65 GMC trans options

[F-85]

I have a 65 GMC short step with V6. I was thinking of maybe keeping the 305 and putting in a better trans. The sm420 sucks!!! Well as a daily driver it does. I was thinking of maybe a M21 or maybe a manual with overdrive. What are some things other people have done?



Josh

[creep]

I believe you would have to buy an adapter plate to use any other trans with the 305. You might be able to find an old factory plate on ebay or there is a company that makes new ones but they are expensive, like $900.

[F-85]

So you couldnt bolt up a M21 to it? Is there another bellhousing that would work?

[creep]

Not that I know of. From what I've read on 6066gmcguy.org the 305 to SM420 bellhousing is the only one that fits and GMCs with a factory auto had adapter plates between the engine and trans.

[mikie 1954]

I'am not positive but i think the m21 will bolt up to your bellhouseing,also a three speed. I have a 64 gmc that had a v6 that i replaced with a 350,had to get a differant bell to bolt to the v8 but same sm420 trany is still in truck.i don't think the differance is in trans patern but in how it bolt to back of the block.

[blackedoutharley]

What exactly is wrong with the SM420?

I have the original trans in my truck (I believe it to be original) and I have put almost 175K on it with no problems and 125K of that has been behind a stroked small block.

I did drain it and replace the oil with Redline MTL long ago.

[BobS]

The bellhousing is different than a Chevy but GM manual transmissions will bolt up. A specific adapter is required for automatics.

Bob

[raycow]

Originally Posted by F-85

I was thinking of maybe a M21 or maybe a manual with overdrive. What are some things other people have done?

I replaced my SM420 with a T5 and I couldn't be happier. It's the best thing that's ever happened to the truck.

Ray

[jbgroby]

I've attached apictures of the plate.

[jbgroby]

There is a guy who seems to always part out V-6 I've bought from him off ebay a few time always a square deal Dirty Dons' in Redding California

YOu get that adaptor and a lot of stuff will work with the V-6


Jake

[slorio]

I replaced my 3 speed with a T85N which is a 3 speed plus an overdrive. Here are some materials on this transmission. Would require some driveshaft work to use in place of your SM420, but would bolt to your bellhousing. The search engine on this website should have lots of posts dealing with this issue.
Regards,
Steve
New Orleans

http://www.speedprint.com/deves50/overdriveindex.php

[raycow]

Originally Posted by jbgroby

I've attached apictures of the plate.

Could you please post a pic of your bellhousing which bolts to that plate? It sure doesn't look like a normal Chevy bolt pattern.

Ray

[F-85]

Originally Posted by blackedoutharley

What exactly is wrong with the SM420?

I have the original trans in my truck (I believe it to be original) and I have put almost 175K on it with no problems and 125K of that has been behind a stroked small block.

I did drain it and replace the oil with Redline MTL long ago.

Nothing is wrong with it. It just sucks as a daily driver. Im not usuing the truck as a work truck. Just a driver. I would like to have a smother shifting trans in it.

[F-85]

Originally Posted by raycow

I replaced my SM420 with a T5 and I couldn't be happier. It's the best thing that's ever happened to the truck.

Ray

What did you have to do for this swap?

[F-85]

I dont think i will go auto right now. Might turn this truck into my Hotrod truck. I have a built 468 Olds that would make this truck scream. But if i do that it would be over the winter.

[raycow]

Originally Posted by F-85

What did you have to do for this swap?

The T-5 bolted to the stock bellhousing, but I had to shorten the input shaft and bearing retainer. You can get an aftermarket spacer plate now that will save you from having to do this step.

In addition, I had to replace the clutch disc and drive shaft and cut a new hole for the shifter a couple inches to the rear of the one for the SM420. It still clears the bench seat ok though.

The T-5 is quiet and shifts easily and the overdrive is great on the highway. Look for one out of a 92 or earlier S-10. Later ones have a different bolt pattern and will not work with a stock bellhousing.

Ray

[F-85]

Originally Posted by raycow

The T-5 bolted to the stock bellhousing, but I had to shorten the input shaft and bearing retainer. You can get an aftermarket spacer plate now that will save you from having to do this step.

In addition, I had to replace the clutch disc and drive shaft and cut a new hole for the shifter a couple inches to the rear of the one for the SM420. It still clears the bench seat ok though.

The T-5 is quiet and shifts easily and the overdrive is great on the highway. Look for one out of a 92 or earlier S-10. Later ones have a different bolt pattern and will not work with a stock bellhousing.

Ray

Hey Ray


Whats the story on the spacer plate? I cant find anything on the Net about it. Who sells it? And what clutch did you end up using? Did you change the pressure plate? Overdrive would be nice!

[raycow]

Originally Posted by F-85

Hey Ray


Whats the story on the spacer plate? I cant find anything on the Net about it. Who sells it? And what clutch did you end up using? Did you change the pressure plate? Overdrive would be nice!

Here are two outfits that sell the spacer plate. There may be others.
http://www.inliners.org/buffalo/index.htm
http://hamiltonintakes.com/products/s10t-5-adapter.html

I kept my stock pressure plate, which is 11", and used a disc for a 4.3 Astrovan. If your PP is smaller than 11", use the stock disc for an S-10.

I forgot to mention one other step. You need to enlarge the T-5 mounting holes to 1/2" so the stock bellhousing bolts will go through them.

Ray

[markeb01]

Thanks for the links Ray, I'm sure that's going to be very helpful to many. Sure makes life easier than doing machine work.

[F-85]

Originally Posted by raycow

Here are two outfits that sell the spacer plate. There may be others.
http://www.inliners.org/buffalo/index.htm
http://hamiltonintakes.com/products/s10t-5-adapter.html

I kept my stock pressure plate, which is 11", and used a disc for a 4.3 Astrovan. If your PP is smaller than 11", use the stock disc for an S-10.

I forgot to mention one other step. You need to enlarge the T-5 mounting holes to 1/2" so the stock bellhousing bolts will go through them.

Ray

Hey Ray


That doesnt sound bad at all. Thanks for all the info.


Josh

[blackedoutharley]

Originally Posted by F-85

Nothing is wrong with it. It just sucks as a daily driver. Im not usuing the truck as a work truck. Just a driver. I would like to have a smother shifting trans in it.

I guess that is a matter of perception. I have an SM420 in my '65 behind a quite healthy small block and at one point it was my only driver (about 85 miles a day commuting) and I have put nearly 125K on this motor.

I did find that switching to Redline MTL synthetic trans lube improved the shifting action considerably and seemed to make the trans quiet down.

If you do this, drive the vehicle first to get the gear lube good and hot then drain before it cools.

[jbgroby]

Ray and others. here are some pictures of the bell housing.

[raycow]

Originally Posted by jbgroby

Ray and others. here are some pictures of the bell housing.

WOW! You have a cast iron Hydramatic there. I had no idea that's what the plate was for. Those transmissions are so scarce they are almost like collectors' items. You are planning to keep it in your project, I hope?

Ray

[jbgroby]

Yep, this is what it looks like mated to a 305 V-6. AND YES, those are stock Valve Covers.