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Transmissions and Rear Ends Three on the tree or four in the floor? Shift it all here.

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  #1  
Old July 29th, 2022, 05:34 PM
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Default Sm420 question

Will a SM420 from a GMC 305 V6 bolt to a 48-54 228 motor and bell housing?
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Old July 29th, 2022, 10:40 PM
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Default Re: Sm420 question

Looks like Novak says SM420 fits all GM bell housing patterns 47-67.......so..............input shaft length and spline count should fit 228?????? Just check the big shoulder diameter.

Last edited by AZKen; July 29th, 2022 at 10:50 PM.
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Old July 30th, 2022, 03:57 PM
George Bongert George Bongert is offline
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Default Re: Sm420 question

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Originally Posted by AZKen View Post
Will a SM420 from a GMC 305 V6 bolt to a 48-54 228 motor and bell housing?
SM420 from the GMC 305 V6 should bolt up to your 228 engine with no problem. Even though there were subtle differences between GMC and Chevy Trucks made from '48 to '67, the SM420 was a standard issue GM Transmission for both GMC and Chevy Trucks.
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Old July 30th, 2022, 05:37 PM
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Default Re: Sm420 question

Thank you George. The SM420 was 47-67 and the 228 was 39-53. So what did the 39-46 GMC use?

Last edited by AZKen; July 31st, 2022 at 03:09 AM.
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Old July 31st, 2022, 09:08 AM
George Bongert George Bongert is offline
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Default Re: Sm420 question

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Originally Posted by AZKen View Post
Thank you George. The SM420 was 47-67 and the 228 was 39-53. So what did the 39-46 GMC use?
I'm afraid that's a question I can't answer, Ken. I don't go back that far. One thing I am reasonably sure of though is that since GM manufactured both Trucks, it is logical that they would have used a transmission that was interchangeable for the '39 to '46 GMC and Chevy Trucks. A search of GM Archives (if one exists) should provide the answer that you are looking for. In my case when I re-powered my '63 Chevy with the GMC 305 V-6, I only changed the engine and bell housing. The bell housing and clutch assembly from the 305 V-6 mated perfectly with the SM420 in my '63 Chevy Truck. Both my '63 Chevy and the '62 GMC Truck that my 305 V-6 came from had SM420 transmissions. And I dare say that the GMC 305 V-6 was a vast improvement over the Chevy 292 inline 6 that it replaced.
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Old August 4th, 2022, 02:43 AM
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Default Re: Sm420 question

The 228 went back further than 47. There is a split in the sn where there is two deck heights needing two different length pushrods. If i remember correctly, the 228 began production in '35-'36 year model. Gmc also produced a 236, 256, 270 (two different block thicknesses) a gi version, and a commercial version. Gm truck division bought from buick their 6 cyl. Casting and rifle-drilled the crankshaft (inserted conn. Rod bearings--not babbit-rod bearings) also full journal oiling.
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