Overdrive help for my SM420

[hotrodalley]

I have a great GMC '66 COE wedge bed car hauler. Everything works great except trying to drive at highway speeds. It also has a 2 speed rear end. Does anyone know if there are any options to replace the SM420 with some type of overdrive. THANKS

[Captainfab]

About the best option for a truck like that would be a NV4500.

[hotrodalley]

Thanks. Do you know if it will fit, or if an adapter is available

[Captainfab]

I'm not well educated on the NV4500's but I believe if you get one from a GM that it will bolt up to the same bellhousing as your SM420. You will likely have to switch to a different clutch to match the input splines of the NV4500. I'm not sure what you would do for the shifter.....how is the shifter placement in a COE? I've never paid much attention to how they are in a COE. What engine do you have?

[dreamtruckproject]

Gearvendors would be the easiest. It bolts onto the back of the SM420 and you don't have to change the shifter placement or clutch setup. The Dodge version NV4500 can bolt up with an adapter if you have the bellhousing center hole enlarged to that of a SM465, but it moves the shifter back approx 5-6 inches IIRC.

[sweet70beast]

When I was mechnicing years ago, one of the older techs used a 1939 Chevy car trans input shaft as his clutch alinement tool. He used it on as new as 98 Chevy\GMCs with the NV4500s in them. The spline count should be the same but I don't know if the input shaft in the same length or not. The center hole on the 420s bellhousings are to small for the 4500. You could find a bellhousing from a mid 70's and up truck because they had enlarged them buy then so they should go right on the trans. I would think they should work with your clutch and pressure plate. Depending on the clutch fork you have, you may have to get one that fits the new bellhousing aswell as a throwout bearing to fit the fork. Hope that makes sense and it helps.

[Captainfab]

Thanks for catching that.....I had forgot about the SM420's having the smaller front bearing retainer. I know the SM465 was available in '69, so a bellhousing from '69 and up should work. IIRC the larger center hole is 5.125"

Originally Posted by sweet70beast

When I was mechnicing years ago, one of the older techs used a 1939 Chevy car trans input shaft as his clutch alinement tool. He used it on as new as 98 Chevy\GMCs with the NV4500s in them. The spline count should be the same but I don't know if the input shaft in the same length or not. The center hole on the 420s bellhousings are to small for the 4500. You could find a bellhousing from a mid 70's and up truck because they had enlarged them buy then so they should go right on the trans. I would think they should work with your clutch and pressure plate. Depending on the clutch fork you have, you may have to get one that fits the new bellhousing aswell as a throwout bearing to fit the fork. Hope that makes sense and it helps.

[62gmc910]

I have seen guys machine the SM420 bell out a bit and take a bit off of the outside of the front retainer on a SM465 ' but I don't know if the NV 4500 retainer has enough meat for that. as i recall the had a hyd throw out system on the front retainer ??? It's been a while

[lmdangerous]

Can you still use the standard clutch linkage ( mechanical), or do you have to go to a hydraulic clutch for the NV 4500??? I wonder if it will fit under the floor board of a 51 chevy truck?????
Thanks LMD

[padresag]

http://www.advanceadapters.com/categ...lete-units/91/
ron

[HEI451]

I had been looking into a 2 speed rear end for my 1964 C30 1 ton Dually Flatbed, but, in conversation with a friend I am doing some magnetos for, he gave me not only a milling machine, but a Hone Over-Drive unit he has had sitting around for years. I haven't got the O/D yet, that will be next summer's project, but, will have to figure out if it needs to be mounted directly behind the granny low trans, or if it can be mounted in place of the center U-Joint mount. I hope it will go in place of the center U-Joint support.

hotrodalley, would you happen to know the two ratios the 2 speed has in it? Just wondering if they are close to the Hone drive ratios.

[blackedoutharley]

The SM420 input shaft is 1 1/8 x 10 spline.

There is another unit called a "brownie" but I can't remember how it mounts. I agree the GearVendors unit is probably the easiest OD unit as far as installation.

[Captainfab]

I believe the 'Brownie' goes between the trans and the bellhousing. Unless that's a different one I'm thinking of.