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MG MGB Technical - 5 bearing crank in a 3 bearing block!!

Has anyone put a 5 bearing crank in a 3 bearing block???

Were there any problems?

Ta

Colin
Colin Parkinson

Won't oil spray out of the two unused journals while you are going along?

I would expect a 3 bearing has a larger centre bearing and wider con-rod bearings.

Can't you just fit a 5 bearing engine?
Martin Layton

You put bungs in the unused main bearings.
Bearing size at this time is not an issue. Idea is to use the 5 bearing B rods (steel)
No cant fit 5 bearing engine. It is a 1600 for a racing A.

Appreciate your comments!

Any more?

Colin
Colin Parkinson

The only issue I can see is that the crank might be a bit weak in the area of the two unsupported bearings.
Dave O'Neill 2

Dave, apparently it is stronger, and very slightly lighter!

Colin
Colin Parkinson

Colin - My understanding is that the 3 main bearing crank was more prone to flexing under high loading (lugging) of the engine. Would a 5 main crank possibly be even more so prone to the flexing problem due to its design being for 2 more supports in the additional bearings? Cheers - Dave
David DuBois

Dave, maye, maybe not!!

That is the sort of thing I would like to know. I would suggest not, as there is far more meat around the webs.

Colin
Colin Parkinson

I'd be interested to know how you get on.
Dave O'Neill 2

Are the bore spacings the same on a 3 vs 5 brg block?
Art Pearse

Not sure why you'd want to bother doing this. In fact the 3 main crank will survive high rpm better than an uncrossdrilled 5 main crank.

The 3 main rods are heavy but strong and should give you no problems.

Doing this raises issues on bearing length, diameter (would you bore out the block to take the larger diameter MGB main journals, or cut down the crank to the smaller diameter existing saddles in the block?), rear main sealing etc. With no useful advantage in sight, I can't see bothering with this project.

Spend the money on a set of racing rods instead.
Bill Spohn

Bill, the 3 main rods are heavy and weak!! See pic
Colin Parkinson

Bill, the 3 main rods are heavy and weak!! See pic

If I could find race rods from stock, I would buy some, but I need to get this car back on track ASAP.
The logic behind this, is to do away with pinch bolt rods, the crank is a secondary issue.

I could get the crank ground to fit the bearings. The bearing length is the same I think.
Rear seal, easy to do,and has to be an improvement? Just need a 3 synchro flywheel.

Art, I believe so.

Any more suggestions?

Thanks

Colin

Colin Parkinson

Have you looked into using cranks or rods from diesel B-series engines?

I know someone who used to race Magnettes and he used diesel parts. I think the diesels had EN40 cranks and stronger rods.
Dave O'Neill 2

Dave, in a word, no!!

I know nothin about the diesel engines, but I can check the parts fiche to look at some part numbers.

Colin
Colin Parkinson

"Bill, the 3 main rods are heavy and weak!!"

I can't disagree with the heavy part, but they aren't particularly weak.

As I type I have on a shelf in front of me a nice S shaped early rod that had a piston break up. The rod punched a hole in the block, broke the crank, but didn't break.

Of course you need to start with rods that still have some duty life left, not something that has been in a racing engine for 40 years. There should still be some on a shelf somewhere or tear some out of an automatic Austin Cambridge that has never seen the high side of 4000 RPM.

Bill Spohn

Bill, I have found that set of rods. Out of an engine with the crank still on standard, and probably Cambridge.
So we are going to get them shot peened and reduce the revs with a milder cam.

Fingers crossed!!

Colin
Colin Parkinson

This thread was discussed between 24/05/2009 and 29/05/2009

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