292 tranny replacment with a 4 speed?....will it work?

[restorationboy14]

Alright I own a '69 GMC c-15 with a 292 inline 6. The 3 speed went kapoot on it amd was wondering if i could put in a 4 speed instead? My yoke is 16 spline. Is there a 4 speed that has a 16 spline? And will it work? There was a floor conversion to 3 on the floor when i got it. So would i have to make a wider hole? And would everything mount up properly? Thanks!!
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[restorationboy14]

I was thinking a SM420 transmission? Whats your guy's take on that? Would it hook up correctly?? Thanks!!!!
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[Lattimer]

Are you saying the transmission input spline count is 16? Those 3-speeds should be 10 spline, at least the one from my 70 was.

You can always change the clutch to match whatever spline count you get for a replacement trans.

The 3-speed in my truck was originally behind a 292, then a 327 small block. My understanding was that all of the 3-speeds were identical. I replaced it with a Saginaw 4-speed form a 1970's car. Do a search on Saginaw 4-speed swap, there are several very good threads. Those Saginaws drop in exactly in place of the 3-speed with only minor mods (you have to get a new shifter and swap the nose cone from the 3-speed over to the 4-speed). I had to enlarge the floor shift hole a bit compared to the 3-speed floor shifter, and getting a shifter to fit with a bench seat is a ton of fun. I had to custom modify the Hurst handle I bought. No driveshaft mods required.

[restorationboy14]

Originally Posted by Lattimer

Are you saying the transmission input spline count is 16? Those 3-speeds should be 10 spline, at least the one from my 70 was.

You can always change the clutch to match whatever spline count you get for a replacement trans.

The 3-speed in my truck was originally behind a 292, then a 327 small block. My understanding was that all of the 3-speeds were identical. I replaced it with a Saginaw 4-speed form a 1970's car. Do a search on Saginaw 4-speed swap, there are several very good threads. Those Saginaws drop in exactly in place of the 3-speed with only minor mods (you have to get a new shifter and swap the nose cone from the 3-speed over to the 4-speed). I had to enlarge the floor shift hole a bit compared to the 3-speed floor shifter, and getting a shifter to fit with a bench seat is a ton of fun. I had to custom modify the Hurst handle I bought. No driveshaft mods required.

The output shaft was 16 spline thanks!!! Howd you modify the shifter??
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[Lattimer]

Originally Posted by restorationboy14

The output shaft was 16 spline thanks!!! Howd you modify the shifter??
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Cut and weld, cuss for a while, cut and weld again....LOL.

If you have bucket seats life is easy, bench takes some work to get handle that will clear the bench and still hit all the gears.

[Shyguy]

Either the SM-420 or 465 will bolt up, but you will need a high hump floor for them.

[restorationboy14]

Hahahaha ok so im 15 and this is my first build so im a rookie lol so my main goal is to keep it all original......except for the tranny LOL what clutch kit do i get?? Thanks!!!
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[restorationboy14]

So a high hump will bolt on? Thanks!!!!
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[da-burb]

Honestly, for the hassle you would be better off finding another 3spd. Changing over to a sm420/465 only gets you a 3 speed trans with a 4th gear you never use. You do need to cut out the low hump and bolt in a high hump and also change the front half of the drive shaft to the correct longer one. Go with a 465 if you must just because it is newer and made up to 1991. I like the reverse shift pattern better as well on the 465's. Away and toward the dash instead of towards the driver and back.(Trouble if you are long legged)

[restorationboy14]

I like the challenge lol So will have have to get a longer yoke on the driveshaft?? Thanks!!

[Shyguy]

Many people don't seem to like either the SM-420 or 465 but I like both of them. I went with a '71 SM-465 to back the '74 mildly built 454 in my '67 swb stepper when we built it last year.

I had a '72 GMC 3/4 ton with a 350 backed by a SM-465 when we lived in California many, many decades ago. The low first gear came in real handy when lauching my 24 ft cabin cruiser.

[RichardJ]

>>change the front half of the drive shaft to the correct longer one.<<

That means what it says. Longer drive shaft. The SM-465 or 420 don't use a slip yoke. The SM-465 will need a different bell housing with 5 1/8" hole. The SM-420 will fit the existing bell. The sm-420 was used in '67 and earlier, so it wouldn't be correct for a '69

If you want easy bolt in, you need a car trans. Sag or Muncie.

[volksworld]

find someone with a 465 and drive it before you attempt to convert yours...the 465 is a stump pulling truck trans ,not a performance 4 spd...what you would want is a Saginaw or Muncie car trans...unless you like to pretend you're driving a tractor trailer...if you're broke put the 3 spd back in...cause a 4 spd for a big block chevelle aint cheap...I don't know whats involved cause I've never done one but a t5 might be a better deal for you...

[Lattimer]

Another thought-----you can get a rebuild kit for the 3-speed for about $100 to do bearings, syncros' gaskets, etc. As long as the housing/gears/case are still ok, it can be rebuilt .

[restorationboy14]

Ok so the 465 is out of the equation. This truck is going to be a puller so that why I wanted to put in the 4 speed. So my only option for a bolt in would be the SM 420. What kind off drive shaft will I need then? I cant seem to find a floor hump either. I checked LMC so im gonna take a trip to the scrap yard. What drive shaft will work?? Thanks a ton guys!!!!

[RichardJ]

With the sm420 you still need a different, longer drive shaft. I would take the 420 out of the equation, because the ones you will find will be older and more worn out. The 420 was made from 1947 to 1967 and there were a couple of changes made in the '50s. I for one do not know how to ID a reall old 420.

