The BSA A7-A10 Forum

Technical => A7 & A10 Engine => Topic started by: tinu on 07.06. 2017 07:11

Title: Special cylinder block gasket?
Post by: tinu on 07.06. 2017 07:11
Does anybody know if this is a kind of special gasket between the crankcase and the cylinder block?
Thanks tinu
Title: Re: Special cylinder block gasket?
Post by: terryg on 07.06. 2017 07:40
I have not seen such a spacer on an A10 before but it's possibly someone's solution to pinking, by reducing the compression ratio.
On the s/a A10 there is already very little space between the rocker box and frame, so not the best approach on those models.
Title: Re: Special cylinder block gasket?
Post by: Klaus on 07.06. 2017 08:20
Verry special *eek*
count the barrel fins, if you reach only 7 you have fit a shorter A7 barrel.

cheers Klaus
Title: Re: Special cylinder block gasket?
Post by: tinu on 07.06. 2017 08:28
I count 8 fins...
Title: Re: Special cylinder block gasket?
Post by: Black Sheep on 07.06. 2017 10:12
These type of gaskets are often seen on Norton twins to lower compression ratios. A bit of a bodge really as it also raises the cam followers away from the cams.
Title: Re: Special cylinder block gasket?
Post by: coater87 on 07.06. 2017 15:03
Could be a few reasons for that I guess.

 Like mentioned, maybe to lower compression.

 Maybe P.O. used a different makes pistons and needed the crown room.

 Maybe a super hot cam at one time and needed the extra valve to piston clearance.

 Without actually peaking inside, it's impossible to tell. And even then you may not be sure. Another P.O. could have changed everything and just replaced the spacer because it was there when he took it apart...
Title: Re: Special cylinder block gasket?
Post by: JulianS on 07.06. 2017 15:19
I use a spacer on my A10 to compensate for a severe skim to the head.
Title: Re: Special cylinder block gasket?
Post by: coater87 on 07.06. 2017 17:44
 Julian,

 Was the head warped that bad, or was someone trying to raise compression and got carried away?
Title: Re: Special cylinder block gasket?
Post by: JulianS on 07.06. 2017 18:05
Regret it was warped and cracked and around 30 - 40 thou taken off.
Title: Re: Special cylinder block gasket?
Post by: chaterlea25 on 07.06. 2017 19:31
Hi All,
I have made such a compression plate to compensate for a heavily skimmed head on a T140
But
On the last two engines I  built  that had the heads skimmed I got thick head gaskets (2mm) from Copper gaskets unlimited in USA
The A10 engine's head was found to be skimmed 30+ thou off square *problem*

I sold a set of 10:1 pistons to a chap and I suggested a compression plate or thick gasket to keep the top end on *ex* *ex*

John
Title: Re: Special cylinder block gasket?
Post by: KiwiGF on 08.06. 2017 00:39
My bike had the same cylinder spacer, it had 9:1 pistons (seized) and I rebuilt it with flat top Pistons (7:1) and no spacer.

I had to get the bottom barrel face machined flat (it was curved by over a mm) but I do not think that issue had anything to do with the spacer being fitted.
Title: Re: Special cylinder block gasket?
Post by: mikeb on 08.06. 2017 00:46
a compression plate, surely. I bought one off ebay a year or two back when rebuilding the a10 when all i had was high-comp pistons, tho found lower comp and didn't use it and was dissuaded from doing so by proper engineers talking about pushrod angles. so they are still about if you need new one - no need to make your own.
Title: Re: Special cylinder block gasket?
Post by: KiwiGF on 08.06. 2017 00:51
Yep, unless the push rods are made longer by the thickness of the spacer the valve/tappet adjusters have be screwed down a few extra threads to compensate, which i guess increases wear on the valves and adjuster. Probably not an issue unless you are doing high mileage though.
Title: Re: Special cylinder block gasket?
Post by: bikerboy on 13.06. 2017 01:38
Check the cylinder head maybe its an A7 head and the guy was worried about the smaller combustion chamber size