measuring below the bottom piston ring travel down the bore, or just above (clean any carbon deposits off) the travel of the top piston ring. Both these readings will be on a relatively unworn part of the cylinder. Now just below the top "unworn" part, is the point where you will expect to find the MAXIMUM wear, unless there is other physical damage such as that caused by broken rings to be seen. Measure this front to back, not side to side and the greatest "lip" which you will detect with a fingernail will be on the thrust face. When running the piston is pushed down and backwards by the expanding gases, creating the most wear to the rear of the cylinder (thrust face), although the compression stroke does create wear to a lesser degree to the front. So supposing you measure say 16 thou total wear, there is a good chance that the wear may not be removed by boring to + 20 thou due to this uneven wear. I would let the person who will do the boring, determine the next bore size, as he will have the experience to measure and form a suitable judgement. He will advise what pistons to get, but you would be best served by an engineer who can supply the pistons for you. That way if + 20 and +40 pistons are available to him, he could try boring to the lower size first to see if it will clean at that and if so extend the life of the cylinder.