Flywheel change = rebalancing?

Jim, I think you have put it quite succinctly. I have an observation though, what about new pistons? Most likely, a vast majority is using new technology pistons, i.e. J&E, etc.
that weight a chunk less. What will happen to the rotating assy., balance wise, when you are altering an important factor into the equation?
Regards, Alberto

 

Alberto - by the way - were you at the San Diego Italian show this past weekend?

Pistons, wrist pins, and connection rods should all be brought to equal weight. Once this is accomplished, then the symmetry of the firing order and connecting rod journal-offset cancels out any absolute weight difference from original factory piston/connecting rod. These are reciprocating, and one is going down as the other is going up, and one is going east as the other is going west, etc., hence, the equal masses of these components, albeit, different than factory, will compensate for the other as long as they are equal.

Jim S.

 

This thread has provided some very interesting discussion and some pictures of great hardware.

The subject of piston and rods weights has been raised and it seems that some confusion has crept in over the difference between a flat plane crank and a multi-plane crank like the Dino. On a flat plane crank like most four cylinder and the 308 V-8 crank, the rod journals and the piston/rod assemblies are offset by a corresponding weight 180 degrees opposite. The piston and rod weights need only be matched and they do not affect the rotational balance of the crank. On a multi-plane crank, things get more complicated because there is not a corresponding mass 180 degrees opposite. That is why these cranks are balanced with bob-weights bolted onto each rod journal. These bob-weights are adjusted to match a certain percentage of the combined piston/rod weight. Greater minds than I have figured out formulas to determine this percentage. The problem is that the mass of the piston/rod assembly is partially rotating mass and partially reciprocating mass. In this case, a change in piston weight does make a difference in the balancing process. The Dino engine is even more complicated because the rod journals are offset. I think the Buick V-6 is also like this.

Yes, I would expect that a brand new Dino front pulley would not have the holes drilled into the offset weight. I would expect that they would be manufactured with a safe margine of excess weight so that they could be drilled to match any crank that they would eventually be mated with. This would be much easier than having to add weight. The fact that various pulleys have differing amounts of drilling would reflect the differences in the crankshaft that they were coupled with.

I am on uncertain ground here, but I am pretty sure that I remember the Fiat Dino flywheel as having a significant offset lump as part of the raw casting. This was on the side of the flywheel that bolts to the crank. Does anyone have a Fiat Dino flywheel to look at? In any case, I know that I have seen flywheels in machine shops that do have this offset lump. It is substantial and it is designed to provide dynamic rotational balance when it is spun with the crankshaft.

I am having a little trouble with the assertion that these offset weights are there only to absorb some bending or harmonics when the engine is running. The concentric weight that is rubber bonded to the front pulley is there to damp torsional harmonics, but it is concentric and does not affect rotational balance. On a very practical level, when the crank is on the balance machine, I doubt that there is enough stress to cause any bending or other stresses. The process is simply to get the whole mass to be balanced as it spins. The offset weights are needed to obtain this balance.

I did have one Dino come to me that had a VERY pronounced and VERY irritating vibration in the 2200 RPM range. The rear view mirror would vibrate. A "balanced" flywheel and clutch assembly was put in this engine. I had to disassemble the engine, re-create the offset weight on the flywheel and re-balance the whole assembly. This cured the vibration. The car had been used this way for years and the vibration was only evident to the driver at that RPM. Oddly, it did seem to smooth out and was pretty good up to red line.

I do think it is good advice to not re-balance these engines in a normal rebuild. They are very good from the factory and the chances of making things worse are there. Just think of the difficulties faced by the guy with a roller bearing crank!!!

 

Received this from a well known dino engine builder in the UK
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Well that's about as good as it gets! Thanks invaluable stuff!

 

Tony

That is very valuable and very interesting!! "From the horse's mouth"

But.... I find it interesting that they consider the entire rod as rotating weight. It just seems to go against logic. If the wrist pin is reciprocating weight and the small end of the rod is around the wrist pin, how can it not be considered reciprocating also?

The practice that I am familiar with is to get separate weights for the big and small end of the rod. This is done by suspending one end of the rod from a wire with the other end resting on the gram scale. This gives two weights for each rod. Work is done to try to get each rod matched in total weight with each small and big end weight as close to equal as possible. In the bob-weight calculation, the big end weight is rotating and the small end weight is reciprocating.

