28 replies to this topic

[nicklouse]

Just picking up on this thread.
Just to confirm can you YES or NO fit a 1275 head onto a 998 without any extra work?
If so on a 998 HLE 1000 can it be done and what head do I need?

both. it depends.

 

if everything is standard and nothing been changed at all then there is a good chance you will not have issues but clearance over the exhaust valve will need to be checked or it can slam into the top of the block.

[JonnyAlpha]

Found a 1275 head, 12g940b for £75 from a 1275 Auto Engine with low milage (25K)?

 

Should I take a punt?

[Cooperman]

There is little or no real point in putting a 12g940 head on a 998 unless it is done in conjunction with a higher lift cam, gas Flowing, improved induction and exhaust, engine balancing and a lower final drive ratio to match the much higher revs being used.
The block will need to be pocketed, the compression ratio calculated and machining done to achieve the correct figure
It is a lot of work to get it right.

[tb205gti]

There is little or no real point in putting a 12g940 head on a 998 unless it is done in conjunction with a higher lift cam, gas Flowing, improved induction and exhaust, engine balancing and a lower final drive ratio to match the much higher revs being used.
The block will need to be pocketed, the compression ratio calculated and machining done to achieve the correct figure
It is a lot of work to get it right.

 

Did it on my 998. standard 12G940, Compression is 10.5:1 static, block pocketed with a dremel (clearances confirmed with clay). Before using the original head it would struggle to get past 5.200 rpm - not it flies past 6.000 and has no problems hitting 6.500 rpm.

Cooper freeflow + RC40, 2xHS2 with MED filter and stacks. The increase in both torque and power is huge compared to the original head. Can't wait to try it with a better cam as well..

 

I'd do it again anytime.

[Cooperman]

Yes a 940 head on a 9998 works best at higher revs, typically over 4500rpm.
The thing is that not many owners want to rev their classic car engine over about 4500 and with a 940 head there will almost certainly be some loss of bottom end to mid range torque.
It depends on how the car is to be driven really.

[ads7]

Does anyone have a Dyno printout of std valve 12g940 on 998 running stage one inlet exhaust only i.e. no cam mods?

Then we can assess the torque and power increase and gains/losses in a particular rev range?

[Algordo1100]

I have done this. Years ago. I had a spare 12G940 unleaded head and a standard 998 A+ with a warped head. It was an experiment.

I didn't pocket the block. I fitted it, torqued it on and turned the motor over by hand. Seemed fine. Finished it off with an LCB, RC40 and an Hif44, electronic ignition.

I ran it like that for years, no trouble and it was one peppy little 998! Revved like anything. Would sit at 90mph all day (On city E gearing). My mate's standard SPI sprite could not catch it (probably a bit tired though)
I really liked that motor. It shouldn't have worked that well but it did. I was probably lucky.

[ads7]

I have done this. Years ago. I had a spare 12G940 unleaded head and a standard 998 A+ with a warped head. It was an experiment.

I didn't pocket the block. I fitted it, torqued it on and turned the motor over by hand. Seemed fine. Finished it off with an LCB, RC40 and an Hif44, electronic ignition.

I ran it like that for years, no trouble and it was one peppy little 998! Revved like anything. Would sit at 90mph all day (On city E gearing). My mate's standard SPI sprite could not catch it (probably a bit tired though)
I really liked that motor. It shouldn't have worked that well but it did. I was probably lucky.

Good to know, thank you.

[JonnyAlpha]

I am in the middle of a 1310 build, but have a spare 12G940 head that I would love to fit to my 998 in the meantime.

Its not the large valve type but has had unleaded inserts fitted.

 

Interesting to hear that you may not need to pocket the block?

 

There is a lad in our Mini Club with the same model / make car as mine (HLE 1000), he fitted a 12G940 head a while back "Cowboy Style" (his words not mine). He took the head off, taped up the bores, marked the block with a 1275 gasket worked out the valve overlap and went at it with a dremel (not sure exactly what shape or pattern.

He then put everything back together and turned it by hand. Fired it up and away he went. 

I am unsure what manifold or carbs he had LCB and HIF 38 or 44 I think. 

 

Anway, on the way to Castle Combe last year he was up with / ahead of the 1275's and on his track sessions was flying!!

 

He is now in the process of fitting twin SU's - not for performance (as I don't believe there would be any increase)- just because he wants to.

 

After doing some research - I looked in my Vizard Yellow Bible and couldn't find any real info on fitting a 1275 head to a 998 however after some searching Kieth Calver to the rescue:

https://www.calverst...0-head-fitting/

Edited by JonnyAlpha, 15 April 2020 - 08:54 AM.

[gazza82]

I am in the middle of a 1310 build, but have a spare 12G940 head that I would love to fit to my 998 in the meantime.

Its not the large valve type but has had unleaded inserts fitted.

 

It depends on the valve sizes and lift .. you'll only really know if you try it on the block and use something to see if the valves touch the block .. turn it over by hand rather than on the starter!

[Avtovaz]

Im not sure the way to go with my car, although i do have a 1293, and i do have a turbo set up which i was going to fit to my 998, i keep thinking a simple 998 probably with some larger pistons in it would be the way to go.

 

I have a 12g940 head in the loft, its had a slight skim, new unleaded valve seats, its all done all ready to go. I also have the standard 998 A+ head untouched, how much work is involved to get the 998 head anything like the 12g940? Id sooner have the 998 head so when i open the bonnet it looks standard, but the other head is just sat there. I want a good drivers car.

 

thanks

[JonnyAlpha]

Im not sure the way to go with my car, although i do have a 1293, and i do have a turbo set up which i was going to fit to my 998, i keep thinking a simple 998 probably with some larger pistons in it would be the way to go.

 

I have a 12g940 head in the loft, its had a slight skim, new unleaded valve seats, its all done all ready to go. I also have the standard 998 A+ head untouched, how much work is involved to get the 998 head anything like the 12g940? Id sooner have the 998 head so when i open the bonnet it looks standard, but the other head is just sat there. I want a good drivers car.

 

thanks

 

to be honest when looking at the Engine (apart from the experts) you'd not really notice any difference whether there was a 1275 or 998 head on the block. 

IMHO you'd be wasting your time trying to get a standard 998 head to perform as well. You could look at getting a 12G295 998 head which I think was the Cooper Head and use that. On a 998 you'd probably need Flat Top Pistons and the head needs to be skimmed to achieve the correct Compression Ratio.

 

I have a 12G295 head that I am considering selling

[Avtovaz]

if i  was to get another head, it would have to be seriously cheep and unleaded really. So if i go 998 n/a, ill put the 1275 head on. The only bit i dont like about that is, i want the car to be as factory looking as i can, may even put wheel trims on it yet, and i know what a 1275 head looks like when i open the bonnet compared to a 998...

[Turbo Phil]

I've just uploaded this, it covers pretty much everything you need to know about fitting a 1275 head to a 998/1098 block.

 

Phil.