Trying to understand the theory aspect of engine balancing is driving me nuts.
I first assumed the weight of the piston,rings,wrist-pin, rod and bearings was supposed to be offset by the counterweights.
Then I remembered not all journals have counterweights.
Hummm...
My machinest told me the big end of the rod and the rod bearing are considered rotating mass. The small end of the rod and everything else on that end was considered a recipocating mass.

When I ask him how do you define what point the big-end and the small-end divide, he states the rod is supported by one end and the other end rests on scales. Then rod is turned around and weighed again. The weight of the "big-end" is simulated with special weights and connected to the journal. Crank is then spun to balance it.

I'm assuming at this point that the recipocating mass componants would be weight matched aginst each other. (I.E. all the pistons, wrist pins, rings, and rod "small-ends" weigh the same)

When clearancing my rod to clear the cam, I gound off a little meat from the rod bolt (head) area.
Cyls 1,2,5 and 6 all got ground.
I wieghed all the rods afterwards, and as I expected the ground rods were a little lighter. I ground the heavy rods to weigh the same as the lightest rod. The top of the rods have a raised rectangular portion. That was the area that I gound on the reduce the rod weights. (The heaviest rod was 1 lb 4.95 ozs.
The lightest was 1 lb 4.80 ozs. They all weigh 1lb 4.80 ozs now)

Another guy at the machine shop said the raised rectangular portion on the rod cap was what I should have ground to match the rod weights.

Another guy from work (with questionable experience) said that the rods and pistons were conencted to the crank and the whole assy was spun.
Surely that's not right. Seems like the piston skirts would bang aginst the rods and destroy themselves.

I was fairly confident I hadn't compromised my balance job too badly, but now I'm beginning to feel uneasy abt the whole thing..

 

Your machinist is right...

Think about what's going on inside the motor. The big end of the rod is going around in circles. Rotating. The little end is going up & down, but isn't rotating (around the crank that is...). Reciprocating.

His description of how to use a scale is exactly normal. That's how everybody does that. The best way I've seen of doing it uses a thing like a lawn mower blade balancer at each end, with some kind of distance gauge that they set to the rod length; that way every rod goes onto the scale exactly the same.

When you ground on the rod bolt head area, you lightened the big end (rotating), so you need to match all the other big-end weights to that one. NOT the small end, which is reciprocating; you didn't change that by clearancing. So you should have ground on the cap. If you ground the small end, you screwed up.

OK, so now we have all the rods the same, and we know their weights... what then? Somehow the crank has to be matched to the big-end weight. Different shops handle this in different ways. Keep in mind that the actual location of the weight is as important to the "moment of inertia" (the measurement you're actually balancing, not the weight proper) as the weight is. Think about a large thin flywheel: if you had a 1 lb. weight on one side a foot from the center, you could balance it with a 1 lb. weight 1 ft from the center exactly opposite, or a ½ lb. weight 2 ' from the center, or a 2 lb. weight ½' from the center, or 2 ¾ lb. weights at some combination of angle and distance from the center, etc. Most of them have weights that they attach to the crank journals that have a movable part that they have previously calibrated so that they know that if it's in one place it has the same moment of inertia (balance) as a 420 gram big-end, if it's in another it has the same moment of inertia as a 421 gram big-end, etc.; they preset these to the known weight of the rods. Then they spin the crank in a fixture which holds it on rubber bands or springs, such that if it's out of balance, it is easy to see; then they adjust the weights until the crank balances, and see how much adjustment of each one it takes to get the best balance; then they drill the counterweights or the rod journal ends to take out that much metal; than they put the weights back to the measured big-end weight, and check it again. It usually only takes 2 or 3 times of this for a good machinist to get the crank to run smooth.

If you change the big end weight by grinding, you need to get your crank re-balanced to the new big-end weight, period. No way around that. For minimum cash outlay you should wait on getting it balanced until after all your clearancing is done; or, avoid grinding on the rods at all for clearance, and grind the block instead, or use a small base-circle cam, as the case may be.

 

Thx RB!

My machinest recommended a place locally. They charge $175-$200 for balancing.
What a pain this eng has been..

I recently over-torqued the rod bolts due to some bad info I recieved. Ordered and recieved a new set of ARP bolts.
Problem is the rod bolt heads, that have to come out, have already been ground for clearance.

Looks like I'll have to get the old bolts pressed out, new bolts pressed in, install the rods one-at-a-time, and re-grind for clearance. Then carry the whole assy to the balanceing shop.

I'm pretty sure my cam (LPE 74211) is not avaliable in a "small base circle" version; and I really want to use that cam..

Since I'm not real crazy abt using weakened rods, especially in a stroker, I'm considering a completely diff set of rods. Some of those that don't use nuts. I think they'll clear the cam without any mods. Since I'll have to get the assy re-balanced anyway, I'm thinking it wouldn't be that much more expensive in the long run, and it would be a LOT stronger..

Any recommendations for aftermarket nut-less rods?? Manely maybe??

 

eagle is a high quality cheap rod, or cost effective rod, i like lunati parts too