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MG MGB Technical - con rod balancing

Having got new pistons for my engine rebuild, I have checked the balance of the little ends. I have 3 that are about 209 grams, the fourth is 202. 7 grams seems a lot to lose. If I aim to balance this out what is the area to take metal off from? My con rods are the diagonal big end cap and plain little end.

I know that this may be futile especially as I am not having the crank/flywheel/clutch balanced. But I thought that every little helps.
Steve Church

Hi Steve

It might be better to balance for total weight of rod rather than each part separately. The rods are usually close as long as one hasn't been replaced in the past (especially with the diagonal split ones).

Peter
Peter Burgess Tuning

If you are sure you are isolating little and big ends and hence your figures are accurate and repeatable, you could ADD metal to little end or the inside of the gudgeon pin with weld, on the lighter one. But Peter will know better if this is an advsable remedy. I used this method to balance piston and pins when I had one light one.
Allan Reeling

As you may be aware. Peter recommends combustion chamber balancing, To as it were get thing running well. I guess if you are not balancing the crank ( and am I correct in recalling that this is probably better than lightening the flywheel?) Then yes if all connecting rods weigh more or less the same this is fine for road use. I too have the diagonal slit rods, which are ponderous in weight.( and here, here you were telling me to ditch them and get the later 18 V ones!)I shal do the necessar to even them but will still balance the crank, leave the flywheel alone good job on the head etc. Mike
J.M. Doust

I have one of Peter's heads so assume the chambers are balanced. Not checked yet. I assumed I don't need to.
Steve Church

Hi Steve

Your chambers should be within 0.5 ccs.

Balancing the pistons and rods has more of an effect on smooth running than the rotating balance.

Peter
Peter Burgess Tuning

Actually, how can one discern whether an engine requires balancing? I suppose, it is a question of smoothness? Someone once told me, all BMW engines were balanced from the factory. certainly any that I have driven do seem smooth. But, to have a truly smooth engine surely all must be balanced and equalised. I can understand Peter that if we have gross discrepancies with chamber volumes then we may have lumpyness.likewise, conrods and pistons, if they are out of wack. But the balanced crankshaft is that more to allow it to rev better?Mike
J.M. Doust

The Factory balanced your MGB engine so it would be smooth. The used parts need checking for balance, unless things have been messed with all should be ok. The flywheel suffers if folks have it balanced when it has a worn ring gear. If someone changes a rod without equalising weights the engine will be out of balance. This does not display as an engine jumping about in the engine bay, it is a high frequency vibration which can, on occasion, be felt through the gear lever. Be aware worn gearbox bearings cause more of a vibration though! We tracked down a problem with a vibrating MGA engine, one rod was 60 grammes out! However the buzz was still present and tracked down to gearbox bearings. Hard engine mounts always make the engine feel worse even in perfect balance.

Peter
Peter Burgess Tuning

BMW produced mostly 6 cylinder engines, which I would assume feel better "balanced" due to there being more energy pulses, for a given rpm, similarly V8's.
Balancing the crank/flywheel/pulley assembly I would assume has more to do with crank longevity than anything else. Evening out the bangs, i.e., fuel/air, combustion chamber volume and valve opening; and having equal..ish reciprocating masses, i.e., pistons, pins and little ends, would affect "lumpiness" more.
Allan Reeling

Peter, do you mean we should not re-balance the flywheel when replacing a ring gear? If we resurface should we re-balance?
Robert McCoy

If your flywheel is untouched since leaving the factory changing the ring gear or refacing should make no difference to balance. Assuming the flywheel is refaced true and the ring gear is fitted correctly.However our flywheel are checked for balance to be on the safe side. So far none has needed balancing unless they have been riddled when balancing against a worn ring gear....one notes extra drilling opposite worn teeth !

Peter
Peter Burgess Tuning

This thread was discussed between 22/01/2012 and 24/01/2012

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