If one thinks of the motion of the rod as the engine turns, it can be seen that the rod 's rotating motion at the crank gradually transforms to reciprocating motion at the piston.

 

The picture of the front pulley seems to say that it must be balanced attached to the engine.
But, the picture of the fly wheel looks like it can be balanced separately.
I will look at my front dampener tonight and get back to you.
I have been balancing engines since the '60's and own one. The next reply will be a definite one. Most of the replies, if not all of them are incorrect. I will give you a definite answer. Especially wrong, is the comment about heavier pistons etc. All v engines except v12's must be balanced w/bob weights mounted to the crank which duplicate the weights of the pistons, rings, pins, rod bearings and weight of the pin end of the rod and the crank end of the rod. Flywheel can be balanced separately but no one knows if that is the way the original factory engine was accomplished in that way.
clay

 

I knew that when I started to rebuild this engine that I would learn a lot- and I have!! Tony- Ron sent me that engineer note as well and I passed it onto Stewsf for his review as he had some opinions on how these engines should be balanced. However, I have not heard back. I am in no way an expert and don't do this for a living so I am learning here- the curve is steep!! I spoke to my machinist and sent him this entire thread (ex todays comments) along with the sheet that Ron provided to both of us. I asked him to review the material and then I asked that Jim Selevan follow up with him to decipher what the machinist did and why and to distill that for this discussion. This gets very specific and I wanted to make sure that the correct terminology was used- so I will leave it to the pros. I hope to have that info up here in the next few days or so. My engine only has the heads (not cams) on along with pistons & crank in. If I now decide to take it all back apart & balance it with the flywheel & pulley there is no easier time than now. Depending on what I hear I may not though. I very much enjoy doing this and since I have another dino to actually drive I am in no hurry, I certainly like learning about all of this & very much appreciate everyone's input & effort. Clay I look forward to your thoughts- Thanks

 

Yes, i know you had a copy as Ron and I talked about this for some time. I too am on a steep learning curve with this, my interest is one of information and knowledge. I dont do it for a living and listen with great interest to those that do.

All i can say is that Ron has rebuilt over 80+ V6 engines over the years and respect is input as well.

I just dont understand why anyone wouldnt rebalance on a rebuild, i reused my old torsional damper and couldn't beleive how far out it was. After witnessing that, i was converted!!!

 

Tony, when you say that the torsional damper was out of balance, are you referring to its "stand alone" balance, or when attached to the crankshaft?

My understanding is that if you spin the torsional damper by it self, it will be way out of balance. It is designed to be out of balance and thereby compensate for crankshaft dynamics.

Jim S.

 

It was on the balance rig.

 

Tony - please forgive my failure to understand. Was it on the balance rig by itself, or attached to the crankshaft?

Jim S.

 

The whole lot was set up on the machine and balanced as one unit.

 

V6's are not naturally balanced due to the odd number of cylinders per bank. With a 90 degree V8 or 60 degree V6 you put bob weights on the crank pins and spin the entire rotating assembly. For internally balanced cranks you can leave the damper and flywheel but it's better to spin the whole lot. The bob weights for the 60 degree V6 should be the rod big-end weight plus bearing shells plus 50% of: the small end, piston, pin, rings and a few grams for oil. Since the Dino motor is 65 degrees you only add 46,3% of the reciprocating parts. You first have to balance the rods end to end and to each other and the pistons, pins to each other. If they are not close, switch pins and pistons to get close to each other and then polish some steel off the inside of the hollow pins.

Or follow the advice given by the Ferrari agent

 

TonyL-

I received a copy of that exact memo when I was rebuilding my motor in '95. It was different then how I balanced V6's in the past so I called Bob Wallace to get his thoughts. His formula percentages were different. I ended up working backwards by adding bob weights to the factory crank and determined how they balanced the motor. I ended up with 762 grams total bob weight and the average balance factor came to 50.24% rather then the 53% I was given from various sources. I did do some corrections on the damper and flywheel.

 

I as I have already stated. The ONLY WAY to balance this flywheel off the crank is not doable. It can be done by matching it to an old flywheel which you know is balanced but the work and trouble is not worth it and it is almost impossible to accomplish accurately. The flywheel and front dampner must be mounted to the crank and spin balanced with bobweights attached.

Your engine is going to shake like hell.

clay

 

The original factory manual does not leave any doubt how to do it. Only question is what is the proper counterweight mentioned ? Image Unavailable, Please Login