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I have a barrel suitable to rebore to +.080, however is this too thin for the cylinder wall or has anyone experience? I think re-sleeving may not be an economical option now?


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IMO re-sleeving is only any good for light usage, the parent metal atound the base flange is weakened and the sleeve does not provide much longditudinal strength. Spare barrels are not that hard to come across, and by the time you have bought sleeves, had them fitted and then bored for correct fit to new pistons, I doubt the cost willl be any less.

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If it’s a nine-bolt barrel, a +0.080” overbore may be ok.

I think it would cause problems on an eight-bolt pre-unit barrel.


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Yes it is a 9 stud barrel, I think I may try it. +.80 pistons are available which would indicate its feasibility.


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I've used +100 before now, with no problems on a 9 stud t120.
Make sure you use decent rings though.

Last edited by NickL; 07/30/21 3:45 am.
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I had 9stud +.060" barrels crack through the right hand side between 3rd and 4th fin from memory. Lucky I noticed it before it came to grief.

Reduce its' red line and regard it as light duty.

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I had a brand new standard barrel crack round one bore!


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I had cylinders bored for new .080 pistons...Engine ran ok but was a vibrator...took it apart and removed material from the pistons so they weighed the same as .040.....less vibration...


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Might be worth using a big set of outside calipers to determine cylinder wall thickness?

There must be numerous makers for these over the years so variation is more than likely.

My failed set broke 30 years ago. They came to me as part of a job lot. Completely unknown history. They were already on .060, well used, then they got used some more, then they broke.

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Triumph's US distributor - The Triumph Corp. sold .080" over size pistons for the 650. They did warn that the largest oversize they could gaurantee that the cylinder would not break was .040". It was the days when you could say, "try it, but you were on your own."

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You might want to check whether the bores are reasonably centered. I have seen BSA cylinders where the wall thickness is appreciably less on one side.

Ed from NJ


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