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MG Midget and Sprite Technical - 1275 onto 1098

I've asked about this before but it got sort of bogged down in lots of suggestions for alternative and better engine upgrades - which really wasn't what I was after! It was a bit like the response "I wouldn't start from here" when asking for directions!

The question is pretty straightforward. Will a standard 1275 cylinder head fit onto a 1098 block? i.e. size and stud positions. and would there be a problem with the valves overlapping the block if they are still being operated by the 1098 cam?
GuyW

Guy

the answer is yes and yes to the first two points.

The 1275 head however is much shallower than the 1098 head. 21.4cc compared with 28.2 cc so the compression ratio would be around 11.1 with the standard dish piston. The valve lift of all standard cams is the same at 0.318"
Bob Beaumont

I think you would need to pocket the block for the exhaust valves, even with a standard cam.

You would need to use 1275 rockers, which will give you .318" valve lift. I've just checked a 1275 head and the greatest distance I found between an exhaust valve and the face of the head was only .307". Some were a lot less, but it's a crusty old head and some of the valves may be stuck slightly open.

Something else to bear in mind is that volume of the combustion chamber is quite a bit smaller on the 1275 head, compared to a 12G295, so your compression ratio would be increased. Pocketing the block would help to reduce it slightly.

Another possible issue is corrosion on top of the block around some of the waterways. When using the 1275 gasket, the fire-ring can overlap some of the corrosion, causing it to blow.

Edit: Bob had answered while I was measuring and typing!
Dave O'Neill 2

There are some discussions on this in the archives, searching for threads containing the words 'pocketing block' results in 11 threads.

As Dave O'Neill mentions the corrosion to the top of the block can overlap the fire ring and result in leakage, ideally the top of the block would be faced but I dealt with the problem in situ as I did the pocketing in situ as well. Shame I can't find the picture at the moment.
David Billington

Forgot to mention those 11 threads were in 'midget and Sprite Technical' .
David Billington

Thanks all. I think that has got me to my destination this time!

I have a couple of spare, good condition 1275 heads and was wondering if one might make a viable swap, and easy upgrade to my 1098. But change looks like being a bit more challenging than just an easy swap. It would make more sence to sell one or both of these and get a head that can be properly reworked
GuyW

I just stumbled across this from Keith Calver, while looking for something else...

https://www.calverst.com/technical-info/engine-small-bore-engine-12g940-head-fitting/
Dave O'Neill 2

Thanks. Interesting read Dave. So block doesn't always need relieving, but most certainly needs checking! He doesn't mention the combustion chamber volume issue but maybe that is covered elsewhere.
GuyW

Guy
To overcome the different combustion chamber sizes you could do the math and use a Cometic gasket with a specific ‘chamber’ size.
I have used these gaskets on a couple of occasions when lowering CR on modified engines when fitting A.CDodd RT cams that are recommended a 9.5 : 1 CR.
Alan
Alan Anstead

This is a fascinating thread.

Would there be any potential performance, rather than practical advantage in using a cylinder head from a 1275cc engine on a 1098cc block in preference to one meant the 1098cc? Some race tuner who wanted a different way to get high compression ratio and this was worth the hassle of pocketing the block rather than skimming a 1098cc head plus porting and larger valves?

Can you install bigger valves in a 12G940 head compared to any 1098cc head?

My guess to answer the first question is perhaps this Is unlikely to have been sought due to other factors such as if the competition class capacity split was up to or under 1300cc favoured 1275cc capacity based engines anyway.

If this head swap had dropped the compression then I suppose it might have been of interest to innovative tuners wanting to turbocharge or supercharge a 1098cc car. They would have still wanted to fit in the relevant competition engine capacity class, noting an equivalence factor (such as 1.4 x notional increase in capacity) might of been applied under completion rules to up the capacity on paper which could move such a. the car up into a larger engined class and be less competitive (if such a mod was eligible and the bottom end of the engine could be made to cope with it).

I always got the impression that the level of tuning and development of normal 1098cc engines was left behind once 1275cc engines were introduced. When I mean normal I am excluding any experimental Morris Engines for factory racing and rallying exploits, anything in Formula Junior and the S engines in Minis. And even for the Minis how much work on 1098cc S engine tuning once the 1275cc S was introduced would have depended on whether a 1098cc cars were desirable purely because of engine class sizes in competition at the time, i.e. if there was an under/up to 1100cc class that BMC or others wanted to compete in for profile and no under 1300cc class. Anything in International Racing and rallying would have needed homologation so cylinder heads would have been listed on paper for that model.

Guy - have you tried posting this question on the Morris Minor Owner’s Club Forum (free to join) including has anyone tried it and how they got on? There are a few innovative tuners on there such as someone who hillclimbs a Moggie and has tried various small bore engine mods.

Cheers
Mike
M Wood

Guy, guess you have read this on MMOC Forum: https://board.mmoc.org.uk/viewtopic.php?t=66340 and an earlier thread it links to: https://board.mmoc.org.uk/viewtopic.php?f=4&t=24559&p=245538&hilit=Declan+12g940#p245538

Well it sounds like those clever Moggie owners have been doing this already to up the performance of their 1098cc cars.

I do love the enthusiasm and ideas of folk still doing stuff to A Series engined BMC/BLMC/BL vehicles.

And we have not even started talking about what engine mods were used in 850cc and 998cc Mini racers as well as in 948cc Frogeyes, Austin A35 and A40 cars.

Cheers
Mike
M Wood

Mike

There are a number of benefits to using a 12G940 head on a small-bore engine, one of which being that the valves are bigger to start with, and that even bigger valves can be fitted, due to the valves being spaced slightly further apart. The latter point is why it's important to use 1275 rockers with the 12G940 head, as the pads are wider. Another one that you mentioned is that it's easier to increase the CR.

When I was racing my A35 (948cc) one of the permitted modifications in the 'roadgoing' class was the use of the 12G295 head. The biggest problem with using this head was getting the CR high enough. Even with flat-top pistons, the head needed a lot of skimming to get the CR up, which could lead to reliability issues. In the 'modified' class, the 12G940 head was allowed, which made things much easier.

Even with the 12G295 head, the block still needed pocketing when using a high-lift cam.
Dave O'Neill 2

Thanks Dave, really interesting insight and information.

I need to delve into Mike Barton’s book on tuning BMC cars to see if he mentions such ingenious head swapping for small bore engines.

I learn so much on this forum.

Cheers
Mike
M Wood

I have a paper copy of the old Speedsport Garton book, but here is an electronic one that has appeared on the web: https://allbrit.de/downloads/Allgemein/TuningBMCSportsCars.pdf which does seem to mention putting a 1275 head (‘HAN9/GAN4’ donor) on a 1098 engine in paragraphs 2 and 3 on page 24 including pocketing the block.

And further to my previous message I meant Mike Garton, not Barton.

Cheers
Mike
M Wood

This thread was discussed between 01/12/2022 and 18/12/2022

MG Midget and Sprite Technical index

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