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Jul 21st, 2025 | | | | Joined: Jun 2018 Posts: 52 Wrench Fetcher | | Wrench Fetcher Joined: Jun 2018 Posts: 52 | I have a '48 GMC 1/2 ton. It has a 4 SPD trans but the smaller 9" clutch assembly. I understand that someone may have swapped in the 4 spd at some point. Was the smaller clutch just used on 3 spds or some 4 spds as well? Will the smaller clutch be ok? I won't be towing with it. | | | | | Joined: May 2009 Posts: 85 'Bolter | | 'Bolter Joined: May 2009 Posts: 85 | If the 9" is working it should be fine if everything is stock. I think the larger clutch requires a different flywheel, the bolt pattern is different and the wear surface is also different.
1949 3100
| | | | | Joined: Jun 2018 Posts: 52 Wrench Fetcher | | Wrench Fetcher Joined: Jun 2018 Posts: 52 | The truck wasn't running when I bought it and hasn't yet. I pulled the trans to check the clutch which I am replacing. I was just curious if anyone had any solid information on the subject. | | | | | Joined: Dec 2017 Posts: 1,609 'Bolter | | 'Bolter Joined: Dec 2017 Posts: 1,609 | If the trans, pressure plate and clutch are out, measure the machined face of the flywheel and the size of the pressure plate. Its probably larger than 9. 55-57 used a 9.5 but the flywheel was actually machined enough to be able to use a 10.00. GM apparently gave us 6 guys the short shrift and used a smaller clutch but a 10 inch clutch and pressure plate will fit. You might find the same with yours. Four speed truck commonly came with 10.00, some with 11 so take a look and see what the flywheel will accommodate.
Mike
| | | | | Joined: Jun 2018 Posts: 52 Wrench Fetcher | | Wrench Fetcher Joined: Jun 2018 Posts: 52 | My truck is a '48. The machined surface of the flywheel is 9" which matches the clutch and pressure plate. | | | | | Joined: Jun 2004 Posts: 8,597 Riding in the Passing Lane | | Riding in the Passing Lane Joined: Jun 2004 Posts: 8,597 | Bigger is not always better. A smaller disc makes shifting easier & is better for the syncros in the trans. As long as it can handle the torque of the engine & the driver knows how to use a clutch.
The syncros have to slow down or stop the disc & gears in the trans.
George
They say money can't buy happiness. It can buy old Chevy trucks though. Same thing. 1972 Chevy c10 Cheyenne SuperIn the Gallery Forum | | | | | Joined: Jan 2013 Posts: 1,518 'Bolter | | 'Bolter Joined: Jan 2013 Posts: 1,518 | 1/2 ton trucks standard size with 4sp is 9", heavy duty available with 10-3/4". Heavy duty requires flywheel with 9 mounting bolts for the pressure plate, instead of 6 for the standard duty. Believe 3/4 and up trucks got the bigger clutch
1953 Chevrolet 3100261 cu inch, sm420, 3.55 rear, torque tube still,omaha orange, still 6 volt, RPO green glass, side carrier spare, all done In the DITY GalleryVideo of the 261 running1964 GMC 1000305 Big Block V6, sm420, the next cab off restoration
| | | | | Joined: Jun 2018 Posts: 52 Wrench Fetcher | | Wrench Fetcher Joined: Jun 2018 Posts: 52 | | | | | | Joined: Jan 2010 Posts: 210 'Bolter | | 'Bolter Joined: Jan 2010 Posts: 210 | Hi GTOKID, Just my 2 cents, but don't put a 10 3/4 clutch in as they're way expensive $$$$$$$$$. Carters and 40's carry them and all you have to do is look at the prices for each piece as they have to be rebuilt. I have a 1.5 1946 and have changed the clutch twice, once after buying and once because the disk rusted to the flywheel. Since your truck is a 1/2 ton, I'd put a 10" or 11" in where you can buy clutch kits all day long provided your flywheel will take the bigger pressure plates, but even then its no biggie to replace that. Just a suggestion, Good luck. | | |
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