The Stovebolt Site Forums The Parking Lot '60-'66 Trucks Transmission Issue ? - Need Help From The Experts

Hello!

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.

Questions:

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

Your gas mileage is good, you don't need to do anything. It will be many years before you get the modification money back on better gas mileage. Do the math. How many miles per year, in town and hiway. Realistic expected mileage improvement. Price of gas per gal.

10,000 miles per year.
average 18 MPG @ $3 per gal
Improved average 21 MPG @ $3 per gal
Delta savings $238 per year.
Good Reman NV4500 $1800
All the other stuff $900
Labor $1500
Total $4200/238= 18 years amortized
Cut out the labor, it's 11 years. At any rate it will all be worn out by then and need done over again just as you get even

I'm guesstimating of course. Just an example using some of your given info. Use your own numbers and everybody will have numbers also.

You don't rebuild a SM420, they are a dime a dozen as well as SM435. $150-$350. New throw out bearing $35.

A 700r4 will solve all your mileage wishes, no clutch to worry about hooking up and you can have a floor shifter stick that looks like your Sm420. Piece of cake install. Much cheaper. Faster off the line and nice to drive.

In your situation, I think an overdrive would be a mistake unless it was a 0.86 OD gear. However a 14% drop in RPMs will not save you much gas.
I've done a lot of numbers matching for OD transmissions because I like T5s. A 0.73 OD gear is a great match for a 3.73 rear axle.
A 0.76 OD is good for a 3.42 rear axle.

Like bartamos has pointed out, the savings will not be that good.

I must ask this . . . How do you like the performance of the 3.08 rear axle? Specifically, I'm curious if the truck seems sluggish in the lower gears. If I recall, the SM420 has a very low first gear. I would guess the 3.08 axle makes it feel less like a granny gear.

If brg. noise is the only thing wrong with the trans. you can replace brgs without a complete tear down of the trans. Its usually the front drive brg. that goes bad & causes a lot of noise in the lower gears & quite in 4th gear. With a 283 the 3:08 rear gear is about all it will pull. Your gas mileage is real good & the engine is not screaming going down the highway.
The NV4200 is a monster heavy trans. you don't need with a 283. If you do add o-drive the T-5 will work good behind the 283 as long as you don't torcher it.

George

Hi guys. Thanks for answering so promptly!

I'll answer in reverse. As for current performance with the 3.08 rear end and the SM420, I don't have any serious complaints. It works just fine in granny gear, seeing as how you want to go slow in that gear anyway. I usually take off from second, using it as first gear. It doesn't come out of the chute really well in second when used as first, which typically causes the people who want to be driving the full speed limit within the first 50 ft to start looking for a way to get around me. Of course, I can punch it and it takes off fairly nicely, but it wastes a lot of gas. 3rd gear, which I'm using is second gear on the street, is a pretty good fit. It gives me good acceleration without having to push it, and when I need it, I can punch it and it goes quite well. 4th gear, which is direct drive, works very well for highway driving. Even when I get over 60 and near 70, with the 3.08 rear end, it is not running such high RPMs that I can't think over the noise. If I had a choice, I would rather have better take off than what I'm getting.

I use my truck pretty much as a daily driver. So, I put at least 10 to 15000 miles a year, as a minimum. I can see how, as has been explained, the ROI on the NV4500 would be extremely difficult to obtain.

I am curious about the 700R4. I wonder how difficult it is to do the swap? I also wonder what the advantages would be over what I have, other than the fact that it's automatic and has overdrive ? Has anyone produced step by step article or video on how to do it?

Thanks for all your help!

Todd, if you do something, are you going to do it yourself? We can't give a bolt by bolt description.
You asked about the "advantages over what you have. Other than the fact it's overdrive and automatic". If you don't see any, then it's none, besides those two.

In general: You remove the trans and bell housing. Remove old trans cross member. Remove anything that is for the manual trans. Install a flexplate, torque converter and the automatic transmission. Provide a cross member for the transmission tail. Have your drive shaft modified to correct length with proper slip yoke for Auto trans.
Provide a vacuum source to A/T. Provide a radiator for A/T with oil line ports. Install Lokar nostalgic shifter on trans. Remove clutch pedal if you want. You probably want the non-computer version of whatever OD auto trans you decide on. It's not very hard to do. The A/T will bolt right up.
look at Classic Truck PN's 79-507 and 79-235 to see what an aftermarket A/T cross member looks like and pad. It's all just searching, reading, dinker, tinker, measure, and don't sweat the small stuff.

"Difficulty" level of the swap is subjective. Depending on your level and experience and if you have a jack and some jack stands. For myself, I know it's a bit of grunting but it is not high tech because all the parts for SBC are readily available. It's out with the old and in with the new. You get all the parts taken off and then you buy what fits per above. You can handle it if you have a good level place to work and a buddy or two. Find a good old boy who is a retired tranny builder to build the 700r4. It does not take a fancy TCI or transmission shop to build one for a 283.
You want a 1987-1992 700r4 or 4L60 (same thing).The 4L60E is the electronic controlled.

Great instructions Bart, but with a 700r4 he won't need a vacuum source but will need a cable hookup to the throttle linkage & should have a lockup provisions.

George

Well, after looking at all of the things I would have to do, and, as has been pointed out so well, the return on investment in replacing the SM420 is probably going to take forever, if at all. Therefore, I'm thinking about just rebuilding it instead. However, I have a question related to a separate problem that the transmission has.

When I am on the highway, shifting from 2nd to 3rd and 3rd to 4th, there is a lot of chatter in the stick shift unless I keep my hand on it. It's not as prominent in 4th gear as it is in second and third. Does anyone know what can cause that, and, how it can be fixed?

Thanks George, was thinking TH350 that I did behind a 454. I think I used a kickdown cable. I believe the 700 needs a TV cable and no vacuum, as said.

Todd, there are some good posts covering stick rattle. The sticks are kind of loose anyway. Some need new bayonet pins/lever fulcrum pins, or new spring or retainer. It can be clutch chatter vibrating it. Broken Mounts. Investigation is necessary.

To reiterate, the ROI was a guesstimate.

It's OK to rebuild the trans if you want. Should be some videos for that. There is a video for everything.

Thanks so much guys! I'll let you know how it turns out.

Thanks Todd, for letting us know what you end up doing. You have passed the trial by fire and a lot of chatter at you. You are a good and patient man. Good person to have as a member.

He’s from Oklahoma what did you expect! 😁

OK is OK.