VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1937-1945 1938 216 Rod / Piston Balancing

Yep, still playing with rods. I have over 40 now and finally spent a few hours doing some serious weighing and head-scratching.

Surprised to find that rod bolts can vary by 7 grams each and that even the nuts can vary by 3 grams.

Gene has confirmed that it is OK to mix forging numbers so I've matched up my best 6 rods that are 0.010 over and have fresh babbit.

The weights from lightest to heaviest are 826,828,828,834,840,866 grams. Average overall weight is 837grams. I can bring up the low weight rods to the average by using the heaviest bolts and nuts I have.

Obviously the 866 is the one forging # that is different than the other 5. It has a fair amount of rough forged areas that could easily be cleaned up and drop some weight.

I figure by weighing all nuts, bolts and dippers (I have over 40 sets), and matching accordingly, that I should be able to get all of these rods "fairly" close in weight, combined with a bit of metal cleanup on the heaviest rods.

My questions:

1) How much difference in weight will cause a a problem? How close together do these weights really need to be?

2) I haven't weighed the cast iron pistons yet, but in the event that I cannot get the rods close enough in weight, would it be acceptible to try to match the complete rod/piston assemblies instead?
I weighed 2 sets of factory rods/pistons and there was almost a 15% weight difference between lightest and heaviest so obviously some variation is ok but how much?

3) My machinist said he could grind some weight off of the rod bottom 2 curved "ribs" that are on either side of the dipper. These appear to be for weight/balance so I assume some material removal is OK?

Any thoughts?
Thanks

There were two diffenent weight pistons used in the 1937-40 engines (also the later).The passenger car pistons weighed 25.5 ounces and the truck (above 1/2 ton) 27 Oz. This is the US cast iron pistons. The lighter being more desireable for higher engine speeds and the trucks had a thicker, stronger top. You might consider matching the lighter pistons with the heaiver rods, or making a piston lighter by triming down the skirt length slightly. I am not sure as to how many grams is acceptable. There are so many other factors that enter in such as the flywheel and pressure plate, harmonic balancer and the crankshaft. These would all need to be perfect.
When I replaced the rods for my '34 and '39 engines I juse weighed them on a baby scale and paired the ligh and heavy on a rod journal that was of the same level. Such as both the pistons would be at the top and one time. My engines are all smooth and have taken alot of high reving with no problems. A professional balancing job would have been more desireable I am sure.
I would not touch the curved ribs on the caps because that could weaken them.
My thoughts :)

I have seen the engine builder at work balancing pistons and rods.
He weighed both ends of each rod and adjusted them all to the lightest. he then weighed each piston directly under each pin boss as well as total and made the lightest the same on each end of the pin then matched all pistons to that. A very time consuming process.
Tony

He also went to .01 of a gram variation.

Performance engines usually try to balance all the reciprocating parts to 1/2 gram which is about the weight of a dollar bill cut in half. The higher the RPM the engine rev's the more important balance becomes. The Chevy cast iron engines being a lot lower RPM running are more forgiving. The closer you can keep the individual parts to there mates the better.

Thanks for the info. I'd still like to find some kind of tolerance for these weights.

In searching race balancing details on the web, I keep finding rod balancing for racing to within 10-15 grams on rods obviously weighing less than these in much higher RPM applications. I've also found lots of notes of modern factory rods (various vendors) being out as much as 40grams between the heaviest to lightest. This is easing my concern slightly since I should be able to get within 10grams on a much heavier rod than the modern rods. I even found some aircraft specs that still had some pretty significant variation.

I've weighed each end separately though the big end with the babbit pretty much dwarfs any tolerance the small end may have. I'm probably being too picky but want to get the best possible match before my machinist matches them...

My information is from having been an Engineer at GM building race & high speed performance engines (built for GM Powertrain Cadillac Northstar engines). I think you will find a low speed engine, as the 207 cu. in 6 cyl. is, won't be anywhere near the 1/2 gram. Chevrolet HP max. peak is only about 3000+ rpm. Chevrolet and other repair (NAPA, Motors, Chilton, Thompson) manuals don't mention weights only fits and clearances. Production and replacement Chevrolet parts tolerances were close enough on weight.

Thanks 35,

Couldn't imagine the patience it would take to get any engine accurate down to 1/2 gram...a few hours of crude weighing was enough for me...