I got a gmc 302 with 6 bolt crank. Is this the same bolt pattern as the early chevy 6's with 6 bolts or are they different? Thanks

awag,
just aquired a 302 with a 6 bolt crank myself. i'd assumed it was the same as chevy but now that you ask i'm going outside right now to find out. hang tight.
robert

awag,
they are indeed different. the gmc pattern is on a 4" bolt circle with 1/2" bolts & the bolt holes are not spaced evenly around the circle. i'm guessing this precludes the use of dowel pin alignment.
the chevy is on aproximately a 3&1/4" bolt circle with 7/16 bolts & dowel pins.
that's leaving me curious as to whether any other engines use the same bolt pattern. i know 455 pontiac used 1/2" flywheel bolts. anyone know anymore about this please chime in.
congrats on you're 302 i was sure tickled to get mine a few weeks ago.
robert

awag, i also checked the flywheel center hole & hub register on the end of the crank & the gmc is much larger, 2&11/16-3/4". i've been looking at some later model pontiac v8 flywheels on ebay & they have a 2&3/4 center hole & larger bolt patternwith no dowel pin holes. wonder if they'd fit? we're not getting any responses here i wonder this should be moved to the engine forum?
robert

Thanks for the replies Robert I have been away from the computer.

I checked my Clymer and California Bill books and even the "55 California Bill book only tells about drilling a '39 Chevy truck flywheel for 8 bolts on a 4 bolt GMC crank. It's time for another Leo Santucci to step up and enlighten us on newer possabalities for our old favorites. There must be newer choices for rods,seals,pistons, valves, rockers,flywheels,etc,etc,etc. Come on guys help us out,Please

beater,
i agree. i like seeing & hereing about the later sixes. ( or just about anything mechanical for that matter ) but what i've always worked on & run in my cars & trucks is the 62 & earlier engines. doc fromahders' series on webrodder.com is a good start. wish he'd write a book.
robert.

There's a guy over at oldgmctrucks.com who has a bunch of GMC flywheels, I got a 6 bolt flywheel from him for my 302. I have an extra flywheel but its for a 13" borg and beck clutch out of a GMC 450 with a clark transmission.

Guys, I'm in the process of building a 321 inch GMC for a 50's style dragster (HA/GR) and have experienced ALL the pitfalls of building a Jimmy.
I've built a complete aluminum roller rocker assembly with 1.85:1 rocker arms. I'm also changing to 1.1" longer rods to get rid of the stock, long, heavy pistons. The head (#983) has been ported, and increased the intake flow from 170's cfm to 250's cfm. I don't know how to post pictures here but will send some if you want to email me. (Goldenri@aol.com)

Ron

Dear AWAG;

Check with Patrick's in AZ.
I'm sure he's 'run into' this and has the answer etc.

Good luck.

Ron,
What rods did you use that were 1.1" longer? I did some checking and Reo/White gold Comet OH-200
9only) seem to be the same size on the mains, but I didn't follow up on them.

ED,
The rods are from a 1950's, 308 Hudson. You have to hone the caps for the slightly bigger SPS rod bolts, narrow the big end to fit the GMC crank, resize the big end, balance the rods and rebush the small end to fit the wrist pins. Then you turn the crank down to the Hudson bearing size and use a certain Chrysler flathead rod bearing.

Unless your going to race this isn't worth the time, trouble amd expense. By the way, what are you going to do with the engine?

Ron

Ed, I haven't measured the rod length in my 200s but I suspect that what you really want is the later 170 or 186 which had shorter strokes (4.125 and 4.25, respectively). All the later Reo/ White eventually got the same rod and main bearing sizes. The blocks were the same. I've got a spare 200 and will try to find some time to measure the rod length.

Ron, how are you coming on the GMC? I have the complete short block done and work on the head has begun. Traded the Garrett GTP-38 turbo for a Holset HSSuper-40.

Gary,
The rods (Hudson) are finally being built, the roller rockers are finished, and I actually think the engine will come to life this winter. The rods have been the big hold-up (as you know).

What are you doing to your cylinder head? Have you had it on the flow bench? I'm looking forward to reading about the Stude when you take it to B'Ville.

Keep me up to date on your progress.

Ron