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MG Midget and Sprite Technical - Centre Main Bearing Strap

While my 1275 engine's stripped down, I've been thinking about adding a centre main bearing strap. I'm not thinking of doing any upgrades this time round, so this would be more for peace of mind than because I'm expecting to have more horse-power. When I'm hill climbing the engine revs to 6500+ for a few seconds at a time but never for any length of time.

Does anyone have experience of fitting these straps?

1) Is this a worthwhile idea?

2) There are two types to choose from: a plain flat bar that will need the bearing cap grinding flat so that it fits or a complete, four bolt replacement cap. Is the four bolt cap worth going for as it will mean drilling and tapping the block?

3) One leg of the oil strainer bolts onto the centre bearing cap. Both of the options I'm considering are thicker than present, so I presume the leg won't fit properly. Any suggestions on how to tackle this one?

Any suggestions much appreciated.

Colin
C Mee

I fitted one to my 1420cc for peace of mind. I used IIRC 1" black bar and surface ground it flat top and bottom and the same with the centre main underside. Fixed with longer socket cap screws. I did it myself as I had access to a surface grinder and other machine tools.
David Billington

I think I have just found the answer to my own question. After loads of fruitless google searches it has just thrown up an item from .... The BBS Archive of course!

In 'To strap or not to strap' Pete Burgess said that he only straps race engines and recommends spending the money on high quality pistons instead.

Thanks Peter.
C Mee

Colin

Have you read David Vizard's book on tuning A Series engines to see what he advised on when to add a main strap as bit more background? (My wonky memory was yes for race engines being buzzed a lot, plus small bore engines being upgraded)

I would be tempted to follow Peter B's advice, but just wonder is it worth considering doing now depending on where you are in the upward spiral of engine upgraditus for competition modification? And that assumes your crankshaft will be always fine in future...

You moving into ModProd class or just getting even more serious about Roadgoung class?

Cheers
Mike

PS talk of spending money on good Pistons instead makes me realise that FW Thirntons are in your part of the world (another option to going to Minispares).
M Wood

Colin
Just a suggestion--
Had you thought of a set of ARP main studs
Given the possible age of yours and the high quality of ARP studs it could be a worthwhile upgrade /option

willy
William Revit

I'd opt for balancing. I run 2 1330's and the difference between the lightened and balanced one and the other (without) is very noticable. The balanced one feels completely smooth and unburstable, and the other relatively agricultural. If the engine is already in pieces it would be the ideal time take the plunge - unless of course you've already done it.
f pollock

Absolutely agree! I got mine line bored, had flywheel lightened and then the crank/flywheel and harmonic balancer all balanced as a unit. I then spent some time weighing and balancing the pistons with their con rods. There's little point in balancing a crank without also getting the conrods matched for weight. But what a difference!
GuyW

I have a strap on my 998, which I think came from Mini Spares. Cap machined flat. It was quite cheap and not worth making myself, but that was over 15 years ago. The mounting point for the oil strainer was hardly any different and easy to accommodate, but I don't know much about 1275s. If you fit a steel cap you will need to line bore it to the block, so it all gets expensive.

There are such mixed opinions about balancing. Vizard said it wasn't worth the money. I had a 1098 balanced in 1972 and it was quite smooth, but my present unbalanced 998 seems about the same to my aged senses. But I do have a crank damper, so not comparing like with like.

Les
L B Rose

I've been running mine without a strap for getting on for 15 years now, not balanced either. I try to change at 7000 but it is quite often higher. But I do think I've been lucky, next time it's in bits (if it doesn't turn itself into bits first) I'm going to fit one. They are cheap enough and machining should be easy to get done.

The only other thing I've done to the bottom end is fit Metro Turbo mains. Not something thats done much now but a local tuner recommend it and I think it might be mentioned in Vizard as well.

Its amazing how strong the A Series is considering its 3 bearing crank and humble origins.
John Payne

Colin

Centre main straps available for small bore (948/998/1098) and big bore (1275) engines from Minispares:
http://www.minispares.com/search/classic/Centre%20main%20strap.aspx

Part numbers CMS10, CMS13 and CMS10BOLT (CMS13 is the big bore one including bolts, which you can buy on their own)

Cheers
Mike
M Wood

I had a Longman prepared 1380 in my modsport engine in the 80's It had an EN40B crank in a thick wall block but no strap . It was regularly taken to 8K rpm Never had a problem in 8 years of racing. Recommend good quality pistons such as Omega.
Bob Beaumont

To my mind balancing is very important if higher revs are to be used on a regular basis - the engines I have had balanced over the years have all had a much improved smooth running that can be felt.
My current engine has been balanced and flywheel lightened and has a strap but I'm not sure I would bother with the strap again unless racing etc.

In the late sixties/early seventies I ran in effect a mk11 sprite engine 948 +60thou + hot cam, home modded head (no money for fancy balancing in those days and no one to do it down in darkest Devon) Managed to snap two cranks - red cranks were silly money. They both failed in front of the front main bearing - so the lack of a damper on the pulley probably didn't help.

R.
richard b

I bought a modsports this time last year with a 1476cc special longstroke crank. It was over-revved to 8k+ on the rolling road due to the tach under reading (not something I'd thought to check) and in first practice at Mondello Park the centre main spun a bearing and disintegrated. The crank was 8 thou out of true.
Crank duly straightened and reground +10, I'm fitting Turbo shells, centre strap, a very expensive KAD harmonic balancer and having the lot balanced expensively.
Belt and braces maybe, but I don't want it to happen again.
David Smith

This thread was discussed between 19/12/2019 and 24/12/2019

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