lowering compression with an extra gasket

[DAVE BRADY]

Hi,

Has anyone every used an extra head gasket or made an extra thick one to lower compression?

Dave.

[rhyatt]

Yes,
It was on my B44 Victor special and lowered it nicely.
My the time I did my maths and cylinder calculations, I needed a .031 piece of copper sheet and traced round a base gasket.

[DAVE BRADY]

Hi,

Is that .031"?  how much did it lower the compression by.  I want to lower 8.25:1 to about 7.25:1 on an A10.

Dave.

[rhyatt]

From memory, whatever the B44 Victor special was (I think mid 9s) it brought it down to mid high 7s.
I then had to make longer pushrod caps

[DAVE BRADY]

Hi,

How much of an increase in thickness was the 0.031 from the standard gasket?  I suppose a small increase in thickness can have quite an affect.

Dave.

[rhyatt]

I left the head gasket as std   but elevated the barrel by this .031" thick copper base gasket.
It was easier to do that and leave the std head gasket and barrel to head spigot as std.
Worked well and reliably

[DAVE BRADY]

Hi rhyatt,

That is useful.  As I will not be replacing a base gasket I will knock a bit off the 0.031 to allow for it.  So I will measure the thickness of the std. head gasket and make a new one at std. + 0.025 ish and see how it goes.

Dave.

[rhyatt]

On the B44 single , I had to extend the pushrod a shade to maintain the angle where they meet the rocker ball ends .
May not apply to your bike

[DAVE BRADY]

Hi,

I will keep that in mind.  I think that on an A10 it may not be a problem but worth checking.  I will measure the existing gasket later today to see what thickness I need.  Thanks.

Dave.

[topher]

Can I ask why you want to do this?

[DAVE BRADY]

Hi,

Of course.  It is my wife's '51 Flash and it has +0.080" pistons giving about 680cc with a 8.25:1 compression.  After many years of kick starting this bike her knee is starting to complain.  The idea is to reduce the compression a bit to make it a bit easier to start.

Dave.

[Allan G]

For what it's worth, when I bought my John hill big bore A65 barrel it came with two gasket sizes. A 1mm and a 2mm. I fitted the 2mm as I didn't want the high compression. That seems fine with no problems. 

I can't remember the company but there is a guy who will make copper gaskets to any thickness and any quantity required. Might be the same person as TT John used for his A10 rocker box gaskets.

Unless you're going to hone the barrel and re-ring it. I wouldn't be so keen on lifting the barrel. That A10 has seen a lot of miles and the wear in the bore will suit what is setup now. Also if you raise it enough the can followers might not follow the full base of the cam circle (if they bottom out that is)

[DAVE BRADY]

Hi Allan,

As you say a lot of miles covered and it still has good compression.  The thicker head gasket is definitely the easier way to go.  TTJohn may respond with info. on the people who do his gaskets.
Cutting one out of sheet copper by hand would be a bit arduous but doable but a spot on laser cut one would be even better - depending on cost.
The gasket I have just taken off is 1mm thick and a new one is 1.04mm, they do not squash much.  I would think 2mm would be about right.   Copper sheet is not cheap so i may end up tying two gaskets.

Dave.

[DAVE BRADY]

Hi,

After a search only the British Cycle Supply Company (Canada) offer a thicker gasket at 0.080.  Not cheap especially with postage added.

Dave.

[Allan G]

Hi,

After a search only the British Cycle Supply Company (Canada) offer a thicker gasket at 0.080.  Not cheap especially with postage added.

Dave.

That might be the way to go. Use FedEx if you get the option. If it suggests USPS it will take a LONG time before you get it and you'll pay more for customs charge than you would with FedEx.