muncie m21 and a np205 transfer case?

[Ricks1971]

Just wondering if anyone has done the mod? I bought the adapter to do this mod. I know the drive lines have to be modified and I need the get the shifters. When I get started I will post pics and post what I needed to do to do this and if anyone has done this let me know if you would of any roadblocks I might get into. Thanks Rick

[special-K]

No,but I wanted to back in the early 80s on a`71 Blazer that had a three spd manual in it.Back then Advanced Adapters couldn`t help me and I couldn`t get enough answers to think it was possible.Keep us informed.

[BuildinA68]

this is a really old post but i am considering this. has anyone done this? would love to have a M21 vs SM465.

[LONGHAIR]

Gearing is going to be an issue with this....That high first gear is not street-friendly in a big/heavy truck in the first place, it can only be worse with 4 wheel drive.

[Mike C]

M21 is a lousy 4x4 truck transmission. Especially the close ratio one. The only application that would make sense IMO would be a mudracer where you could gear to the transmission.

If you don't want a granny gear, look at doing a NP 435.

[BuildinA68]

Yeah that is where I heard of it. I am interested in this setup for street use over the old granny gear tranny. This truck will not see any off roading. Too much money just to tear up. The truck will be on 33's and 3.73 gears, kinda built 350. Plus I like the idea of ripping through the gears.
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[V30 mongo]

Maybe I am just an old fart and don't get it. A 4x4 truck is just that, a 4x4 truck. It was designed for use in tough off road conditions. To make a 4x4 truck into a hot rod car does not make much sense to me. It's like taking a Corvette or Mustang to the mud bogs or rock crawling. The right tool for the right job, worked then, still works now.

[Mike C]

The M21 from GM was only in cars with 3.73 gears or higher because of the 2.21 first gear. And that was with 26" tall tires. The 2.52 first M20 is a way better street setup and 3.36 was standard with 3.07 and 3.55 optional in most cars. Your combo you will be ripping through clutches and not gears.

With 3.73 and a 2.21 first you have an 8.24 overall first gear. With the 3.58 2nd in an SM465 and 3.73 you have a 13.35 first gear overall. The 26" tire has an 81.6' circumference while the 33" has a 103.62 circumference. This is a 23% difference which makes the 13.35 x .77 an effective 10.27 ratio figuring tire size into the mix. With the M21 you are looking at a 6.3 overall first using same numbers. This would suck in a 2000 pound car and would be virtually useless in a 5000 pound truck. Especially on a hill!

With 4.88 in your truck and the M21 you would have a 10.78 first gear and with the 23% reduction for 33's effectively an 8.3 equal to what '69 Z/28 would have had. With 2000 extra pounds but this would require you to re-gear. Mud trucks can run 5.13 or 5.38 gears and they run in low range which lets them pull the close ratio car trans off. So figure 5.38 x 1.97 low range x 2.21 first gear gives them a 23.4 overall first gear and a set of 44's has a 138" circumference or relative to the 26" tire would reduce that to a 9.828 first.

My '69 Camaro has a Super T-10 with a 2.43 first and 4.10 with 28" tires on it which works out to 9.96 first gear with a tire factor of 8% or effectively a 9.16 overall first relative to the 26" tire. It weighs 3450# with me in it.

IMHO, and YMMV, this is going to cost you a good bit of coin, render your truck virtually undriveable, and useless as a truck. With a 360 in my '79 CJ-5 and 3.54 gears on 33" tires, I'd like more gear on the street. Off-road I'd like WAY more gear. The T150 has a 2.99 first gear which I would consider an absolute minimum for a truck first gear. And keep in mind, a CJ-5 only weighs about 3000#.

[BuildinA68]

Mike C thanks for all your input. This info will save me a lot of time and money. That is why I wanted to throw this out there. I have a rebuilt sm465 and resealed np 205 ready to go. Since this is going to be a costly restored truck already, again it will not see any off roading (I have other junky trucks for that), how would my setup do with 3.08 axles? Keep in mind for street use only and probably some type of limited slip unit in the rear. The 3.73's are the stock axles but I have the 3.08's laying around. The 350 I have should be close to if not a slightly over 400hp. I know it migh not be too much of a difference in gas mileage, but that's what I had in mind. And please no comments on the gas mileage, I know it is not going to be a 20mpg truck. Thinking 8-10 with 3.73's vs 11-13 with 3.08's? Again any input is appericated.
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[Mike C]

I ran 3.07 in my Jimmy with 33's and it got terrible mileage, around 10. I went to 4.10 which were too much but mileage improved to about 12. I then added 35's which was just right and mileage went to 14! Personally, I think you will get the best mileage with the 33's and the 3.73 in a carbureted truck. The SM465 is a great transmission for all around use.

How much cam do you have? At 65 mph with the 3.08 gear and 33" tire you are right at 2000 rpm. With 3.73 call it 2450. That is almost identical to my 4.10 and 35" setup. Around town, you will much prefer the 3.73.

My truck has probably a 300 hp .030 327 with a 266 advertised, 210 @ .050 .440 lift cam.

[BuildinA68]

Well that is good news about the mileage, and your setup helps. It has been a while since I built this motor, and I think the advertised lift is 468. But it makes up in the duration. I am not sure what the engine will turn rpm wise except by doing some math. Again your input helps me out greatly. And by the way, I love the Jimmy's in our era. I drool at them out west on eBay.
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[Mike C]

With more cam than mine, you will definitely want more gear and not less. Keep us updated on the project!

