The Stovebolt Site Forums The Shop Area Engine Shop See anything wrong SM420 Overdrive Method

I have asked befor about this, but I think I have a way to attach an overdrive to my sm420. I wanted to bounce it off you guys and see if you can shoot some holes in it.
I am useing a "divorced" overdrive. That is I am going to put a 1ft. drive shaft with a sliding yoke on the output of the sm420 and the input shaft of the overdrive. Then put another driveshaft from the overdrive to the rear end. The overdrive is a 30% borg warner and is the tail end of another transmission. I will have a long bare part on the overdrive shaft, since there is no longer a transmission on it. I have found a yoke to fit the splines on the overdrive input. I will keep the bearing holder from the transmission and put a sealed bearing in it and seal off the rest of the overdrive leaks with a plate. The Overdrive will sit on a cross member. Waddaya think? Any built in problems with this? Thanks - Scott

Sounds interesting. Are you going to be able to go back & forth between direct drive & overdrive? How will you shift? Or is this a permanent overdrive? The SM420 has rather wide gaps in gear ratios. I think 30% would make it hard to accelerate. Unless the motor has alot more power than stock. I think it would work if you can shift in & out of OD, splitting the gears like a stock 3spd w/OD. Just shifting in & out in top gear to reduce rpm's would be great. I have 3 trucks w/SM420, so I'm curious.

You plannin' to do any heavy-haulin' with this OD rig?

Scoot-I am thinking of putting a toggle switch to the OD solenoid and only really using it to lower my rpms on the highway. So, it would be direct drive straight thru the OD most of the time.
Frank- I have been using it to go to get firewood and it is quite a load. The truck handles it fine now. The overdrive primary shaft is quite large and the planetary gears would not be engaged when hauling these types of loads. Do you think it will work to leave it off overdrive for firewood and then engage it for good hwy speed? It looks strong inside except for the planetaries. - Scott

That sounds like a good plan Scott...


Don't know about these later OD's, but the 1930's & '40s units would "Freewheel" when the OD was engaged - this means that when coasting, a series of over-running clutch rollers prevents the driveshaft from driving the engine; in other words, you lose your compression-braking benefit when in OD.

When you pull the lock-out cable, the OD is locked-up in direct drive, and all should be fine for your hauling chores.

Good luck...of all the vehicles that would've benefitted from an OD option, the six-cylinder Chevy cars and trucks from '30s thru '54 would've benfitted most; sadly Chevy didn't begin offering OD until the small-block era, when the need for one was disappearing.

Ford owner could at least opt for a Columbia 2-speed rear; I've never heard of such for a Chevy (or Buick, as long as we're talking torque-tubes...)

Good luck with your project!

Here's what Jolly did.
http://www.6066gmcguy.org/spicer-5831-b.htm

The 3-speed with overdrive I once owned had a reverse lock-out on it. I'm wondering if you need to lock out the overdrive when in reverse. Not sure how you would force that always to happen.

Scott-
It sounds do-able to me. I think we are all smart enough to remember to turn off OD when going to lower or reverse gears. If not, we'll figure it out when the engine dies. I think you should do it. I would like a readily available (& cheap)option for OD. The Spicer would work, but good luck finding one & just how much does one weigh? What tranny are you getting the OD off of? If it works, will you write up a tech tip? Good luck.

I was looking for overdrives and found a big truck with an overdrive that was separated by a bit of space from the back of the tranny mounted on the engine. I don't think that will be a big deal. It was a pretty big OD so I didn't get it. Later on I scored a brownie for my truck but haven't had a chance to mount it yet. Will try to document everything I do for the site.

I am still working on my 47 cj2a jeep. Today I got yokes for it that should seal up the transfer case as it had already had new seals installed, but the old yokes were hogged out where the seals ride and leaking. #@$#$ And as I was doing that, the durn water pump started leaking out the shaft. Pretty soon I am going to have a new 'heep'.... Then I'll scuff and shoot the 59, then I'll do the overdrive.

But speaking of which, I don't have a shifter for it. It only has direct and overdrive, so I was probably going to fabricate a shifter, but if anyone knows of one... or has a source...or...

arnswine, you found a Brownie box. I'd love to find one for my dad's '51 350.