12 G 295 Cylinder Head

Post by **mowoggy** » Mon Sep 29, 2003 2:59 am

Hi

My little 1098 Minor has given me a fantastic run for the last 6 years. Although when I was thinking of buying it I was a little put off by the engine mod's (It's got a Vizard style modified head, Marina Manifold, 1.5" SU Carb and slightly modified exhaust.) it's got plenty of torque, gives 37-40 mpg and is a joy to drive as an eveyday car.

I had the head off the other day and found two cracks, so dug around the shed to find a good one and came across a head from (I think) an MG1100
with a combustion chamber very similar to the modified one but with bigger inlet valves (1 7/32) - the casting number is 12G295. I looked up the spec's in David Vizards book and although he says that it flows very well, I note that the combustion chamber is bigger than the Std 1098 12G202 head (28.3cc against 26.1).

I was wondering about the compression ratio this head would give - I am reluctant to skim too much off it, and wonder if anyone has experience with this head on a 1098 engine?

Cheers

Post by **ianselva** » Mon Sep 29, 2003 8:47 am

You can skim a 12G 295 head as much as you want, I used one with a 12:1 comp ratio on one of my racers . The only practical limit is when you break through the rocker feed oilway.

Post by **rayofleamington** » Mon Sep 29, 2003 12:35 pm

I was a little put off by the engine mod's

With the bigger carb, modified head and exhaust the 1098 is transformed. I can see why you got to like it - my first minor was similar and fun to race around. When I changed back to standard exhaust and carb it was much less sporty.

If you go back to the 12G202 head, your other modifications will not be as balanced, and there will be a reduction in performance.
However if you don't need the sporty behaviour (the standard 1098 set up is liveable).

If you do go back to the standard head, then the 12G295 head is quite sort after. I expect you would get a reasonable price for it on e-bay, (if you wanted to use it to pay for getting standard carbs / manifold and exhaust)

Post by **Peetee** » Mon Sep 29, 2003 2:12 pm

I put a 12G295 on a 948 and the difference was well worth it.
I had the head skimmed beforehand to create an acceptable compression ratio (60 thou) The engineer explained that on the end of the head is a small (5mm ish) round bump in the casting that is the limit to which the head should be skimmed.
I really wanted to keep this head when I sold the car but the buyer wouldn't make a sale without it. They are very rare and it's well worth keeping hold of it. Mine was gas flowed and unused and cost me £ 70+ from e-bay. I thought I did very well and this was reinforced by the number of people who contacted me afterwards to keep them in mind If I decided not to keep it.

Post by **rayofleamington** » Mon Sep 29, 2003 7:15 pm

The 12G940 head (Midget 1275 etc...) seems to work well on the 1098, due to the nice big valves. It's also less sought after than the 12G295 so cheaper secondhand.
However the 12G940 isn't compatible with the 948.

Post by **mowoggy** » Tue Sep 30, 2003 6:50 am

Hi

Interesting to read from Peetee that 60 thou skimmed off the head brought the CR up acceptably. Here in NZ gasoline is 91RON (about 87 MON). I think I'll get the head skimmed but don't want to go much higher than the Std CR for the 1098. The chambers on the modified head that cracked look around the same size as the 12G295. I don't have a burette to measure them though.

Can anybody advise how much needs to be skimmed off to bring the combustion chamber size down to around 26cc?

By the way, I've put a 12G202 head on for the moment, and although it runs well, it seems to be down a little on torque.

Thank you and Cheers

Post by **turbominor** » Wed Oct 01, 2003 11:08 am

Dont forget to check that the exhaust valve does not hit the block, esp if you have different cam or rockets as it comes close as standard. Most engine builders recomend pocketing the block

Post by **Peetee** » Wed Oct 01, 2003 12:50 pm

However the 12G940 isn't compatible with the 948

Not strictly true. If you mean that the bore is diferent than you can discount fitting the 12G295 to the 948 and the 12G940 to the 1098 - but both of these are possible. As Turbominor stated the 1098 block should be pocketed when fitting the 12G940. It is common practice when uprating the 948 engine to rebore it to a 998 and fit the 12G940 head. This is usually done alongside manifold, carb and cam changes to produce a high revving race motor. I would imagine just fitting a 12g940 head to a standard 948 would ruin the mid range torque

Post by **turbominor** » Wed Oct 01, 2003 12:53 pm

Other thing to watch is the compression ratio when swapping to 12g940.

I fitted a metro turbo head to my 1098 and the compression was so high, that i had to return to th 1098 and mod it for turbo use

Post by **rayofleamington** » Wed Oct 01, 2003 1:49 pm

However the 12G940 isn't compatible with the 948

I tried it once - the water jacket holes were out of line and therefore the headgasket didn't seal.

It is common practice when uprating the 948 engine to rebore it to a 998 and fit the 12G940 head.

Maybe my headgasket problem was a one off then?

Post by **Peetee** » Thu Oct 02, 2003 12:30 pm

I tried it once - the water jacket holes were out of line and therefore the headgasket didn't seal.

I believe some of the 12G940 heads were used on blocks with additional studs (11 if I remember correctly). Perhaps you just had a hole or two where you shouldn't have.

Post by **turbominor** » Thu Oct 02, 2003 12:31 pm

I think that is correct for the original mini-cooper 1275 versions, you can just drill and tap you block to add the extra stud

Post by **rayofleamington** » Thu Oct 02, 2003 1:42 pm

I believe some of the 12G940 heads were used on blocks with additional studs

I thought that was only the Mini Cooper ones..I may be wrong.
However this head didn't have the extra stud holes, and had previously been used on a 1098 quite successfully.

Are the Morris 948 and Mini 948 blocks slightly different?

Post by **Cam** » Thu Oct 02, 2003 2:13 pm

As far as I am aware there was never a Mini 948. There was 848, 997, 998, 1071, 1098 and 1275

Post by **Kevin** » Thu Oct 02, 2003 2:19 pm

Are the Morris and Mini blocks slightly different?

Yes they point in different directions

dont they

Post by **Peetee** » Thu Oct 02, 2003 3:29 pm

I tried it once - the water jacket holes were out of line and therefore the headgasket didn't seal.

Were you using the gasket designed for the block or the head?

Post by **rayofleamington** » Thu Oct 02, 2003 8:56 pm

I was using a gasket for the 1100 - (The best compromise I could think of). I re-used the gasket and refitted a standard 948 head, which sealed ok.
(not that I would re-use a gaske these days but I was a poor student/apprentice at the time).

Which choice of gasket is better?

Post by **Peetee** » Thu Oct 02, 2003 9:06 pm

You must use the gasket for the larger bore - be that the block or the cylinder head. Not to do so would leave the gasket exposed (either from above or below, depending on your block/head combination) and lead to it's failure.

Post by **rayofleamington** » Thu Oct 02, 2003 9:24 pm

The 1100 gasket didn't sit proud of the combustion chambers on the head, so I thought it should be ok.
However it blew straight out to the water jacket at first fire up!
As the gasket was still ok and workied when re-used on the block, my only assumption was that the alignment of the water jacket holes.

Anyway, it's all a bit theoretical here as I'm not about to try it again. (I think we've gone off at a tangent due to me bringing up the 12G940)

Post by **Peetee** » Fri Oct 03, 2003 10:00 am

Fair enough Ray.
I must admit to being a bit lost too. All I can say is that a 12G295 with a 1098 gasket works on a 948. But then I did have a gasket failure the first time round. It was a cheap gasket anyway but was the only one available within walking distance

.