The Stovebolt Site Forums The Shop Area Driveline What is an easy swap for sm420?

Hi all,

I have a 56 Chevy pickup with a small block and SM420. Is there a 4 or 5 speed manual transmission that bolts in with no changes to the bellhousing, driveshaft, clutch linkage, shift lever location, etc...?

short answer is no. What is the matter with the SM420? an SM465 is close, but you need a different bellhousing. The center hole in bellhousing is bigger for an SM465 (an 465 is about an inch longer so, you need to shorten your driveshaft.) Still a direct 4th gear though so, doesn't gain you much except a newer tranny. Also, shifter will be a bit farther back.

You're asking for to much.
Let's assume you want an OD transmission because otherwise there's no point in taking out a good SM420.
The T5 is one option for a 5 speed OD, research and read about what's necessary to swap it, it does bolt straight to the bellhousing.
A larger more robust 5 speed OD is the NV4500 which can also be made to fit without unreasonable troubles.

Picking up on one what Grigg said, if your desire is to get the OD gear for more top end, then you might want to consider leaving the drivetrain alone and simply switching out to taller gears in the rear end. If you do it that way, all of the components that you mention not wanting to change will be able to stay the same. Plus the SM 420 is almost bullet proof.

Matt

Yes, gear change in the rear axle would get you down the road faster and would be cheapest and easiest upgrade, after taller tires.

What is the goal/reason for considering transmission swap in the first place?

Hi all,

Thanks for the responses. I rebuilt the SM420 and it works fine. The reason for replacing it is to get 4 (or 5) usable gears that are spaced well for everyday driving. Each change I have made to my pickup has been carefully considered to keep it looking like stock but more enjoyable to drive. I swapped the 3.90 gears out for 3.38 years ago and am running tall tires. Freeway driving is no problem. Just want to have gear spacing that doesn't have me in third by the end of the intersection. Thanks again.

That makes a lot of sense. Like you, I have installed taller gears and larger diameter tires on my truck and can run fine on the highway. It would be nice to have another gear in the SM420, not to go faster, but just to shift into instead of pushing the engine. I agree 100%.

Matt

A 5 speed from a 50's GMC 400 series may bolt to existing bell housing. Usually they're direct drive but sometimes overdrive, they are pretty large, half again as big as the SM420. Clark is probably the brand to look for, some but not all have the GM bolt pattern. Don't think a Spicer bolts in place of the SM420.

You'd want to look up the ratios for what you find and see if the extra gear fills your gap. They will have a low first, like the SM420.

Otherwise you can consider the overdrive transmissions listed earlier and swap back to a lower axle gear.

Hi all,

I think the SM420 was perfectly designed for its original purpose, use in the field. I spent hours walking behind our trucks in the alfalfa fields as a kid, throwing hay bales into the truck as Mom crept along in granny gear. It was the only gear we could use to start out with a load of durum wheat in a soft field with a small 235 or 292 6 cylinder. Not so useful or enjoyable to drive in traffic.

I don't need an overdrive; just better gear spacing. In fact, as Grigg noted, an OD might force me to swap back to the 3.90. A 0.75:1 OD with 3.38 gives an effective final drive ratio of 2.53, extremely high given I don't plan to go over 60-65 regardless.

Was hoping that a car type 4 speed such as a Muncie M20, B-W T10, etc... would be a "bolt in". If not, I will keep the original. Thanks again.

The SM420 has a completely different configuration, length, and shifter location than passenger car 4 speeds like the M20 and/or T10. A driveshaft modification would be required and the shift arrangement would be nowhere close to looking original.
Fred

I wasn't even thinking of car transmission and column shifters, being as you started with a floor shifter. I also don't know anything about those side shifting transmissions.
Might be something to look into.

Take a look at the Saginaw 4 speed conversion information at Patrick's. It addresses many of your questions.

http://www.patricksantiquecars.com/

http://www.patricksantiquecars.com/chevy-gmc-auto-parts.html#chevy9

Matt

Thanks Matt. I will give Patrick a call.

I'm kind of late to the game here, but a Saginaw 4-speed out of a Camaro or S10 will bolt to the bellhousing, then add a T&F Shifter to put the shifter back in the stock location and have your driveshaft made to fit.

Mike B