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

Question

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 cylinder. The 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 "re-round" it, but that's still stressing a part that's been 'stressed' already. BSA Starfire/Triumph TR250 parts are interchangable, but rare enough for me to consider 'fixing' what I have. Or, should I chalk this up to searching the internet for a replacement piston?

Answer 1

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 mallet. The trick was to whack it on both sides of the flattened area. With 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 shape. Add 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 enough. Don't use a metal hammer though. You might clean up the piston with solvent and use a magic marker to color up the areas where you think the interference is. Stick it into the cylinder and the color should be scuffed there the interference is. This will help you know exactly where the high spot or ridge is and where you'll want to strike it at. Don't whack it right on the very edge of the skirt though. Give'er a few whacks, color it up, stick it in, and repeat this until it goes in smoothly. If 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+ degrees. Make sure it's clean and free of oil or solvents because the female of the house won't want the place stunk up. With leather gloves, oven mitts or whatever is necessary, see if the piston will still slip into the cylinder. If you can insert a hot piston into a cold cylinder, you should be in good shape. As far as stressing an old part, pistons are tough characters. I had an industrial engine that ran fine but used lots of oil. When 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 feet. No telling how long it had ran that way.

Answer 2

the only way you may be able to have that fixed would be bringing it to a macine shop. because if is not fixed correctly it would wear funny in your cylinder or it might not combust the fuel correctly. you might find it cheaper, easier, and safer just to replace the piston. Good luck

John

Answer 3

you can get your parts from British Cycle Supply,I deal with them all the time or try Rask Cycle they both have web sights that piston is trash. Worst case scenerio you go to a machine shop and have one made.

Answer 4

Its easier to get parts now then 30 years ago If chunk thats falls of that skirt !!!!