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Ecklers AutoMotive
Username Post: Transmission Dilemma - What Do I Do?        (Topic#353271)
Todd W. White 
Posts: 287
Todd W. White
Loc: Sapulpa, Oklahoma
Reg: 02-08-08
10-02-18 06:39 PM - Post#2747729    


First, here's my setup:

1965 C10
283 CID rebuilt, bored to .060"-over
Holley 1850 4bbl carb
SM420 transmission
3.08 rear end
29.6" tall tires
Current fuel economy: 16-18 in town (w/o much wind). 17-19.5 on highway (also no/low wind)

The problem is that the main bearing on the SM420 is going out, as is the throwout bearing. Clutch (made by Sachs) is good.

* I know I can rebuild it, or I can replace it. If I replace it, I would like to consider a transmission that has overdrive, and I would like a floor shifter, like I have now.

* I talked to "the old guy" at the well-respected transmission parts house/rebuilders in Tulsa, and he told me that, although parts for the SM420 are somewhat still available, they're getting harder to find and more and more expensive when you can find them. He recommended the NV4500 from a 93-95 Chevy/GM 3/4 to 1-ton truck, stating that it would pretty much bolt right up with minor modifications, and that I could change it from an hydraulic clutch setup to a manual one using my existing linkages, with some "tweaking". He did not mention the speedometer - I want to keep my mechanical cluster.


1. With my current setup, I'm getting what I consider respectable gas mileage. Would it be unreasonable to expect better with an overdrive transmission? Will I realize enough RPM improvement that will result in better mpg's without lugging my engine at 60-70 mph?
2. If I did get an OD unit, what ratio would I need to look for in the OD gear? Would the NV4500, as built, work in OD with my 3.08 rear end?
3. If not, what rear end ratio would I need to change it to?
4. If I go with the NV4500, what do I do in order to keep using my mechanically driven speedometer?
5. I know the Dodges and GMs used the NV4500, and I know they can both be used on the '65 Chevrolet, with some "adapting". For me, from my reading, the Dodge version would be a "no" because it would place the shifter way too close to the bench seat. Where would it come up in relation to where it does now on the SM420 if I use the GM-version?
6. What about motor mounts - will they have to be changed, and, if so, to what?
7. What about an additional crossmember - one thread showed pictures without it. Should I plan to put one in if I go with the NV4500?
8. Can I retain my mechanical clutch and linkages, and, if so, how?
9. If not the NV4500, then is there a different one you'd recommend?
10. Lastly, does anyone have any pictures of the swap to an NV4500 (from whatever earlier transmission)?

A big THANKS in advance for all your help! I've read everything the search feature came up with on this type of swap, but these are things that I just couldn't find the answers to, or, in the very least, need clarification on.

- Todd

Ecklers AutoMotive
Frequent Contributor
Posts: 1759

Loc: Lake George, NY
Reg: 11-11-15
10-03-18 06:36 AM - Post#2747756    
    In response to Todd W. White

My experience says the rear gear ratio and tire size won't be happy with o/d, especially with a small block.

Silver Supporting Member
Posts: 277
Reg: 02-05-17
10-03-18 07:09 AM - Post#2747761    
    In response to Shepherd

I have installed both a Borg Warner T5 and a NV3500 in my '65 Chevy truck that originally had a SM420 and 3:73 gears. This was what I learned.
To install the T5, I started with a Camaro T5 that I paid $300 for, then put a S10 tailshaft housing on it and a S10 shifter. It bolted up to the existing bellhousing and worked fine. The issue was that the S10 tailshaft housing speedometer sensor location didn't match the Camaro tailshaft. I had plans to weld a boss on the tailshaft housing to make it all work with the mechanical speedometer, but I ended up swapping to a NV3500 instead. In order to install it, I needed a crossmember from a Automatic. Not sure how hard that would be to find these days, but it wouldn't be hard to fabricate one. The only other thing I remember is having to extend the parking brake cable with a coupler and threaded stud as the crossmember was moved forward by a few inches to hold up the T5. The T5 held up pretty well behind my 327 w/ 300hp cam and Holley 670 cfm carburetor, and I didn't notice any problems with it. Having the overdrive in that truck made a huge difference in highway drivabilty. I assumed I got better fuel economy, but since I didn't have a speedometer, I don't really know.
After awhile I was worried about the longevity of a plain T5 behind the 327, so I went looking for a transmission from a C1500 truck. I found a NV3500 from a '97 Chevy truck for $500. This NV3500 has an integral bellhousing, so I had to convert to a hydraulic clutch. I used a dual master cylinder from a '60-62 truck in order to install the hydraulic clutch in my truck and had a hydraulics shop build me a custom hose that joined the NV3500 slave cylinder to the '60-62 master. I also had to bend the shifter slightly to allow me to shift into reverse without hitting the bench seat, but it wasn't hard to do with a little heat from an Oxy-Acetylene torch. This setup worked well, and I used the same automatic crossmember to hold up the rear. This time, the crossmember moved the parking brake back, which prevented it from fully engaging. I had planned on having the cable shortened, but I never got around to it. In order to solve the speedometer issue, I bought an electronic guage kit. This solved another problem for me since I had installed a fuel tank with a in-tank fuel pump for a TPI retrofit. The sensor was 0-90 ohms, so I needed a fuel sensor that would work in that range. There are companies out there that make a box that will monitor an electronic sensor and drive a cable. They're expensive, but that would solve the electronic speedo problem. Do a google search on cable-x speedometer adapter and you'll find one. There are also aftermarket overdrive transmissions that can be made to bolt right up, but they're pretty expensive too.
The previous comment about the gear ratio sounds like somebody with experience. With my 3:73 gears, an overdrive was very nice to have. My brother wanted to keep his SM420, so he opted for 3:08 gears when he had his differential rebuilt. I can say that his truck won't accelerate as well as mine, but both cruise pretty well on the highway.

Ecklers AutoMotive
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