BSA Owners' Club Workshop Discussions

The BSA Workshop => Twins => Topic started by: chrisx1 on 02 Jul, 2026, 17:23

Title: Compression ?
Post by: chrisx1 on 02 Jul, 2026, 17:23
What compression should I be expecting on a Thunderbolt that's done about. 50 miles on new rebore, pistons and rings? I've got about 100-110
Title: Re: Compression ?
Post by: chrisx1 on 02 Jul, 2026, 17:32
Note: compression checked with engine cold.
Title: Re: Compression ?
Post by: V500 on 02 Jul, 2026, 17:50
Sounds low, and I doubt the rings have bedded in enough yet. Don't be too limp-wristed with the throttle.
Title: Re: Compression ?
Post by: rhyatt on 02 Jul, 2026, 18:08
When you were kicking it over , did you have the throttle wide open ?
Title: Re: Compression ?
Post by: chrisx1 on 02 Jul, 2026, 18:18
No I didn't have the throttle open at all.
Title: Re: Compression ?
Post by: chrisx1 on 02 Jul, 2026, 18:21
The top end is a bit clanky, I'm a little worried a valve maybe connecting with a piston. I've redone the tappets so it's not that.
Title: Re: Compression ?
Post by: DAVE BRADY on 02 Jul, 2026, 18:53
Hi,

The workshop manual tells you what compression reading to expect but a bit more running in will help.
Unless you have fitted very high compression pistons a valve will not touch the a piston.
Re-check the valve clearances at 8 thou inlet and 10 thou exhaust and make sure oil is getting to the rockers.

Dave.
Title: Re: Compression ?
Post by: rhyatt on 02 Jul, 2026, 21:31
Important to have throttle wide open , otherwise there is no air getting in to compression.
It'll make a difference for sure