So I am at that point where I am getting the head rebuilt and closing in on reassembly of the 650 engine with new parts.
Balancing. I know there has been a few threads on this. I have done some research on the web about balance for a two cylinder inline engine where both cylinders move up and down together.

My question is do you leave the balance factor to the engine shop, or specify this factor yourself? I am inclined to specify 58% based on my research.

How necessary have you found balancing to be? Anyone have any before and after experiences?
I did mark the position of the flywheel when I disassembled the engine, but you never know if the flywheel had been removed before.
I am also thinking of using the flywheel out of my 110F engine, as it is a little lighter, and if I get balancing done, will maybe not degrade the smoothness of the engine at idle. Thoughts?

John

I have gone down a similar route.
I had every rotating mass lightened, polished and balanced.
I have a 700cc kit that had guidance and my 500L flywheel has been lightened to 4kg
I have not completed the car to describe the drivng experience as yet - but their advise must of come from a level of design and experience.
There was a considerable quantity of heavy metal inserted into the crank - but i am not aware of the balance factor.
The engineer undertaking the balancing work said the job was the most difficult he'd undertaken due to the quirks of this type of engine.

My 650 engine has a shallow drilling in the back of the flywheel which just happens to be at the top when the crankshaft is at TDC; that may be co-incidental but something worth looking for when aligning it.
I imagine that the biggest issue will arise because of the different weights of non-standard pistons and connecting-rods. Are high-performance pistons lighter as they always look cutaway in the images I see? That would imply that the crankshaft weight needs drilling away.
Can you start to approach this by simple calculation of the differences in weight of the original versus the replacement components?
To me it looks like Fiat are more likely to have balanced individual components rather than get involved with dynamic balancing.
It all seems very challenging to me.

Hi John, good to hear that things are progressing well. I fitted a '110' (500) flywheel to my tuned 652cc engine--it is 2kg lighter than a 126 flywheel. I personally wouldn't go any lighter for a road-going engine. I may have mentioned this site to you before, but look at "Blitz Racing--Fiat 126", and then "Fiat tuning--club 126uk". This site (by Blitz Racing, who accumulated a lot of expertise in tuning 126 engines) has a lot of interesting info in it. if you do get the crank balanced, you will have to supply your engineering shop with the complete crank 'line'--from pulley/filter assembly through to flywheel. Pistons and rods are fairly easy to balance and match--just tedious and use of your wife's cooking scales!

Hi Tom, I have been to the Blitz site and absorbed all their information! Do you know what balance factor your engine guy used?

Peter, that's some good information about the flywheel, I will check mine and see how it lines up with my marks.

Andrew, all of the parts I am using are the original Polish 650 engine components. Only parts being replaced in the rotating mass are bearings, rings and connecting rod bolts, and potentially using the stock 500 flywheel.

John

Hi John; the honest answer is that my engine builder is---yours truly! All I did with this engine was to build up very carefully, but I did not get the crank assembly balanced. My dynamometer guy was very impressed with its smoothness at 5,500rpm (80mph).

Hi John, I have just rebuilt my engine with 80.5mm Italian pistons (= 712.5cc) without any balancing, at the moment I am being 'sympathetic' when driving to allow for some running in, but I can say that when on the move it does seem to get smoother with more revs. I am limiting everything at the moment though.
Just a thought - try having a word with 'Torsten Hanenkamp' of "Il Motore" (web address the same) in Germany, he builds an 80bhp engine running at 8000rpm for racing and should be a source of info Let us know what balance figure you arrive at please.

Ian.

the hobbler said:

Hi John; the honest answer is that my engine builder is---yours truly! All I did with this engine was to build up very carefully, but I did not get the crank assembly balanced. My dynamometer guy was very impressed with its smoothness at 5,500rpm (80mph).

Click to expand...

Hi Tom, well what you did was my original plan. I think I will do the same as you. I am (as Peter pointed out) very meticulous in everything automotive. My plan is to match the pistons, and match and balance the rods as you did, and do all of this myself. I even have my own scale, so no need to steal the wife's!

John

I though I would resurrect this rather than start yet another thread.:bang:

A related discussion about the BIS crankshaft went slighty off-course and the intrigue caused me to look into this subject in a general way.
I can't find any definitive information as to how to calculate the amount of weight that needs to be added or removed from components (crankshaft?) in order to achieve the optimal dynamic balance. There seems to be a "rule-of-thumb" proportional method which gives a starting point but it is always explained that each engine type or configuration will need adustments to this.

So although it seems clear that balancing is desirable and can be achieved, I am sceptical about how advisable it is to proceed with a process that appears to be "science" and yet turns out to be more of an "art".

I have read a few anecdotal posts on this Forum which indicate that in practise, most of our engines get smoother as they approach optimal RPM and I can vouch the same for my own. So I think that this supports what most of us do when building our engines in that in the first instance it's worth assembling an engine "raw" and observing how well it runs "unbalanced".

In the event of a rebuilt engine showing signs of being seriously out of balance, then for anyone who is keen and adept at engine assembly, the prospect of dismantling in order to sort that out should not be too daunting.

As far as I am aware, in motorsport, engines are regularly dismantled and rebuilt and whilst I am possibly a bit naive due to lack of knowledge, at the lower levels of the sport I am dubious that regular re-balancing is done as a standard procedure.

It would be brilliant if anyone could post a link to a video of their own, running engine which has benefitted from being balanced. I know it is difficult to convey an impression of smoothness or roughness in an engine but it would help to see what we are aiming for.

Build your own engine balancer:


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