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Question:

Can I repair a BSA B25 (67mm) piston?

I dropped the dang thing while scrubbing the carbon buildup off the crown, and it landed on it's skirt and flattened it ever so slightly - but enough to keep it from being reinserted into the cylinderThe piston crown fits into the cylinder nicely about 4/5 the way in, then hangs where the flattened section begins.It's aluminum, and I think I can quot;re-roundquot; it, but that's still stressing a part that's been 'stressed' alreadyBSA Starfire/Triumph TR250 parts are interchangable, but rare enough for me to consider 'fixing' what I haveOr, should I chalk this up to searching the internet for a replacement piston?

Answer:

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Back in the good old days when brand new pistons were sometimes out of round, the old timers would whack them with a hard rubber or wooden malletThe trick was to whack it on both sides of the flattened areaWith your piston, a few thousands of an inch is enough to mess it up, so it wouldn't take much at all to get it back into shapeAdd to that the fact that metal has a memory and has a desire to revert to its orginal shape, a few light taps might be enoughDon't use a metal hammer thoughYou might clean up the piston with solvent and use a magic marker to color up the areas where you think the interference isStick it into the cylinder and the color should be scuffed there the interference isThis will help you know exactly where the high spot or ridge is and where you'll want to strike it atDon't whack it right on the very edge of the skirt thoughGive'er a few whacks, color it up, stick it in, and repeat this until it goes in smoothlyIf it seems to fit ok when cold but you're not sure what'll happen when it's good and hot, stick the piston only in the oven (without rings) and get it thoroughly heated up 400+ degreesMake sure it's clean and free of oil or solvents because the female of the house won't want the place stunk upWith leather gloves, oven mitts or whatever is necessary, see if the piston will still slip into the cylinderIf you can insert a hot piston into a cold cylinder, you should be in good shapeAs far as stressing an old part, pistons are tough charactersI had an industrial engine that ran fine but used lots of oilWhen I took it apart, I found pieces of piston skirt in the oil pan and the skirts of 6 of the 8 pistons literally fell to pieces at my feetNo telling how long it had ran that way.
Anne Marie-Anne of Green Gables Jason Michael -Jason and the Argonauts

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