The Stovebolt Site Forums The Shop Area Engine Shop GMC flywheel bolt pattern

Early GMC 248 and 270 crankshafts have 4 bolts (not 6) that attach the flywheel. Has anyone attempted to re-drill the flange for the 6 bolt pattern? Yes, I'm aware that the ignition timing mark would have to be altered due to different clocking of the flywheel in relation to the crankshaft! I'm bidding on a couple of GMC cranks on Ebay, both with 4 bolt flanges.
Jerry

I have a 4 bolt crank in a 1940 GMC with a 6 bolt flywheel. 2 holes will line up and had to re-drill and tap the other 4. You will have to find top dead center for your timing mark. This gave me a 12 volt flywheel and starter. Have 4K miles with no problems.
Joe

Thanks! I've purchased one 4 bolt crank, and have an offer in process on a second one. I'll be increasing the stroke on both of them by 3/8" by offset grinding the rod journals and using a smaller rod bearing and a different piston. If I can find some steel core cams, I believe it will be possible to run roller lifters.
Jerry

Jerry,
What rods will work with that combination?

Ed

Original GMC rods- - - -I just put permanently installed split bushings in the big ends, and use a smaller diameter rod bearing. A Federal-Mogul 9320 CP rod bearing for a Continental engine allows me to offset grind the crank and get 3/8" extra stroke.
Jerry

Great idea!

Ed

That same rod bearing fits several truck and agricultural engines- - - -Case, Mack, Buda, etc. It's about 1 15/16" ID, compared to the GMC bearing which is 2 5/16". That allows for a 3/16" offset on the rod journal, allowing a 3/8" stroke increase. The bearing is almost exactly the same width as the GMC bearing.
Jerry

GMC-IS-Me, I have a question about your drilling and tapping your crank to flywheel. I am repowering my 1937 Chevy 1/2 ton truck and the 216 engine specs on engine show that is has a 4 bolt pattern ( 1937 216)to the flywheel and the donner engine is a 1956 235 with a 6 bolt crank shaft bolt pattern. In order to use the original 3 speed - torque tube differential I need to use the 216 flywheel and bellhousing. Can you explain the pain - process you went through to drill yours? in my case I would be drilling the flywheel for four bolts.

Thanks

RonG

RonG,

I know for a fact that a 47- 55.1 flywheel and bell housing will fit on a 1958 Chevy 235, so you should be able to bolt on a Chevy flywheel, either 6V or later 12V ring gear on that 6 bolt crank. The only real difference is the number of teeth on the ring gear to match up with the 6V or 12V starter. HERE is some info on flywheels and starters.

I would think it would be easier to use a 6 bolt flywheel which should be readily available rather than redrill your older 4 bolt flywheel.

In the link it says when measuring the flywheel teeth to measure 9 teeth i think it should be 10 teeth the picture also shows 10 teeth???
Jay D.

it says "measure from the top of any tooth to the top of a tooth that is nine teeth away". The one you start with and nine more teeth.

klhansen, jat d, and Bill Hanlon,

Thanks, that's great information and encouragement that I am not looking at a disaster. I have disassembled the 216 which was quite the task. As you know unlike modern engines the clutch and flywheel need to come off to get to the Bellhousing bolts and my 216 was seized. After pulling the head and pistons I was finally able to turn it to get to the clutch bolts. However, a note in the 1937 Chevy 1/2 ton truck shop manual I bought from Chev's of the 40's has this note on page 146. It states: "NOTE- On 1/2 ton trucks it will be necessary to block the clutch levers with 1/32" flat washers so that the pressure plate assembly will clear the Clutch housing." But, it doesn't go into detail on how or where to put the washers.... This is a 1937 Chevy 1/2 Ton Truck. I ended up having to remove the clutch pressure plate "release sleeve" retained by three springs to get enough clearance to remove it.

I will be pulling the 1956 235 bell housing next to see what my next steps are. The 216 has a 139 tooth flywheel designed for a 6 volt starter and the 235 probably I believe a 163 or so flywheel with a 12 volt starter and has more modern flex clutch and throw-out bearing, unlike the carbon throw-out bearing and spring pack pressure plate. I'm contemplating using the 163 tooth flywheel, modern clutch, just need to mock it up in the old bell housing to see if the clutch fork with work, or if I need to consider a custom fork. Btw, the 3 speed transmission is a different bolt pattern then the 1956 bell housing and the Brake master cylinder and clutch pedals and safety break lever mounts under the floor to the side of the transmission. I want to try and keep that.

Thoughts?

Thanks

RonG

The 12 volt flywheel ring gear has 168 teeth, not 163. There is also a different starter pinion gear, which has the same 9 teeth as the 6 volt starter, but the tooth spacing is different.
Jerry

Hotrod,

Thanks for pointing that out, I've got the 12 volt starter for the 235. I'll mock it up to understand if it will work in the 216 bell housing.

Thanks,

RonG