Compression Ratios, rebores and head skimming

[Steve Simmons]

"Best" would be a porting job and larger intake valves. But whether it's worth the cost and expense is a matter or personal tastes. I had 32/35.5mm valves installed on the last head I took for modification. By the time I was done, the parts and labor for the head was approaching $2K USD.

[SteveW]

I've now gone back and measured my original skimmed cylinder head volumes so that I could accurately determine the compression ratio. If anyone needs to do it, it's quite easy. I got an old CD case and drilled two holes in it, I then took a beaker of water, weighed it on some digital kitchen scales, carefull filled the combustion space and then re-weighed the beaker. I could have just used a graded syringe but I didnt have one and I though that this method would be more accurate (plus I'm an ex-research chemist, so I like doing this sort of stuff!).

With a bit of maths (math if you're from the USA), I got a compression ratio of 9.5:1 assuming a 1mm head gasket. This is about what was previously mentioned, but it's good to confirm. Valve clearance is going to be fine, but I could do with dropping the compression a bit. Does anyone know where I can get a thicker than standard banana head gasket in the UK (preferably a composite, rather than solid copper), all I can find are round hole ones.

[Tom Lange, MGT Repair]

If you compare the two pictures above, you can quickly see the difference in head thickness, and therefore compression ratios. One indication is that the bottom of the spark plug hole has been cut away, leaving a sharp edge that can cause running-on.

As a rule of thumb, if you can see the recessed area along the pushrod tubes (as with Steve's head), the compression ratio will be manageable. But if the entire recessed area has been machined away as in the second photo, the compression ratio will be too high for a street machine - above 9.5:1 - unless some remediation is taken: a compression plate, a solid copper gasket, or two head gaskets.

I've done three engines in the past decade for owners who needed to re-use their thin heads, and two gaskets have not caused a problem. I DO re-torque them as much as 8 times in the first few thousand miles, and use improved head studs with a quarter inch of thread added to accommodate the thinner head. Head studs and nuts are key to stopping blown head gaskets.

Steve - what 35.5mm valves are you using? Are you using stock XPAG 34mm exhaust valves?

Tom Lange
MGT Repair

[Steve Simmons]

Steve - what 35.5mm valves are you using? Are you using stock XPAG 34mm exhaust valves?

Using stock TC 32mm valves on exhaust. The specialist I used claims these engine are already over-exhausted and going larger would be detrimental. Old school thinking was always to go bigger, but modern thinking seems to be moving toward smaller ports for higher velocities. Carbs also, using 1.25 where 1.5 sometimes used to be installed. Efficiency of flow for even mixture rather than a stampede. If I ever get this head installed, we'll see how it goes.

The 35.5 valves came from the builder. He may have cut them himself. Or maybe a 36 that he lightly shaved to help flow around the edges. Due to the small chamber size, the larger you go the more you can disrupt air flow at the sides of the chamber. So apparently there is a point of diminishing returns. I'm not an expert on head porting, just relaying what I've been told.

[i.thomson]

I personally would be loath to stay with the small valves especially as I would be replacing the valves and guides anyway along with fitting hardened valve seats for future proofing. Having gone that far it makes sense to me to at least tidy up the casting flash inside the ports and match the manifolds to improve the gas flow a bit. This is quite a bit of work I know but those rough and misaligned ports offend me.
Ian Thomson

[SteveW]

I few posts ago I asked a question and it's got a little lost in the subsequent discussion so I thought that I would ask it again.

Does anyone know a UK supplier where I can get a thicker than standard banana head gasket? I would like to reduce my compression a little from 9.5:1. I could get a cylinder head saver to use with a standard gasket or stack two standard gaskets, but I would prefer to do it in one by using a thicker composite gasket if possible.

[Steve Simmons]

I tried to find one a while back with no luck. I could only find solid copper in anything thicker than stock. I was hoping Cometic would have a multi-layer gasket, but they don't currently have the tooling to do them for XPAG. Maybe if someone ordered 1000 then it would happen.