Crankshaft Balancing Question

I would definately get it balanced, long job to take it out if you dont!!

Make sure you get all the crank components from clutch to & inc torsional damper

Tony

 

TonyL - I respectively disagree. If the crank and associated parts were balanced prior to the 1st undersize modification, I do not believe that at 7,000 RPM the cost justifies the benefit. It should be fine. Ten-thousandths was removed in a symmetrical fashion from each journal. I can't imagine that this will make a difference.

Jim S.

 

I am a little confused by the question. It sounds like the engine was balanced during the rebuild and the crank was not damaged by the "oil incident". It seems as if everything is ready to go with a set of new bearing shells. Certainly, just replacing the bearing shells will not warrant doing the balance again.

Just a few thoughts. I assume that you are talking about a 246 Ferrari engine. This crank is externally balanced and the job needs to be done with the front damper and flywheel in place. The person doing the balance will also need to have the formula giving the percentages of rod and piston weights to make up the proper bob-weights for the rod journals during the process.

 

Sowest - I read it as though the crank and associated parts were balanced prior to the oil failure, and then the crank was brought to first undersize as a consequence of the oil failure. In my first post I suggested that removing 10 thousandths should not present a concern (symmetric and applied to all journals), but I likely am in error in the 1st undersize is probably 30 or 60 thousandths (?). Nonetheless, I would not rebalance after simply bringing all journals to 1st undersize.

Jim S.

 

Hi Jim

I read the original post a couple of times and I am still not sure of the sequence of events. I do agree with you that, no matter the sequence of events, it is unlikely that the balance needs to be addressed. As time passes, I find that I am more and more reluctant to bring anything to local machine shops. I can't sleep at night wondering what will happen to these special and valuable parts.

 

Jim - I always respect your knowledge on these cars too, but my only concern was:-

1. Why not do it, while the engine is stripped, does a couple hundred bucks matter that much. Checking everything is OK prior to reassembly, doesnt hurt in my opinion as the force of any imbalance multiplies with speed2

2. The grinding has removed material (billet steel) and replaced it with larger bearing caps, these are not the same weight as the material removed. Therefore this MAY / COULD upset the balance. Anything outside the centre of gravity / axis line is affected by weight, regardless if we think it is removed in equal amounts around the circumference

" machining the crankshaft to the first under size rod and main bearing specifications "

I took it that the rod and main bearing had been machined too. This will affect the balance.

Regards

Tony

 

SoWest - I share your recurring nightmare.

Tony - your explanation does lend me pause...I was thinking of the material moved from the main bearings, but had not considered your observation about the difference in weight at the off-axis rod journals.

The weight of the material removed from each rod journal is rather small. In addition, the weight of each new large-end bearing shell is likely close to the same. Thus, the net change in balance is likely to be small, if detectable. Consider, for example, the weight of the bearing shell compared to the weight of the rod, bolts, wrist pin, and piston.

Nonetheless, your comment is well received, and understanding the obscessive compulsive nature of our group of friends here, many would send the crank off for a rebalance. I likely would not, for the reasons that SoWest suggested. There is more opportunity for things to go wrong.

But your observation based on the physics is sound.

Jim S.