10 replies to this topic

[beentheunseen]

The 12g295 head i bought off ebay just arrived! I want to fit it to my 1989 998cc mini spending minimal amounts of money in the process, but wanted to seek advice on what I should check/replace before I do the swap.

It seems to be in really good shape - no evidence of valve recession, and only minimal play of the valves in their guides, though approximately how much play would be acceptable? Obviously I will lap the valves into their seats before refitting them, and will replace all of the stem oil seals/o-rings, but I don't think there's any need to replace any valves or guides... I need to confirm the chamber volumes and do the compression ratio maths, but i've been told they're down to 22.6cc already, and the mating face is perfectly flat so i don't think I'll need to do a skim.

Anything else i should check for or consider replacing?

Thanks, Ben

[Cooperman]

Your 998 will have dished pistons whereas the original 998 onto which the 295 head was fitted had raised-top pistons. So you will need to measure and calculate the compression ratio to be sure that it's within acceptable limits. A good figure to aim for is 10:1 or thereabouts.
Do the sums accurately so that it's correct.

[lrostoke]

Your 998 will have dished pistons whereas the original 998 onto which the 295 head was fitted had raised-top pistons. So you will need to measure and calculate the compression ratio to be sure that it's within acceptable limits. A good figure to aim for is 10:1 or thereabouts.
Do the sums accurately so that it's correct.

Think you could be wrong on the dished pistons. Sisters got an 89 and brothers got a 90, both are flat top pistons.
But you are right about a non modified head needing skimming to get the compression back up, the 12g295 as a bigger volume combustion chamber than the later standard 998.

[jaydee]

My 998 has flat tops too.
Some 998s have dished pistons, probably the low compression engines.

[liirge]

The stock 998 cylinder head has 24.5cc Chamber volumes, for reference

[Cooperman]

There are so many variations of piston nand head that a comp ratio check and calculation is vital when changing to different heads or pistons.
The 998 has had raised, flat-top and dished pistons on different models over the past 48 years from Riley Elf onwards, thro' Cooper 998, and all standard production 998's including those fitted to vans, pick-ups, etc.

[lrostoke]

Just a thought but could fitting a 12g295 to a 998 with dished pistons be a bit of a problem, I've read if you skim to much off the head to raise the compression you can cut into the oil ways ??

[blacktulip]

Just a thought but could fitting a 12g295 to a 998 with dished pistons be a bit of a problem, I've read if you skim to much off the head to raise the compression you can cut into the oil ways ??

its true, thats why i put my old head back on my early 998. the amount that needs taken off the 12g295 for dished pistons will go into the oil gallery. an 89 mini would have flat top pistons im sure. the best thing to do is to check your engine number on the engine guide in the FAQ or for 100% accuracy- take the head off and look.

[Cooperman]

If you measure the total thikness of the head then poke a vernier depth gauge down the oil feed hole and subtract the two dimensions it gives the remaining depth. As new, you can normally work on a total remaining thickness of about 0.180". This allows a safe total of 0.120" to be skimmed from a new 295 leaving 0.060" of metal. However, as they are old heads the remaining thickness should always be checked.

[logan]

yes i think that their is problems to get over as you say as ive just done the same build and when i did the calcs it shown that i needed 80thou off the head which probably would have gone into the oilways,my combustion chamber was 27cc on my 295 head which is just below what it is from stock i believe but when i did the calcs on another head i have with 22.5cc i was still struggling to get the required compresion ratio.like the good man himself cooperman says their is diffrent pistons out their but getting hold of them is a diffrent matter.

[valve bounce]

if you have dished pistons then there wont be enought metal left on the head but if you have flat top pistons then it will work out about prefect,
if you take a spark plug out then using a tourch you should be able to see if you have flat tops or dished