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MG Midget and Sprite Technical - 1098 head, or not

Hi everyone,
I've a 1965 Mk 2, 1098 midget, but at some stage in its life its had a swap for a 1098 Morris Minor engine.
I've the offer of a 1275 head for a Midget, but is it interchangable.
Any help appreciated.


It is possible, although not necessarily straightforward.

You would need to use the 1275 gasket and also the 1275 rockers.

Also, the 1098 gasket has some large coolant holes in it which can cause circular patches of corrosion on the block face. The fire ring on no. 1 cylinder of the 1275 gasket overlaps one of them - possibly no. 4, also - which prevents it sealing.

The only way to correct this is to skim the block to remove the corrosion.

You would also need to work out the compression ratio with the 1275 head.
Dave O'Neill2

Your 1098 probably has a 295 cylinder head with a chamber capacity of 28.3cc.
Your 1275 head is probably 940 cylinder head with a chamber of 24.1 cc so that the compression ratio will be raised.
When fitting a big bore head to a small bore engine there is the possibility that the cylinder bores may need pocketing to avoid valve strike.
As Dave says you'll have to do some measuring and calculating. Helpfully Minispares quote the gasket volumes of various head gaskets that you will need to factor into your calculations.
Alan Anstead

Hi again
As an afterthought you might have a 202 head on your Minor engine. You'll have to check the casting numbers on top of the heads.
Alan Anstead

Dont forget you also have to pocket the piston tops and I want to say notch out the clys at the deck for valve clearance

Of course you could take all that cash and a 1098 cly head and let peter burguss work his black magic and have something really special


Im going out on a limb and guessing a BMW motor cycle cly. Head KT100 / KT1100, wont fit on a 1098, or would it ?

Prop and the Blackhole Midget

Why do you think it is a minor engine
And why do you think fitting a 1275 head (12G940 is guess?) will do it any good?

If you have a minor engine with 12G202 head the head is your least problem....
The 12G202 is a very good base to tune

For example My old 998 mini engine had a modded 12G202 and made as much power as a std 1275

So what is te real problem/wish
Onno K


The piston tops don't need pocketing but the block may well do with the larger valves in the 1275 head.

I've done this conversion and IIRC it was just the exhaust valves that required pocketing of the block and I did that in situ with a valve seat cutter held in a jig bolted to a head stud and plenty of grease around the piston tops to trap swarf. I did have a problem with one patch of corrosion as Dave mentioned above causing a slight weep and pressurising the coolant system but that was corrected by a small patch of weld to the block face and dressing it back in situ with no further problem. Knowing the issue beforehand woukld help avoid it. IIRC the cam in the engine was a Piper 276 and the transformation was amazing as it started to pull like a train above about 3.5kRPM due to the better breathing.
David Billington

Fhanks david,

To be honest, all I know is from what little ive read....I knew something had to be cut back for the valves to work as intened

Personally Im still a fan of having peter burguss reworking a 1098 head with lots of black magic and skip 1275 experimental stuff

Prop and the Blackhole Midget

Consider this:

I have just removed my Richard Longman 202 head.
Polished; ported; 32mm inlets CR 10.43 /1.
With other mods (567 cam) on base 948 engine on rolling road it shows 62BHP (a 1275 is 60/65BHP) at 6000rpm.

I substituted a stock 295 head
Unpolished; unported; 31mm inlets CR adjusted to 9.5 / 1 CR. Now showing 63 BHP at 5400rpm on same rolling road.

Alan Anstead

That the CR drop and and smaler ports help with your type of cam?

If the head has to big ports it can make less power because gas speeds are lower.
There for the results can be better with a un modded head.

That is exactly why I asked what the real problem /wish is.
You always need to consider the whole picture
Onno K

Problem was an 'expert' (not Longman (nor me)) decided to raise the CR. But that is another story!
Alan Anstead

Alan, my book lists 21.4cc for the 1275 head, and my memory seems to concur with that.
Paul Walbran

Oops! I think you may be right. Several heads on the bench for measuring lately I may have made a mistake.Where's my copy of Vizard?
Alan Anstead

My memory might be right?! Scary.
Paul Walbran

Thanks all of you for the help.

I bought the car from the UK, but it's got a problem with N1 cylinder, burning oil. I changed the valve seals (3 days before the Circuit des Remparts Rallye,) and its improved it but not cured it. The next job is to renew the valve guides.

The compression test reads: N1=90, N2=91, N3=90 and N4=92.

Eng N 10MA-U-L, which appears to be a Low compression Moggy Minor series 5 engine.

Is the Kent MASC attending the Remparts this year ?

We did the Remparts in 2013.
This year I am doing Spridget Treffen, Bressuire Historic GP then LeMans Classic incorporating a Loire Valley Tour.
Alan Anstead

I only missed Remparts in 2013 will be there this year again.
We are planning to bring a complete team this year with several race cars! Our hero racer made it to the pre war podium last time (2012) so let's hope for a repeat.

So You are enjoying the car and it is just burning oil.
At what rate is it burning oil?
If it is less than 1L per 1000KM then just leave it as is.

But if you want to do the work and get a bit more power out of it....
new high CR pistons
some mild head work
266 cam

Onno K

It's burning that much that it goes onto three cylinders after about 500kms and when I pull the head, the crown of the N1 piston has thick, hard burnt oil deposits covering it.
I'll look out for you in the pits....Is the car still orange?

Our club ran 25 cars in the Rallye on the Saturday. It's a shame that we didn't know you were coming over, as we're only 15 mins north of Angouleme.
Fortunately, my better half works for the Remparts every week and this years event is well underway, with places filling fast.

In 2010 I bought a 1275 Mini GT cylinder for 20 off ebay. A lucky last minute bid. This had been overhauled with unleaded conversion.

This went on the Morris 1000 engine which is installed in my 1943 Singer Le Mans.

Apart from using a 1275 head gasket, I made no other alterations as previously discussed. I was probably naive and probably fortunate. Four years on and there has never been any problems. Water, oil and pressure all stay in their allocated areas.

Did it make a difference? Not as much a putting a HIF 44 Su on.

Roly Alcock

good grief!

Should have said 1934 Singer Le Mans
and qualified Morris 1000, as a 1098 engine.

Roly Alcock


Good point on the details as you've reminded me that the 1098 engine I put the 1275 head on was fitted with a Dellorto 40 DHLA carb so maybe the breathing also helped in the conversion. I always found the Dellorto worked much better and pulled better at lower revs and above but maybe the SUs were knackered. The 6 Dellortos I have cost on average 7.50 each courtesy of rusty Alfa Romeos in the late 1980s, early 1990s. The first one I bought apparently came off an Alfa Romeo and was fitted to a Harley Davidson but scared the owner so much he put the standard carb back on the Harley. I couldn't stomach the cost of a 45 carb for my 1420cc build so used one of the 40 DHLAs with both chokes feeding a plenum, worked OK but I was told it would not achieve maximum power but it went OK.
David Billington

This thread was discussed between 30/01/2014 and 04/02/2014

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