The Stovebolt Site Forums The Shop Area Engine Shop GMC 270 Connecting Rod Length

Does anyone know the spec for connecting rod center-to-center distance on a GMC 270? This is an older engine with dome top pistons.

I've replaced the rod end bushing and still need to bore and hone them to size so I thought I might as well maker sure the C/L distance is right.
Rough measuring it looks to be a little under 7". Who knows if these rod have been resized at some point.

Mr. AI doesn't seem to know....gave me two numbers which were both wrong.

1951 and 1957 GMC Maintenance Manuals do not specify rod length.

All the GMC rods I've measured have been within a few thousandths of 7 inches. Reconditioning will shorten the rods slightly. There is a custom length big block Chevy V8 rod that is 7.1" long that can be used when changing the stroke on a GMC crankshaft that has a 2.2" rod journal diameter. It uses the same wrist pin diameter as a GMC 248/270/302. A 4" stroke 270 or 302 crank can be stroked to 4 1/8", and more if the rod big end is adapted to use the 2.1" 350 V8 rod bearings.

www.ebay.com/itm/137163248466?

Jerry

Just to back up Jerry's length info, a search through my older GMC Books of Parts (1940-46) shows all the GMC engines (up through the 302) used the same GM# 2135418 connecting rod. Using that info, I looked in the earlier GMC Shop Manuals I have (1940-45) and they show the stock length specifications as 6.998" - 7.003".

All my references after the 1945 Shop Manual stopped listing the rod length. It only listed large and small end dimensions and bearing/bushing info. Guess maybe they figured you didn't have to keep restating what hasn't changed.

The AERA connecting rod reconditioning manual I use also does not list a length on GMC six rods. When I ran the connecting rod reconditioning department of a big automotive machine shop in Nashville Tennessee back in the late 1990s, we always used 7" as the standard dimension for those rods. Same length as flathead Ford V8 rods, BTW!
Jerry

Thanks for the info. I suspected they were supposed to be 7" and your info backs that up. I'll have to measure my rods to see how close they are. If only a little off I can bore the small end bushings to correct the error.
If off too much I can either trade out the rod or bore them to get them as close as I can.
I doubt if a few thousandths out of spec would really matter much.

Unless you're building a blueprinted racing engine that's going to turn warp speed, a few thousandths of difference won't matter. I've always measured a set of rods to find the shortest one, and reconditioned all the others to that length. Then we would deck the block to get zero deck height, and balance the rods to a +/- 1/10 gram weight match. (A dollar bill weighs one gram). Then that $8K engine would last half a season on a 1/4 mile dirt track, cycling from 3K to 8K RPM twice a lap. Totally worn out (or blown up) in less than 500 miles!
Jerry