Bonneville America Forums Forums Technical Forums General Tech cylinder rebore

I had the cylinder barrell on a 71 BSA sandblasted and powder coated. Exterior looks great, but I had thought they would tape up the barrell before sandblasting. They did not, and the inside is now no longer smooth. Do I need to get this rebored? What do I need to know about doing this? Is it the case that I can use the existing pistons with new rings?

oh, I think it will need to be rebored!

If the scarring is very light you might be able to get away with just honing the cylinders but a next size up bore job, new pistons and rings would insure longer engine life. You can't really know until the surface is smooth and you can measure the gap in a new set of rings placed inside the cylinder and see if it's within the acceptable tolerance.

if funds allow i would rebore and put a slightly larger piston in. you may be able to get away with rehoning but make sure you have a very competant (spelling) guy do it

Frank

I would mic the cyl. side to side and front to back at top middle and bottom of ring travel and see what numbers you have. If you have some room then give it a hone and mic again. If you can't get a cross hatch pattern within tolerences then bore. I would have thought they would protect the cylander walls and the gasket surface too.

Thanks for the responses -- talked to a machine shop and its one size up on pistons and rebore.

As I said, though, they did me a favor by not protecting the insides. There clearly is corrosion in this thing from where the pistons sat for 20 years, and I couldn't really tell when it was still mucked up on the inside.

Meant to add -- anyone know where I can buy pistons for a 71 BSA online?

Google - Ohio Valley BSA Club. There is a link to many of the officers. Very knowlegable folk and I'm sure someone can direct you.

Further to that (will try the Ohio guys) anyone done a 60s or early 70s BSA or Triumph big bore conversion? If I have to bore this out, might as well bore it big.

Rask cycle for parts or machine work.

But are you sure the cylinders are not made of aluminium and rebore/rehoning is not possible without a good Nichel-Silicium threatement?

The shop will know how to handle the boring job when the cylinders are delivered.