BSA Owners' Club Forum

The BSA Workshop => Twins => Topic started by: DEREKANDERSON8 on 16 February, 2017, 17:13:14

Title: Bsa valve seat depth
Post by: DEREKANDERSON8 on 16 February, 2017, 17:13:14
This may not have a simple answer but i will ask, i am having new valve seats cut on A7 cylinder head (for A10) and the guy doing the work asked if i knew what depth the valves go to in the head, is there a BSA value for this? or is the the other way of looking at it is how far does the stem stick out of guide at rocker end.  I guess this info would be on the engineering drawings but i appear to have mislaid mine.

The valve seats have worn and he is fitting new inserts and recutting them.

cheers

derek  :-\
Title: Re: Bsa valve seat depth
Post by: Arthur on 16 February, 2017, 17:37:47
You would normally use the original seat profile as a guide for the new seats. Is there a reason this can't be done in your case?

Title: Re: Bsa valve seat depth
Post by: JulianS on 16 February, 2017, 18:10:43
I have had a number of seats replaced over the years and I was never asked that question, each time the engineer knew what to do and the  job was well done with no issues.

I thought that there was probably some engineering good/normal practice here when the new seats are fitted and cut?

Cut too deep you get poor gas flow and run out of tappet adjustment.

Not deep enough its possible  valves might touch piston with high comp piston and/or high lift cam or again you could run out of tappet adjustment.

The only figures I have seen are for fitted valve spring length.

I would consult a different engineer.
Title: Re: Bsa valve seat depth
Post by: DEREKANDERSON8 on 16 February, 2017, 19:14:58
Thanks for quick reply both.

The guy doing the job is very competent and i have no doubts from that perspective. I think the reason is he doesn't do a lot of motorcycle stuff its all heavy duty diesel engined jobs, he just wanted to make sure i think. The valve seat as is had receded quite a bit so valve was very low in head.

I will let you know how it goes but i don't think there will be a problem.

Cheers

derek

Title: Re: Bsa valve seat depth
Post by: Arthur on 16 February, 2017, 19:51:51
Derek.

He should be able to allow for the valve seat recession and be able to work out how much valve stem should be projecting above the end of the valve guide based on the current seats and valves. This dimension should then be achieved after replacing and re-cutting the valve seats (and replacing the valve guides if necessary).