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MG Midget and Sprite Technical - Swapping heads

Can one fit a standard 1275 head to a 1098 block? I thought Vizard would explain but he isn't that clear about it. I suspect it may depend on valve lift and therefore which cam is installed?
GuyW

It can be done but the block needs pocketing, I've done it in situ on the one I did using a jig fixed to a head stud and a valve seat top narrowing cutter. Unfortunately I can't find the photo at the moment but IIRC I've posted it in the past. Bit of a nightmare trying to find it in the archive though. IIRC I was using a 276 cam and it gave a great improvement above 3500RPM. Had an issue with a rust pitting on the top of the block coinciding with a fire ring leading to pressurisation of the cooling system but that was dealt with.
David Billington

Guy

Surely a modified 12G295 or 12G202 head is the way to go?

There is some info on using a 1275 head (12G940 casting) on a small bore engine on Keith Calver’s website: https://www.calverst.com/technical-info/engine-small-bore-engine-12g940-head-fitting/

Cheers
Mike
M Wood

" Surely a modified 12G295 or 12G202 head is the way to go? "

Surely, yes Mike. Except I have 3 spare 12G940 heads!

Thanks for the Calver link.
GuyW

And the 12G920 has larger valves already. (yes, you can make the 12G295 larger, but so too the 12G940)
Aoart from the block pocket there is a height difference on the valves. But worth perservering with if you have one immediately to hand.
Paul Walbran

Sorry, ignore that about the valves. The valves are a few mm longer, but the chamber is shallower, so they offset. I was thinking of when using 12G941 valves in 12G295 head.
Sigh.

The main other issue can be CR, as the chambers are (rounded) 28cc and 21cc. Provided that is allowed for it is a gain.
Paul Walbran

Guy

Any use? https://www.ebay.co.uk/itm/134103308202

https://www.ebay.co.uk/itm/284025043010?hash=item4221363842%3Ag%3AyycAAOSwgZNhbuD6&LH_ItemCondition=4

https://www.ebay.co.uk/itm/284554708183?hash=item4240c844d7%3Ag%3AXRYAAOSw-wNhbuHX&LH_ItemCondition=4

Cheers
Mike
M Wood

I tried this once on a Morris 1300/Austin America. I was able to obtain an 1100 with manual transmission to replace the 1275 with a dead automatic. Replacing the cracked head on the 1100 with the head from the 1275 was the plan. At first, it bent pushrods but after replacing a couple, was OK. No doubt the valves made their dents in the top of the block. The compression ratio was clearly increased as the engine had pinking problems. Otherwise, it ran like a scalded ape! The mild white foam that collected under the valve cover from a tiny bit of water entering the oil was easily vented from the 1100 side cover to the road.

Some things are not meant to be.
Glenn Mallory

This thread was discussed between 07/05/2022 and 11/05/2022

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