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MG Midget and Sprite Technical - 998cc upgrade to 948 engine

Hi

Anyone done an upgrade to 948cc Sprite engine, such as rebore using 998cc Mini flat top pistons or rebore and using oversized 948cc pistons (up to 995cc)?

Not crazy budget, so not building a Formula Junior engine or trying to replicate works experiential racing engines in Sebrings. Old days I would be looking for 998cc Mini Cooper pistons (D shaped flat top). I am wondering what standard new and secondhand pistons and rods can be used, as well as what upgraded pistons that Minipares offer.

I realise that going 1275 (1293) with a ribbed gearbox would be a sensible choice, and that my standard MkII close ratio smooth case gearbox may not enjoy an upgrade.

Cheers
Mike
M Wood

When I raced my A35, it was standard practice to rebore to 998 and use Metro flat-top pistons. In those days, they could be bought from Roversport for about £4 each.

There are two types. The early ones have a fully floating gudgeon pin with circlips, so you need the 998 Mini or 1098 midget or Minor rods.

Later A+ pistons used an interference fit pin, so you need the rods from an A+ 998 Mini or Metro.
Dave O'Neill 2

My 948 is at about 1046 as you can bore +0.060 and beyond fitting mini pistons using the 998 conrods.
Alan Anstead

Rebore to std 998 Mini flat top pistons, alternatively use 6cc dished ones.Do leave the old rods in the bin, buy the later 1098 ones! I beefed up my Sprite to 65 HP doing this (and some other things) With comp ratio of 8:1 and a SW 5-7 from Swifttune (or Kent 256) it is a joy to drive (with 12G295 head). Realize that you have to upgrade your clutch.....
K van Beijmerwerdt

A 6 spring stock 948 clutch will cope with 65Bhp.
There are a plethora of camshaft profiles to choose from many that rarely get a mention on this site depending upon your driving style and usage.
Alan Anstead

Thanks for these very helpful replies.

Best wishes
Mike
M Wood

Exactly what I did. I have a spare set of 1098 rods if you are interested. What will you do about carburation, ignition etc? Received wisdom these days would be single SU HIF 38, but I slavishly followed Vizard's recipe and have an HIF44. Standard ignition advance curve would not be right, and I got points bounce over 6000 rpm, so I have a 59D4 from 998 Metro. It had to be modified as it's retained differently in the A Plus block. I am on my second 998 Metro cylinder head (first one cracked), with big valves and 10.5 CR. Yes the standard clutch will just about cope, but I had the flywheel drilled for a 1275 clutch, which is a vast improvement. Your Mk II smoothcase gearbox will be OK - I had one in my first Sprite with 1098 Morris Minor engine. Just don't overstress the synchros.

Les
L B Rose

Hi Les

A set of 1098cc rods would be great - please e-mail me off-line at mike DOT mikewood AT gmail DOT com

I will probably re-use my twin 1 1/4" HS4 carbs (but do have some HS2 carbs and an Alexander manifold, but probably best suited for a modded 1275 engine)

Thanks for the advice on gearbox, clutch and head.

Cheers
Mike
M Wood

Mike - did you get my email about the rods?

Les
L B Rose

This thread was discussed between 11/05/2019 and 18/07/2019

MG Midget and Sprite Technical index

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