The Stovebolt Site Forums The Shop Area Engine Shop GMC experts- - - -got a question

I'm crunching numbers on altered bore and stroke GMC's with the idea of substituting newer, more available and less E$$$PENSIVE internal parts for rebuilding and modifications. MY SWAG measurements on a 248 I've got in the shop makes me believe the deck height from the crankshaft centerline to the top of the block is close to 11.250 inches. Am I somewhere in the ballpark with that measurement? I'm looking for a way to use big block Chevy H-beam connecting rods with a stroke increase, and pistons with the right compression height to get close to the top of the block. I believe I can increase the stroke of the 270/302 crankshafts by at least 1/4", maybe more. 228 and 248 cranks can also be stroked by the same amount, since the crankpin diameter is the same.
Jerry

Jerry - I haven't yet found a specific GMC manual to confirm, but a 2020 GMC engine conversation in the HAMB made reference to a 1955 Wayne Performance Parts catalog indicating the original 248 deck height is 11.25 inches.

Perhaps one of our GMC engine builders can provide more details/confirmation?

Ran across this on Inliners.org

Deck Height 11.156 228-236-248-256
Deck Height 11.250 256-270-302

I'm no GMC engine builder, but the numbers make sense except for the very rare 256 engine being shown with 2 different deck heights.

Given the 3/16" difference in stroke between the 228/248 and 270/302 engines and the fact that they all use the same rods, the deck height of the 228/248 would need to be 3/32" (0.09375") shorter than the 270/302.

Thanks, Bill! I'm tinkering with rod lengths and piston compression heights to see if more modern, less expensive parts might be used for building up GMCs like I can do with Chevy stovebolts. Offset grinding the crankshaft and using a different rod/piston combination might open up a lot of possibilities on these old engines.
Jerry

I have been messing with GMC engines for almost 70 years. Parts are getting really scarce. I suppose there isn't that much interest in GMC any more. Especially the 228 and the 246. I have two trucks that were born with a 236, but were retrofitted with a 248 and a 302. I have never seen a 256 in real life. I think it will be very hard to find something to use in a 228 and 248 1952 and older with the big crown pistons because the combustion chamber is so large. If I were to rebuild a 228 or 248 now, I would be afraid to bore it for fear of not finding pistons. I would just leave them standard. If the pistons have worn skirts, I would consider knurling them. That worked for us 60 years ago and it would still work. I am going on 82 years old. I don't know if I will ever build an engine again. And I have lots of NOS parts in case I ever want to build one.

I've found a piston/rod/stroke combination that looks like it will work on most of the GMC engines with up to a 1/4" increased stroke. It involves using a custom length H beam rod for a big block Chevy V8, one that's rated for over 1000 HP on drag race engines. Those rods are surprisingly affordable from some of the vendors on Ebay and the BBC rod bearings will allow for a pretty good increase in stroke by offset grinding the crankshaft. Pistons for Japanese industrial Diesel engines are included in the mix. The reciprocating assembly also ends up considerably lighter weight. The 256 engine was mostly used in Canadian military vehicles. I've rebuilt a grand total of one of them, and had to modify the pistons to accept different thickness rings.
Jerry

OK- - - -I just made some more accurate measurements. The compression height of my 248 piston is 2.345. Adding the 7" rod length and half the 3.813 stroke comes out to 11.252 inches. Rounding off for possible accumulating measurement errors, it looks like at least my example of a 248 has an 11.25 deck height. "Your mileage may vary!"
Jerry

Jerry, you'll need to be sure that your stroker engines don't have interference with things like the camshaft, like I found when I stuck a 235 crank in a 216. If they have a lot of interference with the cam, there's only so much you can whittle off it. Not sure how you can check that beforehand, as it would depend on the thickness of the big end of the rod, and the spacing of the cam from the center of the crank.

Since the material to change the stroke is coming off the bottom side of the crankshaft journal, the cam/rod relationship won't change much, if any. Also, I compare the dimensions of the rods I use against a stock stovebolt rod, and whittle on the rods if necessary. In most cases, the new rod has a slimmer contour than the original. I always static balance the rods to +/- 1/10 gram (a dollar bill weighs one gram) so any alteration of the rod contour gets compensated for during the balancing process.