VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1929-1932 Engine balancing

So I am having a reputable machine shop here in Vancouver WA work on a replacement engine for my 29 coupe. The replacement engine has new pistons, newly babbited connecting rods, and newly babbited main bearings. Shims on all of them appear to be about .010". I asked him to check out the balance on the crank and all the rod/piston assemblies. Funny thing is even though they all looked good there was 40.5 grams difference between the heavy and the light. 1.5 oz that's quite a bit. So he separated the pistons from the rods and swapped pistons onto different rods. That got it down to a 13 gram difference, getting closer. He said even the piston pins were different weights. I would have thought since everything was new and from the same manufacturer it would have been close. Good thing I had him check it out. The cam had a new gear on it but the lobes had not been ground. Since I had the lifters from the original engine resurfaced I am having the cam ground too. They said it would be done in about a week and cost $99.00.

I went through the same thing recently with my 31’ engine. My shop trimmed the ends of the piston pins to lighten up the small end. The issue is rods are all different through the years with they way they were made. Just look at the web of the rods and you’ll see differences. The pistons are cast and are not close in weight either. The new drop cast aluminum pistons are no better either. The new set I bought were way off.

When I did my 38 216 it was the same. I literally had 40ish rods side by side from lightest to heaviest labelled with the weights in order to find the 6 closest and then had the machinist work with those 6. The differences were huge. Even the closest 6 varied considerably and had to have some areas ground to close the weight gap.

They had no intention of high RPM when these were built....