7 replies to this topic

[black olive]

whilst Im putting and RC40 and manifold, and HIF38 on the mini, its worth changing the head. so much conflicting information out there. options appear to be 12g295 which is expensive and requires skimming with the flat top pistons, 12g940, but differing opinions on whether the block needs pocketing or not with a standard cam, J5058 which apparently is a better flowing standard head, or a bespoke ported head with bigger valves.

I have access to a slark stage 4 head, but he wants £450 for it and I feel it is TOTALLY overkill for an engine in otherwise fairly standard tune.

Ive also been reading up on diy porting etc, and it would appear to be a difficult head to diy, and costing a lot by the time you have bought materials, and had seats cut, inserts fitted etc etc

advice please, preferably from someone who has been there ??

[LTSwaby]

im putting 997 cooper head on my 998 the casting number on that is 12G202

[steven]

the 12g295 is considered to be the better head but needs to be skimmed and you have to calculate the compression ratio, the 12g202 is a better head than the standard 998 head but it's not as good as the 12g295 the 12g940 requires the block to be pocketed, I suppose it depends on how much work and money you want to do and spent

[Cooperman]

The 12G940 only requires the block to be pocketed if you fit a higher lift camshaft. A Kent 266 or MG Metro both have 0.318/0.320 inch valve lift and so long as the distance from the installed valve face to the level of the valve head is equal to or greater than 0.320" there is no problem. The gasket thickness can be considered as the safety factor. If that measured distance is only a small amount above the head face it is possible to 'sink' the valves into the head, but again it's a machine shop job.
It is, however, necessary to measure & calculate the compression ration whatever head is to be fitted. it is the only way to build the top end of an engine correctly. Guessing just does not work with engines.
You can end up with several visits to the machine shop as first of all the head needs to be skimmed flat and the valve seats cut to be in line with the presumably new valve guides. Then the head is 'trial assembled', the volume measured with the valves lapped in and the distance from valve face to head level accurately measured. If the volume is too great the head must be further skimmed by the measured amount and the new valve face to head surface level calculated. If it is less than 0.320" the valves must be sunk into the head to obtain the 0.320" dimension and the valve seats again re-cut and valves re-lapped in, so it's the machine shop again.

[blacktulip]

what type of pistons have you got? if flat you can go for a 12g295 skimmed to suit and a decent cam would be beneficial with it.

[black olive]

Im guessing flat top. ill have to take a plug out and have a look. can do the skimming etc myself no probs, god good machining facilities. looking at Vizard's bible, will be looking out for 202 or a 295 I think. I would put a cam in if the engine were coming out, but just want bolt on mods at the moment

[black olive]

managed to pick up an unleaded 12g202 head for 50 quid. :)

[madaboutcherry]

http://www.ebay.co.u...=item2c6e6dd95b