Reducing the compression ratio

[K1100]

Can you reduce the compression ratio on an A10 (1954) by fitting more than one copper head gasket?

[RichardL]

Yes, but I can't say if it is a good idea. For some discussion and math regarding how much volume will be added to the combustion chamber, look here: http://a7a10.net/forum/index.php/topic,3744.msg25829.html#msg25829

Richard L.

[bsa-bill]

better option is a plate under the barrels, trouble with another head gasket is it's also another joint.

Do you know what compression it has now, just if your going to lift the head and maybe barrels then I would think there's a good chance someone here will have raised the compression on their bike and have a set of good used pistons going, need to know size of course but definitely the very best option 

[a101960]

You could try here:

http://myworld.ebay.co.uk/coppergasket63?_trksid=p2047675.l2559

This Guy would make you a copper gasket/shim plate to go under your barrels. I have just ordered a set of copper rocker cover gaskets for my A10 from him. Although fitting two copper head gaskets would give you a reduction in the compression ratio, l am with Bill. Two gaskets would probably cause a leak. The crankcase to barrel joint is definately the best option.

John

[RichardL]

So, John, if one was going to buy a custom-made copper shim to go under the barrels, why not just get a thicker custom head gasket?

Richard L.

[bsa-bill]

why not just get a thicker custom head gasket?

Cause we didn't think of that    - nice one Richard

[a101960]

So, John, if one was going to buy a custom-made copper shim to go under the barrels, why not just get a thicker custom head gasket?

To be honest Richard, I can think of no good reason not follow your suggestion. I suppose by using a standard head gasket it would make replacement easier if a new head gasket was needed at any time. I am thinking that a head gasket is more likely blow and get damaged in the process than a gasket at the base of the barrels. However, other than that your question is perfectly valid and seems sound in principle to me.

John

[muskrat]

 G'day fellas, I agree the best (easiest) option is to get Lani to make a thicker gasket.
I have been playing with my cafe to reduce pinging I reduced the comp ratio from 10.5:1 down to about 9.5:1 by making another gasket out of 1mm soft alloy sheet (read backers tray). Added it plus the copper gasket and used copper gasket cement on both. Torqued head to 35Ft/Lb no leaks. I know it's not the done thing but it can be done.
 A comp plate under the barrels is the best option but no thicker than 1.5mm as the followers won't drop far enough to meet the cam. Even less if the cam has been re-profiled.
Cheers

[Joolstacho]

Yup. I also needed to reduce the comp ratio on my SR, (ex grass track motor) I fitted lower comp pistons and made x2 alloy plates to fit under the barrel- I think they're each about 40 thick (2 so I could adjust ratio if needed) -I encountered the cam follower problem, but can't remember how I fixed it... That's a lot of help eh! 

[chaz]

interesting, on a recent T120 Triumph rebuild I had the option of 3 thicknesses of copper head gasket. no mention of it on the majority of retailers sites only on one. good job as I couldnt get the push rod tubes to seat without leaking or distorting. did BSA do a part number for alternative copper gaskets?

[BSA_54A10]

No BSA only made one thickness of head gasket.
The thicker ones are aftermarket.
Lanni does 5 diferent thickness standard and thicker ones to order.

[bsa-bill]

I couldnt get the push rod tubes to seat without leaking or distorting.

Well Chaz if you will get involved with engines that have internal bits stuck on the outside what do you expect

[chaz]

Bill..
it pays for work on the BSA, now having just finished one square 4 engine Ive got another to do.
just got the replacement rocker cover studs for the A10 so hopefully will be running again soon

[mayes]

I have tried this on one of my bikes not really sucsesful the best option is to find what pistons you have and go to standard if you have high comp as these tend not to run so well with modern fuel as stated more joints to worry about hope this is helpful John