I would consider the 465 a much better trans. You will need the correct bell housing. You need a cast iron bell with the mount ears and provision for bolting the starter motor. A later aluminum bell has no pads for the mounts and you will need a bolt-UP starter. You would have to check if your 292 block is drilled for a bolt-UP starter.

[Vintage Windmills]

So what are you looking to pull with the truck? How many miles and what type of driving will you be doing? If we understand your goals, then we can suggest a good trans for your application.

Think of the SM420 and SM465 as performing about the same. The comments on the 465 also apply to the 420.

If you're not pulling heavy, may want the mid 80's new process 4spd with the overdrive 4th gear. A 292 would have the torque to run that nicely I would think unless you are pulling heavy trailers (over 3000lbs). Here is some info on this 4spd with OD:

http://www.classicjunkyard.com/new_process_a833.htm


http://www.customclassictrucks.com/t...n/viewall.html

[Don Quixote]

Restoboy, you are getting a ton of good advice that I would not dispute. That said, I can speak from personal experience. When I bought Rocinante in 1996, she had a 292 with an SM465. I ended up swapping the 292 for a 350 because of a lack of rebuild parts in my neck of the woods at the time. You will find that the bellhousings are interchangable as to this day I am still running my "292" bellhousing with my SM465. I only changed the engine. The only thing I will dispute is what was said about the slip yoke. I do have one with my setup. Back ing the day, it was common for oilmen and farmers to swap out the drivelines for heavier duty ones to hold up on dirt roads better. Mine is such a conversion. The carrier bearing (sits between the front and rear halves of the driveshaft, if you will) is giant and rather difficult to find but still quite feasible and if you are building a puller, it is a good idea. If you find a same year range C30 out to pasture, you will probably find all you will need. It was a common setup for a 1 ton to have a 292 with a SM465 and a heavy duty split shaft.

[restorationboy14]

I'm so thankful for all the responses!! The 292 was rebuilt in `98 and has 5726 miles on it. This truck then sat for 14 years so I rebuilt the carb and got the engine running. This truck is going to be hauling about 2 or 3 head of cattle ( 2500 lbs) so that's why I want this tranny. Can you explain the whole process of what to do?? Thanks!!!

[restorationboy14]

How do i check if my block is drilled for a bolt up starter?? Thanks guys!!

[Don Quixote]

Chances are, it's not. On the inlines, typically the starters bolt up to the bellhousing. Even though I have been running a 350 since 2000, I still use the 292 bellhousing and a 292 starter. I just didn't see a need to change everything. That said, I do prefer the sm465. I could have put in a nv4500 but I just didn't see an overall need to but years ago I did change the rear end ratio to 3.73. The 292s didn't like to rev up really high do I found that 55-60 was my comfortable cruising max with the 292. The 3.73 gave me the ability to cruise at about 70 comfortably although when I am loaded with my cargo trailer on (that itself runs around 15000lb) I stick to 65ish.

If you are able to put in the high hump then I would go for it. If nothing else, many, many years will go by before you ever have to worry about transmission work.

On my sm465, an old farmer ran the fields and dirt roads for 25 years before I bought the truck with 198,000 miles on it. I now have about 440,000 on the truck and I travel just about all of North America with and without my trailer. I rebuilt my transmission for the first time about 2 months ago. I got 41 years and 430,000 hard miles out of it and they require just about no maintenance as long as you change the oil about every 30,000 miles and run 80w/90. By the way, I only paid $550 to have someone else do the work.

[restorationboy14]

Originally Posted by Don Quixote

Chances are, it's not. On the inlines, typically the starters bolt up to the bellhousing. Even though I have been running a 350 since 2000, I still use the 292 bellhousing and a 292 starter. I just didn't see a need to change everything. That said, I do prefer the sm465. I could have put in a nv4500 but I just didn't see an overall need to but years ago I did change the rear end ratio to 3.73. The 292s didn't like to rev up really high do I found that 55-60 was my comfortable cruising max with the 292. The 3.73 gave me the ability to cruise at about 70 comfortably although when I am loaded with my cargo trailer on (that itself runs around 15000lb) I stick to 65ish.

If you are able to put in the high hump then I would go for it. If nothing else, many, many years will go by before you ever have to worry about transmission work.

On my sm465, an old farmer ran the fields and dirt roads for 25 years before I bought the truck with 198,000 miles on it. I now have about 440,000 on the truck and I travel just about all of North America with and without my trailer. I rebuilt my transmission for the first time about 2 months ago. I got 41 years and 430,000 hard miles out of it and they require just about no maintenance as long as you change the oil about every 30,000 miles and run 80w/90. By the way, I only paid $550 to have someone else do the work.

Ok so with the SM420 I can keep the bellhousing as is but need to install the floorhump and a c-30 driveshaft? Will the 4 speed increase gas mileage or power or both? Thanks a ton!!!!

[Don Quixote]

You are pretty much right but do not expect any mileage improvements. Those transmissions are heavy and the moving parts are heavy as well.
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[restorationboy14]

Originally Posted by Don Quixote

You are pretty much right but do not expect any mileage improvements. Those transmissions are heavy and the moving parts are heavy as well.
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will I lose mileage??

[Don Quixote]

It'll be 6 in one hand, half a dozen in the other. The real advantage lies in the fact that you won't spend a lot of time working on the tranny. Just don't be thinking that you just built a 1 ton out of it. It'll still be a half ton, just with a really beefy drivetrain. Bear in mind though that if you do put the high hump in, you will need to replace the floormat as well. Is your ride a column shift or a floor shift? I don't remember what you said you had in it and if I look, I'll lose what I just typed :P