55.2-59 Sm318 to Sm420 swap in 59GMC

[72 k15]

I would assume its a pretty easy swap im getting sick of the first gear in the 318. I know the tranny hump will need to be replaced with the sm420 one but anything else? Driveshaft length the same? Mounts?

[1project2many]

Depending on the three speed, you may find the bottom bolt holes are a problem. Light duty three speeds were bolted to the bellhousing from the rear. Heavy duty three speeds and 4 speeds are different. Upper trans to bellhousing bolts are installed from the rear while the lower bolts are installed from the front, inside the bellhousing. SM318 Should be a light duty transmssion.

Driveshaft may not be the same. Does your transmission currently have a slip yoke or bolt on rear yoke? How about the one you want to install?

If you can find a heavy duty three speed it has different gear ratios in 1 and 2.

1st 3.17
2nd 1.75
3rd 1.00
R 3.76

[mr48chev]

You will have to change the driveshaft as the transmisisons aren't even close to being the same as far as length or the way the driveshaft hooks up
Info on the 420 here https://www.novak-adapt.com/knowledg.../manual/sm420/

As far as gears, no one I know ever starts out in low gear (granny) with an SM 420 unless they are pulling a grade or pulling a heavy load. 90 % of the time you start out in second gear. Second gear is 3.6 though and is better than either first gear offered in the 318 as there are two first gear ratios on the 318, 1st gear ratio Suffix A equals 3.03:1, Suffix B equals 2.42:1.

318 on left, 420 on right.

I'm thinking with the truck bellhousing you use a bolt and nut with the 318 on the two lower bolts and the two lower bolts on the 420 thread into the two bottom bolts on the trans.

[72 k15]

Originally Posted by 1project2many

Depending on the three speed, you may find the bottom bolt holes are a problem. Light duty three speeds were bolted to the bellhousing from the rear. Heavy duty three speeds and 4 speeds are different. Upper trans to bellhousing bolts are installed from the rear while the lower bolts are installed from the front, inside the bellhousing. SM318 Should be a light duty transmssion.

Driveshaft may not be the same. Does your transmission currently have a slip yoke or bolt on rear yoke? How about the one you want to install?

If you can find a heavy duty three speed it has different gear ratios in 1 and 2.

1st 3.17
2nd 1.75
3rd 1.00
R 3.76

Im pretty sure it has a slip joint, I can go crawl under it tonight, Did they make a 3 speed the was synchronized in all 3 gears? I dont mind the 3 on the tree but I hate having to basically stop for 1st gear

[1project2many]

Originally Posted by 72 k15

Did they make a 3 speed the was synchronized in all 3 gears? I dont mind the 3 on the tree but I hate having to basically stop for 1st gear

Not as far as I know. The HD trans was synchronized in 2 and 3 only. I learned to double clutch on the downshift but it's not for everyone.

If you have the SM420 then by all means use it. But it's noisy in 1, 2, and 3 (direct drive is very quiet). Very noisy. I ended up adapting a shifter from a GMT400 so I could use the rubber isolated base to keep gear noise away. Mr48 is right about Creeper gear being nearly useless. If you have to buy the transmission I would look for an SM465 In some ways it's the "Hey, we could have done this better" version of the SM420.

You might luck out and find a fully synchronized 3 speed. In trucks they started around 65-66 depending on the transmission model.

Good luck, and don't forget to update this thread if you swap the trans!

[72 k15]

Originally Posted by 1project2many

Not as far as I know. The HD trans was synchronized in 2 and 3 only. I learned to double clutch on the downshift but it's not for everyone.

If you have the SM420 then by all means use it. But it's noisy in 1, 2, and 3 (direct drive is very quiet). Very noisy. I ended up adapting a shifter from a GMT400 so I could use the rubber isolated base to keep gear noise away. Mr48 is right about Creeper gear being nearly useless. If you have to buy the transmission I would look for an SM465 In some ways it's the "Hey, we could have done this better" version of the SM420.

You might luck out and find a fully synchronized 3 speed. In trucks they started around 65-66 depending on the transmission model.

Good luck, and don't forget to update this thread if you swap the trans!

I might go the synchronized 3 speed route then, I actually like the 3 on the tree. Any idea if it will just swap over? I would assume its pretty much the same tranny.

