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Can anyone out there tell me when or what year GMC changed from a 4 bolt flywheel to a 6 bolt flywheel and when did Chevy change from a 4 bolt to 6 bolt? And the difference between a 153 tooth flywheel and a 168 tooth flywheel.

I've got three different Chevrolet flywheels on hand and one on my truck and all of them are 6 bolt flywheels. At least one of them is a 139 tooth flywheel which is for use with a 6 volt starter. The others are 168 tooth flywheels and they are for use with 12 volt starters. The crank on my old 52 model 216 was also set up for a 6 bolt flywheel.

6 volt and 12 volt starters are kind of hard to tell apart when looking at the teeth, but they are definitely different.

GMC may be, and probably is, a different situation, especially the earlier years, 30's and 40's.

Joe

GMC went to 6 bolt beginning with the '54 248. They and '55 1st series flywheels are 139 tooth and use a 6V starter. If you put a later 248, 270 or 302 in a 6V truck you'll need to find one of these flywheels or convert to 12V because they have 168 tooth flywheels.

Never heard of a 153 tooth flywheel. Are you sure it's GM?

The 139 tooth ring gear will fit on a 168 tooth flywheel if you have a machine shop turn the flywheel down to fit the gear. However, the converse is not true.

Hi All,

This came up because my 270 uses a 4 bolt fly wheel and I was trying to find a larger clutch and fly wheel setup. My 270 has the 9 1/8 inch clutch.

I wanted an 11 inch clutch for towing. Then someone came up with a 4 bolt, 13 inch clutch and flywheel out of a school bus...weeeeeee, I bought it.

Then I found a 4 bolt, 11 inch clutch and fly wheel, which I also bought.

Then I found a 11 clutch and flywheel for my 248 which has a 6 bolt crank.

I only have one chevrolet 4 bolt flywheel. It came off a 1937 216 in a 1941 chevy 1/2 ton truck. Other than that all the Chevrolet flywheels I have are 6 bolt ranging from 1946-1954.

I'm a little lax on GMC info.

Jeff