Can use this head on a 948 block?

[dukeminor1971]

Hi - I have a 12g295 cylinder head which I want fit to my 948cc engine.

I've measured the head height (correctly I hope) at 2.66" the original height would have been 2.75". A massive 90thou skimmed off. The inlet valves are larger at 33mm (originally 31mm I think) I was told that the valve chambers are 26cc (originally 28.3cc I believe). The head originally had been run for some 2000 miles like this on a 998 mini.

I'm at a bit of loss now trying to work out if this cylinder head can be run on my standard 948 block and was wondering if anyone more technically minded would have any idea?

[philthehill]

For the normal fitment of a 12G295 head only 0.060" is required to be removed from the lower face of the block.
0.090" is to me a bit excessive.
The head should fit even with the larger valves as the valves are centred on the 948cc bores.
My only concern would be that the compression ratio will be too high and with those larger valves you may not be able to get the best out of the head without doing additional modifications to the engine. You may find that it runs worse.
Is your engine in good condition? You do not want to fit this head and then find that is knocking hells bells out of the bearings and place additional loading on the rings and pistons.
Your choice but I personally would only fit the 12G295 head in question to your 948cc engine if the engine was being overhauled or had been recently overhauled.

Phil

[dukeminor1971]

For the normal fitment of a 12G295 head only 0.060" is required to be removed from the lower face of the block.
0.090" is to me a bit excessive.
The head should fit even with the larger valves as the valves are centred on the 948cc bores.
My only concern would be that the compression ratio will be too high and with those larger valves you may not be able to get the best out of the head without doing additional modifications to the engine. You may find that it runs worse.
Is your engine in good condition? You do not want to fit this head and then find that is knocking hells bells out of the bearings and place additional loading on the rings and pistons.
Your choice but I personally would only fit the 12G295 head in question to your 948cc engine if the engine was being overhauled or had been recently overhauled.

Phil

Thanks for the reply.
The engine is running fine. I was hoping that the larger valve chamber (26cc compared to 24.5 for the 948) and a thicker gasket would take up some 'of the slack'.

[liammonty]

For the normal fitment of a 12G295 head only 0.060" is required to be removed from the lower face of the block.
0.090" is to me a bit excessive.
The head should fit even with the larger valves as the valves are centred on the 948cc bores.
My only concern would be that the compression ratio will be too high and with those larger valves you may not be able to get the best out of the head without doing additional modifications to the engine. You may find that it runs worse.
Is your engine in good condition? You do not want to fit this head and then find that is knocking hells bells out of the bearings and place additional loading on the rings and pistons.
Your choice but I personally would only fit the 12G295 head in question to your 948cc engine if the engine was being overhauled or had been recently overhauled.

Phil

It's conjecture to go by how much has been skimmed from the head - the only meaningful measurement is the combustion chamber. In practice, I suspect you will find that it is almost impossible to skim too much off a 295 head when using standard dished pistons - you end up breaking through the oilway before that happens in most cases. I'm running flat pistons in place of the original dished ones in my overbored 948, and the 295 head still required a hefty chunk removing from it to bring the CR up to around 8.5:1 or just over. You can measure the chamber size yourself readily enough - it's detailed in the Vizard book - you'll need a piece of glass and a syringe, burette or similar to measure the amount of fluid it holds. It's straightforward enough. Or you could get your local engineering shop to do it. I wouldn't discount using it until you're certain based purely on the head thickness.

[dukeminor1971]

For the normal fitment of a 12G295 head only 0.060" is required to be removed from the lower face of the block.
0.090" is to me a bit excessive.
The head should fit even with the larger valves as the valves are centred on the 948cc bores.
My only concern would be that the compression ratio will be too high and with those larger valves you may not be able to get the best out of the head without doing additional modifications to the engine. You may find that it runs worse.
Is your engine in good condition? You do not want to fit this head and then find that is knocking hells bells out of the bearings and place additional loading on the rings and pistons.
Your choice but I personally would only fit the 12G295 head in question to your 948cc engine if the engine was being overhauled or had been recently overhauled.

Phil

It's conjecture to go by how much has been skimmed from the head - the only meaningful measurement is the combustion chamber. In practice, I suspect you will find that it is almost impossible to skim too much off a 295 head when using standard dished pistons - you end up breaking through the oilway before that happens in most cases. I'm running flat pistons in place of the original dished ones in my overbored 948, and the 295 head still required a hefty chunk removing from it to bring the CR up to around 8.5:1 or just over. You can measure the chamber size yourself readily enough - it's detailed in the Vizard book - you'll need a piece of glass and a syringe, burette or similar to measure the amount of fluid it holds. It's straightforward enough. Or you could get your local engineering shop to do it. I wouldn't discount using it until you're certain based purely on the head thickness.

The chamber was measured at 26cc

[liammonty]

Excellent - you’ll be fine then. Chamber size matches that of a standard 12G202 head now (as used on 1098) and is a little larger even than your 948 head which should be around 24.5cc. So no concerns regarding the CR being too high. If the head has been skimmed the amount you’ve measured, though, it doesn’t sound as though there would be scope to remove any more from it due to the likelihood of breaking through the oilway in it. I would give it a go as is!