[poison heart]

I have a few flywheels laying around and I thought one was for a 55-57 and the other was a 58-60. I looked at the date code on both and they were both 57 which would be both 58s. They're the same diameter and theyre both 176 teeth which would be correct for 58-60 (and maybe up to 63????). One has the big hole in the middle for the crank and the other has a small hole in the middle where the pilot bearing goes. Then I noticed that the one with the small hole for the bearing was cast GMC and I remembered the flywheel I put in my 59 was a GMC flywheel and had the bearing hole.

My question is, do all 59 cranks have the hole for the pilot bearing or just the sticks? Could I have used either flywheel on my 59?

[Jack Gifford]

Originally Posted by poison heart

... the other has a small hole in the middle where the pilot bearing goes...
My question is, do all 59 cranks have the hole for the pilot bearing...

Well... I've been sick, and thought I was recovering, but it seems that one of us hasn't got our head on straight. I've never seen a Pontiac flywheel that didn't have a large hole to fit over the "register" diameter on the back of the crankshaft. So how could any flywheel have a "small hole in the middle"?

Regarding whether all '59 cranks were bored for a pilot bearing, I THINK the answer is yes, but could be remembering wrong.

[poison heart]

Well I know the GMC flywheels had the small hole since I know the one in my star chief is a GMC and this one is cast "GMC" also.

Also, the GMC bellhousings are completely different than the Pontiacs. So Technically it's not a Pontiac flywheel, it's a GMC

Here is a picture

It is cast X XX 7 and has the cool "GMC" logo cast into it on the back

Also, note the different pressure plate bolt pattern on it. I had the flywheel in my starchief redrilled for the regular pontiac style pressure plate.

[Bill Hanlon]

Comparing 1960 Pontiac Master Parts Catalog to 1958 GMC Master Parts Book (sorry, my 1960 GMC book is at home):

Flywheels
523861 55-57 Pontiac all standard trans
531439 58-60 Pontiac all standard trans
2331204 55.2-59 GMC models 300 and below with standard trans
2331217 55.2-59 GMC models 350 and above with standard trans

Crankshafts

Both books list the same crankshaft for 55.

For 56, GMC book shows 521601 while Pontiac book shows 521602, 1 higher. Probably the same crank with one book mistakenly listing the casting number instead of the part number. I've seen this mistake before in GM parts books.

Pontiac book shows 530048 crank for 57 while GMC shows that part number in 58. I'm pretty sure the GMC book is correct here GMC shows 57 crank 523206. No reason that I can think of that they shouldn't be the same.

Pontiac book shows 528527 crank for 58, GMC shows 530048. It makes sense that 58 cranks are different because GMC only bored the 58 block to 3.875" instead of Pontiac's 4.0625", which would require different balancing.

[badlefthand]

Oldsmobile had a 176 tooth flywheel,14 1/2" 57-59 and I have read that some trucks had the olds engine.That one looks like a ford clutch fit to me

[poison heart]

Interesting, thanks Bill. I'll get the numbers off of them and see what I can come up with

Well I know pontiacs were 176 tooth also. I'm thinking these are probably 14.5" also since I have a 12" clutch in my star chief.

I've never heard that trucks had the Olds engine. Can someone verify this?

[Bill Hanlon]

Larger (bigger than 500 series) gasoline V8 GMC trucks from 55.2 thru 59 used Olds based engines. I think it was 324 inch in 55/56 and 370 inch (Olds called it 371) in 57 and later.

[poison heart]

Interesting, thanks again Bill

[Jack Gifford]

I guess I'm looking at stuff I'd never seen before. How does the GMC flywheel register onto the crank- does it have a shallow counterbore on the front? And does the GMC crankshaft have a much shorter register-diameter than a Pontiac crank?

[poison heart]

IDK but the GMC flywheel fits perfect on the stock Pontiac crank. That's what's in mine right now

I think the engine side of the flywheel has an indention for the crank just like how the pontiac flywheels have the big holes for the crank register.

I'll take a picture of the back of mine.

Jack, don't you have a Pontiac powered GMC? Is it not a 4 speed manual?

[Bill Hanlon]

Jack's GMC uses a 55/56 Pontiac slant-pan dual range Hydra-Matic.

[poison heart]

Ah I see

[poison heart]

Jack, here are some photos I took today. Sorry for the quality but my phone was all I had.


The one on the right is the GMC, the one on the left is the Pontiac. I did not find a part number on the pontiac flywheel.


The crank register holes appear to be different sizes but I measured them and they are both right over 2.5" It must be an optical illusion.


Part # on the GMC flywheel


Date code on the GMC flywheel


Can't really tell but thats the fancy "GMC" cast

And surprisingly enough, a 62 crank will fit the GMC flywheel but will not fit the Pontiac one. The 62 crank has a weird shaped flange and does not fit the circular indention in the back of the flywheel. The GMC flywheel does not have an indention so it fits fine.

[poison heart]

Also note that the GMC part number does not match any that Bill posted, but it only one number off the 350 and above GMC. HHMMMMM

I wish I had the part number on the flywheel in my car right now just out of curiosity. I know it's a GMC flywheel though as is the bellhousing. I think they're from a '59 since the heads and intake were date coded 59.

[poison heart]

And another thing, I should have taken a picture of these next to a 62 flywheel just for reference.

Does anyone have pictures of a 58-60 Pontiac flywheel for reference?

[Bill Hanlon]

Originally Posted by poison heart

Also note that the GMC part number does not match any that Bill posted, but it only one number off the 350 and above GMC.

2331218 is the casting number for part number 2331217. This is for 350 series and up trucks. It uses a 12" clutch. Smaller trucks used an 11" clutch.

[poison heart]

OH ok, why wouldn't they make the part numbers and casting numbers match? Wouldn't that be easier?

The one in my 59 uses a 12" clutch also. I guess it's probably the same exact flywheel then.

[Jack Gifford]

Thanks for all those pictures. That's more than I ever knew about GMC flywheels!
Although my '59 came from the factory with a 336 V8 and 4-speed granny-low manual transmission, I never saw all the pieces. Some owner before me had swapped in (cobbled in, actually) a '72 350 engine/bellhousing/flywheel against the 4-speed box. And, as Bill Hanlon noted, I'm putting it back together with a Hydramatic.

[poison heart]

I see.

There's a local guy right now trying to sell a 57 GMC with a Pontiac motor and a 4 speed. Wants $600 for the motor and tranny. I wish I could pull it off but not for $600


Pretty unique to see a 4 barrel early pontiac especially in a truck.


I don't know where I got this but that's a GMC bellhousing and flywheel. Notice the bellhousing is completely different and has the mounts on the bottom

[Rocky 389]

Poise.....that one GMC flywheel with the small center register hole is weird! Are you sure it isn't an old pilot bearing [or bushing] siezed into the hole?
I'm using a 57 GMC flywheel on my 370 but it looks like the other one [conventional] in your photo...and it uses an 11 inch clutch, "Long" style, meaning it has even spacing between the pressure plate bolts and the pressure plate has 3 fingers