Hi all,
I am restoring a WM20 and now that I have nearly all external parts cleaned and primed I am looking at restoring the engine and gearbox. I have removed the head and valves and the inlet valve 's sealing surface is pitted very badly yet the block face is good but will need a valve grind. I have ordered a new valve from the UK. My knowledge is now lacking. I have removed the piston and the top ring disintegrated in my hand, obviously it will need rings. The gudgeon pin sleeve in the piston has some wear and so I will have to sleeve the piston, the gudgeon pin seems true. The bore is actually quite good, with the smallest of lips where the piston finishes it upward movement. How do I know if and when I need a new piston, I cannot quite follow the logic of oversize pistons or rings. Is there a formula that can be followed to determine this ? Any help would be appreciated
Cheers
Kevin

email (option): myf1@hotmail.com.au

You need to take your piston and barrel to a motor engineer to measure properly with micrometers. The piston should be measured at the widest part at the bottom of the piston (skirt) and on a new engine (or fresh rebore) the barrel should measure 0.003"-0.005" bigger, this is the clearance calculated to allow for expansion of the piston. The BSA standards book quotes 0.007" is acceptable at engine overhaul and condemned at 0.017" (0.001" = one thousandths of an inch).

You can not guess at the wear or ovality and taper of the bore by feeling the ridge at the top. Ron

email (option): ronpier@talk21.com

Thanks Ron, I will take the pot and piston to an engineer and let the professionals do whats needed.
Regards
Kevin

email (option): myf1@hotmail.com.au

The illustration below shows where the bore should be measured to check for ovality and parallelism....When the bore wears it wears into what is essentially a 'barrel' shape due to the uneven loadings exerted on the piston by the connecting rod/crank....That is why multiple measurements are needed to establish the extent of the wear....Ian

email (option): ian@wright52.plus.com