GMC Medium Duty Flywheel Size SAE 2

[tommr]

Anybody know if the medium duty 401M flywheel is the same diameter as the light duty 305 flywheel? same tooth count? I have an SAE 2 flywheel housing. thanks

[FetchMeAPepsi]

I'm pretty sure the flywheels are different from 401 to 305. I think I remember someone saying the 401 was internally balanced or something, and the 305 was not.

[bigblockv6]

The 401 and 305 both have internal balance crankshafts, the difference is bigger truck applications have larger diameter flywheels yet 305, 351 and 401 engines share interchangeability with bottom end components and flywheels.

[jrmunn]

I am trying to follow this to know what fits on my own applications (64 GMC 1500 pickup with 305E, 66 GMC 5500 truck with 351C, and a rusted and stuck GMC 401M out of a pickup under a tarp). As I read the messages above, it sounds like the 305E flywheel will also bolt onto the 351 and 401 engines, but the 351 in my truck probably has a larger diameter flywheel?

[tommr]

just to clarify I was trying to ascertain as to whether the SAE 2 apps use the same diameter flywheel as the light duty apps. thanks .

[bigblockv6]

Checking my parts books, the best that I can come up with without physically seeing your truck is that pickup truck and medium duty applications up to 1974 6000 series all use the same ring gear. That would tell me the flywheel diameter is the same for the 351C but there are 12 and 13 inch clutches so the flywheels are drilled differently.

[tommr]

interesting. I appreciate you looking that up. do you think all medium duty motors used an SAE 2 bellhousing or do you think some used conventional bell housings? the replacement ring gears are showing slightly under 16 inches. my flywheel measured slightly more than 16 inches. thanks.

[bigblockv6]

Which flywheel measured over 16 inches? Some medium duty trucks do have the larger diameter flywheel and bell housing, that can depend on gvw or tractor application versus flat bed or box van. I only found that true for the late 6500 series which is also medium duty. At one time I did have a 478 flywheel out of a 1963 6500 series with the bigger bell housing yet the flywheel was exactly the same diameter as the 305E flywheel in my pickup truck, unfortunately I couldn't use it because the side facing the crankshaft had a huge crack so that was a grenade waiting to happen and ended up with a custom built flywheel. So as you can see anything is possible.

[tommr]

This is a bus application. was a repower by the bus builder using mid 60s GMC 401M from a school bus. im going to take this apart again and replace the pot style flywheel with a flat style and a stamped steel pull style pressure plate. im thinking the SAE2 flywheels are slightly larger in diameter than the other medium duty and light duty apps. its possible this flywheel was a conversion flywheel and not even a gmc piece. been thinking of having a flywheel built but saw some used ones that I thought might work. thanks.

[bigblockv6]

The mid 60's 401M was actually in the Heavy Duty application category, 7500-8500 series. That pan type flywheel looks like a real heavy piece. Anyway use what you feel is best for your application, I spent $1100 on a custom flywheel for my 478M and that was quite a few years back.

[tommr]

Yes I want to lose this flywheel.. I don't need the extra rotating mass. got prices under $1000 18 months ago for a copy of this pot style flywheel. swapping out the non synchro Spicer 4 spd for a 5 speed OD Spicer. wasn't thinking heavy duty applications. thanks.

[Funky61]

Here is the flywheel that came on the 401M;
16" across tooth to tooth diameter.
Not as concave as your tommr and used a 13" clutch.
Unsure of what size truck it was off of; Older Gent said a tractor trailer

[tommr]

thanks for taking the time to post those flywheel pics. I think V6 has it right that I need a GMC 7500/8500 flywheel. thanks much.