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A re-bore takes so very little material away, that I think your mechanic is telling wee stories. Classically, pistons are offered in oversizes and there will be a range available, typically in +0.25, +0.5, +1.00 mm on the standard diameter. You choose according to the amount of scoring on the cylinder walls, then look up the recommended piston-to-bore clearance, then you take your barrels to a machine shop, and they do it for you. It is not too expensive.... in 1997 it cost me about Ł20 per bore, and about Ł72 per piston (Mahle) and there were 3 of them.
Westerbeke will not make pistons, I understand. Pistons are made by others. Typically, if you turn the piston upside down and clean off the gum a bit you can see the part number on it and the manufacturer... Mahle is one of them I have seen.
Be careful, there may be a slim possibility that the motor might already have been re-bored, so you will have to measure the bores at present, then choose.
Could you send a wee picture of the piston and the bores? I was interested in what was meant by a "cracked" piston, and bores rarely score unless there is a lubrication failure, so it would be interesting to see them. Has the piston shed lumps of itself into the crankcase?
You might get away with just a single new standard piston (and no re-bore) if the scoring isn't too bad, which will be about $100 plus your gaskets, and for an older engine, it can run for years yet. As long as the bottom end is sound and you have not complained of excessive bottom end noise???, you will probably be OK.
I don't know your mechanic, but I have seen often in the past a doom-prophet promising to fix a motor at great expense when there isn't a lot wrong with it. Be careful.
Take some pictures for us. It would be interesting to see what this "pitting" looks like.
Last edited by Rockter; 01-23-2009 at 12:39 PM.