[BuildinA68]

Thane again and will do. It is a slow process right now because I have my 2 wheel drive truck in progress too. Going with a 6.0 and 4l80e, lowered with all the goodies. That one is at the body shop now getting the aftermarket body panels to fit. They suck. Again thanks for the input, I can go back with my original plan now!!
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[SONOSADSACK]

I had fun with mine while driving it on the road like a muscle car.
Casting: 3925660 Years: 1968 - 1970 Ratios: M20 2.52 1st , M21 2.20 1st, M22 2.20 1st
I really do not know much about it as it was there when I picked it up. By the looks of it, it was there for many years too. I sold the trans to a guy building a Chevelle but I think I still have the advanced adapter and bell housing.

[BuildinA68]

did you have to change out the output shaft? oh thats right it was like that when you got it. do you know what gears were in the truck? which transmission was it m20, m21, or m22? after talking to a few people they strongly suggested a m20 and low gears. what size tires were you running? how much would you want for the adapter, just in case?

[SONOSADSACK]

When I ran the casting number on the trans this is what I came up with “Casting: 3925660 Years: 1968 - 1970 Ratios: M20 2.52 1st , M21 2.20 1st, M22 2.20 1st”
This is all I have left over. You can see some wear on the gear but if you are interested enough in it to pay for shipping you may have it. Just send me a pm.

[Mike C]

That should be the reverse gear. It is not constant mesh, so you can see the wear that occurs over time going into reverse. All three transmissions used the same case, so when you ran the #'s, that's what you got back. Most common was probably the 2.52 gear wide ratio M20 (M20 in trucks same code on the buildsheet as SM465 which stood for wide ratio 4 speed) M21 in high perf cars that had 3.73 or higher numeric gears. M22 the reduced gear angle heavy duty trans that everyone claims to have (at least on magazine cars!) but were pretty uncommon relatively speaking.

[special-K]

I have a true M22 from a '65 GTO going in my '67 GMC shortbed 2wd. That's a totally different application,though. I wanted to do this swap in a Blazer I had that came with a 3spd. I figure the M20-22s would be a better transmission being 1st gear is lower than a 3spd and 4 gears gives closer ratios between gears. The factory figured 3spds would do the trick,so I'm sure a 4spd car trans would,even better. The best trans for a 4wd truck? No. But,the best for shifting and an improvement over a 3spd? Yes.

[Mike C]

Your M22 should have the 2.20 first gear, I think that was the only iteration. So much of it depends on tire size and rear gear. Figure 4.10 and 26" tall tires would be fine in a 3600# truck. Maybe even 28" tires. But obviously engine displacement and cam characteristics play a big role. A 302 Z/28's characteristic were different than an LS6 Chevelle yet they had the same transmission. So tuning the gear is the secret.

But it is still a crappy transmission for a truck that sees any real truck use like towing. I find myself using 1st in my SM465 in my dually and my Jimmy when starting with a loaded trailer. Especially up hill! Also not near low enough gear for any crawling off-road. My A1 has a 2.97 first gear and 5.38 with 33" tire and a 3.50 first would be handy and anything less undriveable. (I ditched the 35's just for that reason.) But part of the A1's issue is the 72 hp F-head....

For 4 wheel drives, the car transmission would be killer in a mud racer or sand buggy where wheel speed and fast shifting is important or, as you mention, a street truck that is more of a Hot Rod than a hauler.

[LONGHAIR]

That was my point from the begining. The over-all gear spread is not enough. If you do get it geared low enough to take off well, you can't go out on the highway. It will be screaming it's guts out.

[V30 mongo]

You all are thinking on the wrong side of the transmission. How about this: a Muncie type 4 speed into a SM465. You would not need the reverse housing, as the SM465 has reverse. You would leave the SM465 in 2Nd or 3Rd and use the Muncie. Kinda like a big truck transminnion but only smaller size. I guess you can call it a 16 speed forward/ 4speed reverse. There is a company that makes a small two speed unit to bolt to the front of a SM465. Maybe two of them can be stacked together.

[Mike C]

Easy way to do that is just add the 203 gear box to the 205 for the standard 203/205 doubler on the back of the M22 car trans. Then you would have a 1.96 multiplication factor on every gear.

[special-K]

I was just saying is,without getting technical,the car Muncies would be an improvement over the factory Saginaw 3spd. 30 years ago I has a 350hp/3spd/Dana20/posi '71 Blazer with 13/37 Swampers and felt an M22 would be an improvement. Due to Advance adapters' lack of knowledge on this swap,I decided to buy a donor Blazer and go with a TH350/NP205. Probably was a good thing they were no help. But,I was just into letting it rip in the mud and driving it around.

[BuildinA68]

Ok I don't want to make anyone mad, and I do appericate all of the valuable information. What if the truck was geared 4.88 and the m20, and 33's was used, what would the rpm's read at about 70 on the highway? Again thanks for all the information, kinda doubt I am going to do this, but........
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[Mike C]

4.88 rear x 2.56 first in the M20 (later ones were 2.52) gives a 12.49 x 15% reduction for 10.6 first gear. Compared to 3.5 x 3.07 or a 10.75 for 2nd gear in an SM465, so it would be quite driveable around town. It would turn about 3250 rpm at 60 mph.

The M21/22 gets credit as the closest ratio 4 speed transmission ever available in a production car, and they rock in the environment they were built for: road racing.

You can get closer ratio HD trans (NP435 or T18) and obviously it's yours to do with as you please, but I think you will regret the car trans if you ever plan to drive the vehicle on the road much and use it like a truck. If you want to build a killer autocrosser then go with the car trans. Or in a mud truck, go with the car trans. But if you ever plan to just trail ride, or tow your boat, or pull somebody out of a ditch, you will regret not having the gearing options that the SM465 offers.