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MG Midget and Sprite Technical - Flat top pistons. Worth it?

After another blown head gasket ( this one lasted 6000+ miles) my 12G295 head is at the machine shop for resurfacing. The block will follow when I’ve removed it from the car. Hopefully this will solve the problem.
The engine is a 10CC 1098 overbored +40 thou”so 1133cc.
The pistons that are in it at the moment are the 5 ring type and have been in the engine since 1970, so I think I will change them to the later 3 ring type.
But the question is do I fit dished ones or flat top? I won’t be racing the car or doing track days, so is it worth going to the extra expense of flat tops, or will they be an improvement seeing as how the 12G295 head has the largest combustion chamber of all the A series heads, or will there be more chance of HGF again with flat tops?
Bernie.
b higginson

Flat top pistons will increase the compression ratio and potentially the greater risk of HGF. There is no point in increasing the CR unless other modifications are planned to ingntion, carburation and exhaust.There is also evidence to suggest that dish pistons can creat a better swirl effect and improve combustion.
Bob Beaumont

Didn’t the 1098 have flat tops as standard?
Dave O'Neill 2

There has been a development. The machine shop has just told me that the head is cracked, so that’s the end of that.
Plan B. Except I haven’t got a plan B.
I so p***ed off because the engine is the original one from new and I really wanted to keep it together with the car.
I have yet to take the block to the shop for surfacing, but if that too is cracked I’m up the proverbial without a paddle.
Bernie.
b higginson

Bu**er, that's bad news.

I have a spare 10CC engine if you get stuck.
Dave O'Neill 2

There are a couple of heads on evilbay but will require a refurb.
Bob Beaumont

Where is the crack?
Some are fixable with the right skills and process, though at a cost that exceeds that of another head.

What type of head gasket have you been using?
Paul Walbran

Bernie
There is a Motorcycle garage in Kent that did a remarkable job on restoring a badly damaged Spridget head for one of my group. It was so badly damaged that we thought it was scrap.I can get the contacts for you if you are interested.

The problem with buying heads off eBay is that they can also be cracked.

If you go for flat top piston with any head you will need to measure combustion chamber and piston top to top of block factoring in the bit beside the piston down to the first ring and also the gasket volume to work out the CR. The CR may be too high for your cam and you may then be blighted by pinking.
Alan Anstead

www.goddenengineering.co.uk are the firm
01622-870171

Alan Anstead

Dave. Thanks for the offer. I’ll be in touch if my block is cracked. I won’t know until the middle of next week.

Paul. I’ve used a couple of different gaskets, the last one being from Minisport of Padiham. I was assured that “ this is the one the racers use”. But I think in my heart of hearts I’ve known for a while that the head and block could be out of true.

Alan. Thank you for the info. I may take you up on that if all else fails.

Tonight I have had an offer of a block and possibly a head from Steve Keil, a local MASC member so all is not lost yet. I’ll see what next week brings and report back.

Thanks to all for the helpful suggestions.
Bernie.

b higginson

Blimey Alan that's a name from the past. I remember Don Godden doing grasstrack in the sixties.

Trev
T Mason

Continuing from Bob's comments about dished pistons improving swirl, for a given CR on the same head, a dished piston sits further up the bore. This increases squish and improves combustion to the extent that lower octane fuel could be used. Theoretically. In practice we use a higher CR on the same fuel 😂

The Metro engines ran 10.5 CR with dished pistons virtually right up to the top of the block at the outer edge, on a very standard 252 deg inlet profile and advance curve to suit and no ill effects other than using an anti run-on valve.

My 1950cc MGB with 11.5:1 and zero deck height, when fitted with 56 deg inlet closing and advance curve to matc, will run happily on 98 octane, taking a foot-full from about 1500. Large ports and low gas speeds dictate part throttle only below that.


Paul Walbran

The reason for my interest in gasket type was that we've found some types (not all) of copper/steel sandwich gaskets blow between cylinders due to poor quality filler material.
I have yet to identify the source as we see only the results from previous fitment.
It became an issue after asbestos was banned and alternative fillers had to be found.
One that persistently fails is a pinkish material that looks like that used in SU carb kits. Good for carbs but can't hack the extra heat between the cylinders.
Paul Walbran

Bernie, don't hesitate to go to Goddens as Alan suggests.

http://www.goddenengineering.co.uk/index.htm

Not just ANY bike place. Velocettes and Vincents. Seriously good engineering

I too can personally recommend them. I took what I thought was my knackered pre crossflow Ford Capri cylinder head to them, for unleading in very bad seats. PERFECT JOB. A well known and respected classic Ford specialist in Maidstone recommended them to me I think. They do a lot of heads I discovered when I went there.

anamnesis

When I had mine 1098 engine done the guys recommended not going over +20 thou.
Has the boring to +40 left too little metal and caused the crack? Just wondering.

I got 180 compression on all four with these pistons.

Hope you get yours sorted soon

James Paul

Sorry just read you message properly and realised you head is cracked and not the block. 🤦🏼

Be interesting to know where it had cracked. If between the chambers then would over boring of the block increase the temperature stress on the head gasket and the gap between the head chambers?



James Paul

Turns out that my block was indeed cracked, so I have got a fresh 12G295 head from a Mini guy in Barnsley and a block and crank from Steve Keil.
I will buy a set of flat top pistons and send the head to Peter Burgess for refurbing and tuning.
b higginson

This thread was discussed between 25/09/2025 and 07/10/2025

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