The Stovebolt Site Forums The Shop Area Engine Shop The Connecting Rod are Supposed to Match ???

Good day to all

One of the things I am having the machine shop do to my 261 parts is perform a balance on the engine. (I may not have the term correct here) I am having them do this because while cruising up the highway on the former build the engine would vibrate considerably. Definitely not the smooth runner the 235 in my sedan is. I also have the theory that a balanced engine will last longer and be more efficient.

One thing the machine shop found on my former "Farmer John" overhaul is the that the connecting rods are mis-matched. They all have the same casting number (3836964) but varying "GM" numbers and letters. All but one of the rods is marked GM 30 either "A" or "B" and the one rod is marked GM 58 "A". I am betting the GM number and letter is a revision number. The GM 30 rods weigh-in from between 828 grams to 836 grams. The rod marked GM 58 A weighs-in heavier at 851 grams. I bet the GM58 rod is where my vibration was coming from.

I am now delving into areas that I haven't delved into before.

The Question:
Shouldn't all the rods (with the same casting number) have the same GM number and letter? Can the letter vary slightly so long as the GM numbers are the same?

Any insight here would be appreciated

Thanks in advance
Walt Williams
La Verkin, UT

The only thought I have is that when researching in a GM supplied Master Parts Catalog there are many variations of parts numbers to casting numbers and replacement numbers for ALL parts spread over years. I does seem someone replaced one of your rods at some point, but obviously you and your machinist know to weigh and balance each rod. Even just doing at home rebuilds, I weigh the piston and pins and rods and try to get them in close tolerance to each other. It's not ideal as a machine shop would do, but I'm comfortable with it.

the only problem I can see with your situation is there will be a problem getting rid of 20 some odd grams of material, there just ain't no where to take it from. Even 8 grams difference is quite a bit. (the value given for the remaining 5 rods extrapolated from your statement) I got mine to within +/- 2 grams, which still isn't all that great, but refer to the above statement. So yes I would say your vibration issue is a balance issue, or at least contributing to it. The number isn't as important as the weight. I had a full set of matching number rods, one was messed up and I had one extra that I had taken to the supposed premier shop in the county, (turns out the dad had a very good reputation, while the son who was now running the place was busy ruining the same reputation) to get cut for inserts where they promptly ground the sides away for some inexplicable reason, then declared it "too narrow" and only charged me $50 to ruin my piece. So I ended up sending 4 rods to Egge for machining, and of them one was no good, and of the remaining 3 one was a near exact match in weight.