mini 1275 head

[fatal8446]

as i understand it the 1275 mini a+ head i have can be bolted onto my 1098 to give a bit better pefomance is this

a) true

and

b) just a case of bolting it on without having to modify anything

???

[PSL184]

a - true
b - what is the casting number of the head - 12g295 or 12g940 ?

If the number is cam4810 it's pants.....

[fatal8446]

i will pop over my garage in the morning and have a look, i like you... ur an imsomniac too lol

[PSL184]

Its the only time its quiet in this house ;-)

[billlobban]

You will however have to skim the head if its a 295 - Combustion chamber volume Standard 26.1- 12G 295 28.3cc. Note also that the 12G940 comes with 2 different valve sizes. There can be problems with fitting the larger valve head. But someone will no doubt be able to advise you if you get to that point

[mike.perry]

Is there a table anywhere of head numbers and specifications?

[linearaudio]

If you type "940 head on 1098, some observations" into search, I documented my experience of doing this. The main concern is whether the exhaust valves will hit the top of the block on full lift, in which case you either recess them into the head by about 1mm or grind a ledge into the bore face to clear the valve. Some say this is almost a certain problem, however I have done two conversions now and found plenty of clearance without resorting to any cutting! Others agree, so it is down to luck on your particular build. Makes a very useful improvement in power and, with the bigger carb/exhaust etc a huge improvement from lower mid range all the way up!

[Peetee]

The 12G295 head was never used on the 1275 engines. The 1275 used various versions of the 12G940, differences being the size and material used for the valves and the requirement or not for a thermostat bypass hose. Why they decided to stamp them all as '940' is a mystery and makes identification difficult if you can't put your hands on them.

[fatal8446]

right the head casting number is 12g940g it has the two different sized valves.

[billlobban]

I probably didnt explain very well
There are 2 (that I know of) different 12G940 heads The difference is in the valves
1 has 1 5/16" inlet and 1 5/32" exhaust
Other 1.4" inlet and 1 5/32" exhaust
Both supposed to be great heads on a 1098 (BMC swears by them) but please note the post by lineraudio above

[fatal8446]

will have to trial fit it and see if it clears by hand

[Declan_Burns]

You should do some calculations prior to fitting. See my earlier post.
http://www.morrisminoroc.co.uk/index.ph ... ght=declan

[fatal8446]

thanks for the help guys, as i say will have a test fit of this sometime this week weather permitting,

how did you get your valve movement measurements, just lots of weight onto the top of them?? or turn the engine by hand with the head fitted and a rule next to the valve stem?

[Declan_Burns]

You should really invest in a caliper dial gauge. A rule is really not suitable for such measurements. Measure the distance from the valve to the cylinder head as I mentioned. This will show you if a valve needs sinking or not. You may be lucky. Do check the the danger areas by cranking the engine slowly with blu-tack inserted between the valves and the block prior to torquing down the head.

[linearaudio]

thanks for the help guys, as i say will have a test fit of this sometime this week weather permitting,

how did you get your valve movement measurements, just lots of weight onto the top of them?? or turn the engine by hand with the head fitted and a rule next to the valve stem?

Ideally use a dial guage, but this is a biggish investment for a one-off. If you do as I suggested, bolt it all down, take carefully to full valve lift, and with a suitable lever you can see quite clearly whether you have the required sort of clearance- needs a bit of judgement, thats all. Look for about 1mm of clearance.

[fatal8446]

ah right so measure from the mateing surface of the head back into the valve got ya. guna stick it on my engineering flat and go about it with a feeler guage make sure she is straight, will do that today. However the weather is bogging today so i doubt ill strip the head off.

when test fitting( not torqued down) i can use the old head gasket yes?? as this will be a better representation on the compressed (new) gasket correct?

[Declan_Burns]

You need the 1275 gasket to go with the 12g940 head. Mine was 1mm thick. The 1098cc gasket is smaller so if the valve hits it you will clearly see.

[linearaudio]

Various different types of head gasket, from memory the composite type around 0.95mm, the copper type around 1.1mm, but they all seem to compress to about 0.85mm when torqued.

Another way to approach it is to bolt the head on with NO gasket. If the valves clear, then when you add the thickness of the head gasket you will have a nice margin of safety!

[MarkyB]

Sounds like a good plan but take it easy when you are trying it out with the starting handle.
Don't go at it as if you are trying to start the engine, just turn it over gently and see if there is any resistance.

I'll own up too bending a couple of valves on a highly modified head by not following any of this advice .
In my defence, I was young, and it seemed like a good idea at the time.

[stuart_k]

I had that head on my 1098 Midget when I bought it. When I took the head off it was clear that the valves were making their own pocket. I think it would be best to pocket the block.

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