MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG Midget and Sprite Technical - 948 engine with +60 pistons

Yes, Yes, I know with a little online searching and a calculation I should be able to get the answer, however, for those of you that probably have the information tucked in the back of your mind, Ill ask anyway.

After stripping my frog the 948 engine has +60 pistons in it - the question is, what does this actually equate the engine cc to?

Also the bores look ok but as +60 seems slightly excessive (I think the original manual says you can go to +40) should I be looking at liners and taking it back down to a relatively "normal" bore?

PS - do folks normally let their insurance company know if there has been an increase in the cc of the car if the bores are machined to take larger pistons??

Thanks

Alan
a reid

I want to say its a 998

I certianly wouldnt tell the insurance company... how would they know if you dont tell them
Prop and the Blackhole Midget

Alan
At 2.477 + .0.060 you are just under 2.542 by 0.005 in.(2.537)
If you want to have a rebore you can keep going and use 998 (2.542) pistons.
To go with the 998 pistons you can do away with the 948 pinch bolt con rods and use the 998 items. My 948 is at about 1046 on overbore with the relevant oversize 998 pistons on 998 rods.
Alan
Alan Anstead

Found this on Minifinity.

998 +20 = 1014
998 +40 = 1027
998 +60 = 1043

1275 +20 = 1293
1275 +40 = 1312
1275 +60 =1330

http://www.minifinity.com/forum/viewtopic.php?f=5&t=577
Lawrence Slater

I told the insurance company I had a 1275 engine in the frog and there was no difference in the premium
Bob Beaumont

The price of liners + boring of them is rather expensive.
mog

When I was racing my A35, the rules said we could overbore a maximum of +.060".

At that time, 948 pistons were getting difficult to find in any size, so we managed to get a dispensation to overbore to +.065" and use a standard 998 piston.

As far as insurance companies are concerned, or even the DVLA, the nominal engine capacity is still the same. If you fitted a 1098 or 1275, it would be a different matter.
Dave O'Neill 2

All,

thanks for the input - first stage is to find out what state the bores are in and what if anything is required - if nothing then I guess i simply re-ring the existing pistons. If I can get away with a "slight" re-bore then it sounds like 998 pistons and con rods are the way to go.

Already twitching at the anticipation of the additional POWER.....

Alan
a reid

lol, I just used the same rods, I know later rods better, but the old pins fitted the mini pistons fine.
mog

948 +0.060" = 995cc.

We ran this setup for about 22 years before the switch to the current 1380.

Combined with a flowed 12G295 head, Cooper 997 cam (2A948), HS4 SU and a big(ish) bore exhaust with LCB manifold gave a good performance in our Minor.

Motorway cruising was not so great as we still had the original 4.555:1 diff, but for everything else it was pretty good.

The advice to go to +0.065" and 998 pistons/rods is good, as the reason we had to go to +0.060" 45 years ago was that one of the gudgeon pin pinch bolts worked loose and scored the No. 4 bore very badly.
Richard Wale

This thread was discussed between 27/07/2015 and 29/07/2015

MG Midget and Sprite Technical index

This thread is from the archives. Join the live MG Midget and Sprite Technical BBS now