[mr48chev]

I've got a full syncro Saginaw 3 speed out of a later Chevy truck in my 48 and that has been one smooth working transmission. My buddy gave/traded it to me years back when the Muncie 3 speed I had in it needed serious work.

It is similar to this one out of a 68 Chevy and they came in 66 to early 70's Chevys.

[72 k15]

Originally Posted by mr48chev

I've got a full syncro Saginaw 3 speed out of a later Chevy truck in my 48 and that has been one smooth working transmission. My buddy gave/traded it to me years back when the Muncie 3 speed I had in it needed serious work.

It is similar to this one out of a 68 Chevy and they came in 66 to early 70's Chevys.

Any idea of the model of it the one so it makes a little easier to track down.

[mr48chev]

68/ though early 70's cars and pickups is what they came in.

The best part is that unlike the 318 you can shift into low smoothly while still rolling up to a light.

They shouldn't be all that hard to find in the Fargo area as there should have been plenty of them sold in pickups in that area around that time. The "I want a new truck but don't want to pay for a bunch of extras" trucks.

I've got some extended family in the Fargo area either Davison or Dunn.

[72 k15]

Originally Posted by mr48chev

68/ though early 70's cars and pickups is what they came in.

The best part is that unlike the 318 you can shift into low smoothly while still rolling up to a light.

They shouldn't be all that hard to find in the Fargo area as there should have been plenty of them sold in pickups in that area around that time. The "I want a new truck but don't want to pay for a bunch of extras" trucks.

I've got some extended family in the Fargo area either Davison or Dunn.

Do you know the model number, for example I have the sm318 would just make it easier to find if I knew. Any idea on the length being the same

[mr48chev]

Sorry, I gave you all the info I have and you will just have to do the same searching that I would have to do to give you an answer.

Maybe I should put that in my signature at the bottom of the page, = I don't know everything, but I do know how to search effectively.

[72 k15]

Originally Posted by mr48chev

Sorry, I gave you all the info I have and you will just have to do the same searching that I would have to do to give you an answer.

Maybe I should put that in my signature at the bottom of the page, = I don't know everything, but I do know how to search effectively.

Well thanks for your help, I find the search engine hard to use on this website but ill give it a try.

[sack54]

https://www.ebay.com/itm/35346886637...MAAOSwMNpgg1XR

[mr48chev]

Originally Posted by 72 k15

Well thanks for your help, I find the search engine hard to use on this website but ill give it a try.

You have to go out past this board to find that info. Outside what little info I knew from the one I have the other info I gave came from an Ebay add. . I don't know the designated model number of that trans but it can be found easy enough. It was just a Full syncro Saginaw 3 speed to me. I've got a Hurst 3 speed shifter on mine that came from the same buddy. We horse traded back and forth for over 30 years before he got dementia and ended up in a nursing home.

[1project2many]

Well thanks for your help, I find the search engine hard to use on this website but ill give it a try.

Ok, super secret tip for searching this forum. Use google, then add "site:67-72chevytrucks.com" to the end to search this site. For example:
1968 three speed site:67-72chevytrucks.com

BTW, it really looks like the Saginaw 3 speed transmission is the one to get. Searching FB Marketplace, Craigslist, and Ebay all get results for that.

Good luck.

[mr48chev]

You don't want the 55/60 Saginaw 3 speed it is the 66 and up transmission that is the desirable one.

This chart shows the Saginaw 3 speed 343 and the Muncie 351 transmissions both with 7 bolt side covers and fully syncro. https://documents.holley.com/techlibrary_manual.pdf

I can find the Sanginaw 343 listed for Camaros and listed for trucks and cars but no actual definitive list of donor rigs and have been searching for about a half hour.

All I can really say about them is that they shift smooth and they are heavy. Heavy when you are laying under the truck on your back and have to stab one by hand without help and I did that more than once with mine but probably couldn't manage it now.

[72 k15]

So I pulled the engine and trans out yesterday, the tranny bolts to the bell housing on top from the outside the bell housing and the bottom from inside. Also not sure if this 235 bell housing will even fit my 270 now either so might be getting a new one of those.

[1project2many]

The truck bellhousings are like this. Two bolts from back on top, two on bottom from front. Car bellhousing is all four from back. Mixing and matching can mean using nut and bolt combination or, maybe (I've never tried this) helicoil. Also need to watch the diameter of the large center hole which locates the transmission using the input shaft bearing retainer. Most transmissions and bellhousings from this era match up but once in a while they don't.