Piston & Ring Sizes - How Much to Bore

[KJDonovan]

I need some advice here on my project bike (72 Jackpiner).  I am getting ready to replace the top end and the jug I have has a score in it that's about 20-30 thousands deep.  This score only runs about one third of the way up and I was thinking of filling it with a small bead of TIG weld before having it re-bored, but now I'm second guessing myself.  

Currently the inside diameter of the jug measures 2.560" where there is no score.  If I were to fill the score with weld I would guess when its re-bored it would measure 2.600, removing .020 from the ID.

The second option would be not to fill the score and remove .035 all the way around, this would leave me with an ID of about 2.630"

Anyone have any ideas on which would be the better option?  Additionally, I am not sure what size ranges piston & rings come in.  Does anyone know the initial size, and then increments in which they increase in diameter to accommodate boring?

Any advice would be appreciated,

Thanks,

KD
[?]

Kevin J. Donovan
Foster, Rhode Island

[Rocket]

KJ
Standard bore for the 175 is 63.5mm and the piston sizes go up in .25mm.  Example the next size is 63.75 up to I believe 64mm.  I don't know about welding in the cylindar and then boring.  I would be afraid of the weld breaking free at some time.  I would feel more comfortable boring the until the groove had been eliminated.
Rocket

[wildman]

A good boring bar operator can adjust the cut to favor the side with the score mark and just kiss the opposite side enough to get a round hole. Ideally, the operator would bore the cylinder until the scratch disappears, then orders (or has in stock) the next nearest oversize to bore to. Know also, it's not always necessary to remove ALL of the mark if it's in a non-critical area, where it didn't affect compression or ring life. Not ideal, but might save an oversize. Good luck, Wildman

1975 250 Cross Country, 1974 175 Jackpiner, 1975 125

[firstturn]

Kevin,
  Rocket and Wildman have just given you the best two answers that one could get from years of experience.  When I had a boring bar I did do some off center boring to save cylinders that had an exceptional port job.  Just my take.  Great answers.

Ron Carbaugh

[OUCWBOY]

Worst case is to get the cylinder resleeved and start with a new sdt bore.

Donny Smith