3OTT question, Standard vs. H.D.?

[JJorgensen52]

I'm getting ready to swap a 3 speed (column shoft, I know!) into a factory 350 '72. I've been digging into the order guides a bit, and there are references in some places to a "3-speed H.D." for 350 and 400 powered trucks, but the ratios are the same.

Does anyone know what, if any, difference there is? It seems like possibly it references the SM330, which has the fifth mounting ear (though the bell housing for that didn't appear until '73 or '74). Search hasn't turned up anything useful, nor has Google.

Any thoughts?

Thanks!

[weim55]

Yes, there is a difference. Six cylinders and 307s got the Saginaw, 350s and a big blocks got the huskier Muncie. They both look very similar in appearance with the same number of side cover bolts but side-by-side they are easy to note the differences. I have both of these transmissions back at the shop, I could post pics in the next day or two if you need.

Steve weim55 Colorado

[weim55]

Muncie

[JJorgensen52]

I see!

Is there a major strength difference between the two? I have two Saginaw equipt trucks, but no Muncie. The 350 truck has a pretty stout motor in it.

[weim55]

The Muncie is definitely a stronger box but the Saginaw should certainly suffice. As long as you're not doing clutch dumps, burn outs and things of the like that Saginaw should live just fine. Saginaw comes in a variety of ratios and the most common is the 2.85 first unit that came in most of these trucks OE. I'd stay away from the 2.54 first version as it's a bit sluggish off the line with the tall tires most of our trucks have.

Steve weim55 Colorado

[FabricatorKevin]

weim55 - I think my big block ‘69 C20 has the Muncie 3 speed, based on your (excellent) picture. Here is what I have (sorry about the pics being rotated)

[weim55]

Originally Posted by FabricatorKevin

weim55 - I think my big block ‘69 C20 has the Muncie 3 speed, based on your (excellent) picture. Here is what I have (sorry about the pics being rotated)

Yup, that's it! From personal experience I will go out on a limb and say this is the strongest side shift box GM ever made through the muscle car era. Stronger than the M series Muncie 4 speeds and the T10s. This (M13 3 speed) is one super husky box that will take unbelievable punishment! I assure you that big block and the extra heft of the 3/4 ton is no strain for it. There's a reason GM trusted (warrantied) it for the application........

Super cool truck you got there BTW............

Steve weim55 Colorado

[JJorgensen52]

https://rover.ebay.com/rover/0/0/0?m...2F283250950514

Looks like this one is a later unit? In the pictures here I don't see that reverse light switch on the side of the box.

Going to have to keep my eyes peeled for the right Muncie box.

This thread is most informative!! Thank you.

[weim55]

[QUOTE=JJorgensen52

Looks like this one is a later unit? In the pictures here I don't see that reverse light switch on the side of the box.

Yes, this (ebay unit) is a later ('73 + square body) unit. the pics show a july 1973 casting date and the tailshaft has a boss for a transmission mount. '69 to '72 does not have a boss for the transmission mount as the bellhousing is the rear mount for the powertrain. Wondering if you could use this later box w/ your current automatic crossmember and not have to go through the hassle of converting your truck to the bellhousing-crossmember style. Just a thought....

Steve weim55 Colorado