12 G 295 Cylinder Head

[MikeTrav68]

A footnote on non-identical heads I forgot last time. The heater outlet on 12g940 is closer to the outside than the standard 1100 head. You need to put a nick in the rear engine steady bar to clear the valve.

[MikeTrav68]

IMPORTANT 2nd footnote. I had the AF460 (copper) gasket fail. Usual point, between two centre cylinders where the head is thin and the siamesed exhaust port causes more heat. Got a race headgasket cos my head has been cut too close to the gasket when it was ported, just where it failed: C-AHT188. Mini Spares swear it's stronger, folded copper inside fire ring and more sealant; should be at the price! Starts life and looks like an AF460 at 5x the price. But they enlarge 3 stud holes, one is a datum for 12G940, just what I needed for my close cut head, so I called them to find out why and decide which to use. In the conversation, they mentioned water pump clearance, I measured and found a problem: the back edge of the pump hits the 12G940 where it tapers under the thermostat. Difficult to see but swipe a feeler guage along and it can be found. I had to grind at least a mm off the pump to clear the head. Hence gasket problem! It stops the head torquing down properly, even though no gap is visible.

A last point on first fitting whilst I remember. The thermostat outlet rotation is different on 12G940. I cut the rad top outlet short, close to the radiator, and used most of a bottom hose. Looks like it should have been that way in the first place

[rayofleamington]

The thermostat outlet rotation is different on 12G940.

Yes - it's an easy way to spot the large block cylinder head as they have a different rotation of the thermostat housing.
The nicest way is to get a B series (Marina) thermostat housing which aligns the pipe exit back near to where it was originally. The downside is that these are not easy to find.

[les]

As a point of interest the C-AHT-188 gasket was/is the competition gasket for the 1275 cooper 's' which otherwise tended to blow between cylinders 2/3, if modified to any extent.

[MorrisMartin]

Ive just put a modified and skimmed 12G295 head on my mini (yes I know its not my morris but I need you suggestions!) Now I have put it on it seems little faster than before and down on torque. any ideas why?

[turbominor]

think that is down to the large combustion chamgers onthe 12g295 lowering you compression , as i think they were used in conjunction with hi compression pistons.

most people get 60thou taken of the 12g295 to get compression up.. How much did you get skimmed off?

[Kevin]

Yes, and what size engine have you put it on.

[MorrisMartin]

I dont know how much has been taken off as it has sat in the shed for the last 25 years! It is down to the tab bit that sticks out though. ( if that makes sence!) It is on a 998 with flat top pistons. Is now running better since introduction of new HT lead but still dosent seem a great deal faster.

[Peetee]

The engine will be down on torque in the mid range. This is down to the reduced gas speed passing through the inlet valves. the engine is demanding the same amount of gas (bore and stroke is the same) but the larger inlet means the gasses don't have to travel into the combustion chamber so quickly. When you reduce the gas speed you reduce the inertial ramming - this is where the gas velocity effectively drags in more mixture after the induction stroke has finished.
As the revs increase the extra power will be felt. there is less time to fill the cylinder so the extra inlet volume starts to do a more efficient job.
That's a rather simplistic way to look at it as camshaft, inlet manifold and carb changes also play a part - but you didn't mention them.

[MorrisMartin]

Ahh I see! It does seem better at higher revvs. I have a MG metro inlet and a LCB exhaust manifold with maniflow exhaust. The carb is standard HS4 but has different needle. I also seem to have a random flat spot which seems to occur only when it wants too but I had this before the new head as well. I seem to get a backfire when going from crusing to accelration now too.

[Cam]

I seem to get a backfire when going from crusing to accelration now too.

Could be timing that could. Have you got a good dizzy in there with a working vac advance?

Have you any clue which cam is installed? Does it have a very lumpy irregular tickover or does it tickover sweetly?

As Pete said, the cam also has a large effect especially if it's a high performance cam as these cause the inlet/exhaust valves to overlap at low revs which is inefficient but is ideal at high RPM due to the inertial ramming effect that Pete spoke about. So depending on which combination of these items you have, will determine your engine's performance at low/mid/high RPM.

My tuned 1380 can easily be beaten by a standard 1275 under 2000 revs, but then goes on to produce FAR more power (over twice as much) at 7000 RPM!!

[doobry]

. I seem to get a backfire when going from crusing to accelration now too.

I had that once with a faulty rotor arm - I very much doubt that is likely to be your probolem but it goes to show how hard it can be to figure out missfires! If you find a very clued up rolling road chap he opught to be able to sort it out, but not cheap tho!!

[Peetee]

I also seem to have a random flat spot which seems to occur only when it wants too

This could be that the distributor advance curve is no longer suitable because of the other changes you have made. I had a tuned 1300 engine (not an A-series admittedly) which had several visits to the rolling road and then a vernier camshaft drive fitted but nothing would cure the in-gear flat spot. If you accelerated from a standstill it would pull like a train from 2500 rpm all the way to the red line. However, ask it to do anything when cruising at 3000 rpm and it would just bog down, make a sound like a holed exhaust and then crawl up to 3600 rpm before it started to get lively again. I could only put this down to a standard advance curve in the dizzy as modified A-series engines always seem to need their dizzys changed.

[Kevin]

It might be quicker to get advice from a mini site as this is what the problem relates to and I would think the mini tuners would have come across this problem many times before.

[MorrisMartin]

Thanks for your suggestions. I will try them. The cam is still the standerd item.