5 replies to this topic

[kenny998]

I've got a 1982 998 HLE with a blown water bypass hose. As i'm going to have to take the head off to change the hose i was wondering if i should fit a 12g202 head that i've got in the garage and if this would go on without any modifications and if i would see any improved performance.

Cheers

[Spitz]

the 202 head has a slightly larger chamber capacity than a regular 998 head ( not as big as 295 though ) so you should have it shaved to ~ 24cc for best results I imagine.

[Cooperman]

The 12G202 was an early 997 Cooper head and the 997 had higher compression pistons than the later prodution 998 engines. So if you just fit it the ompression ratio will be very low. You need to take the old head off and o the compression ratio alulations to establish how much you need to have skimmed off the 202 head to make it what you want, probably 10:1 would be about ideal. Until you do the measurements you don't know how much to machine off as it may, or may not, have been skimmed down before. You should not just maching off any amount before measuring.
Comp ratio calcs are not as difficult as some people think. All you need is a straight edge, a set of feeler gauges, a 5 cc shrynge from Boots-the-Chemist and a calculator. PM me for the formula if you want to.
As far as I am aware, the 12G202 was identical to the 12G295 except that the 202 did not have the small registers machined in the manifold face/inlet ports to take the minlet manifold alignment sleeves.
There are not many 202 heads which have not already been skimmed at some time.

[Spitz]

the 12g202 had smaller intake valves than the 295. It's chamber capacity is 26.1 as opposed to the 295 's 28.3cc.

Yes...you should measure the chamber before doing anything to the 202 as I alluded to in first post about shaving to ~24cc.
You can clamp a plexiglass or similar flat material over one of the chambers with two small holes drilled in it. Fill with a measured amount of fluid from a syringe ( I use ATF ). This should get you a very close measurement to determine if it is original size.
( obviously spark plug and valves need to be in place )

Edited by Spitz, 06 October 2010 - 12:33 PM.

[Cooperman]

the 12g202 had smaller intake valves than the 295. It's chamber capacity is 26.1 as opposed to the 295 's 28.3cc.

Yes...you should measure the chamber before doing anything to the 202 as I alluded to in first post about shaving to ~24cc.
You can clamp a plexiglass or similar flat material over one of the chambers with two small holes drilled in it. Fill with a measured amount of fluid from a syringe ( I use ATF ). This should get you a very close measurement to determine if it is original size.
( obviously spark plug and valves need to be in place )

You are quite right. I just looked up my very old data sheets and the 202 had 29.4 mm inlets and 25.4 mm exhausts, whereas the 295 had 30.86 mm inlets and 25.4 mm exhausts.
That old 997 was a really 'nasty' little engine. I re-built one recently and had forgotten how nasty it really was with its small bore and very long stroke. No wonder they used a lot of oil.

[ado15]

The 12G202 was also used extensively on the 1098cc engines (Austin/Morris 1100, Morris 1000, Mini, etc). This was a longer stroke than the 998 so the larger chamber worked fine. It has larger inlet valves than the 998 as already pointed out, but the ports were not that great. You could clean out the casting roughness in the ports to gain a little better flow, but the 202 doesn't lend itself to extensive modification.

Edited by ado15, 06 October 2010 - 02:14 PM.