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