dropping the tired th350 for an m21.

[lowblackc10]

Hi all, I'm new to this site and have a few questions. I have an early 70's muncie m21 that I want to put into my 84 c10. It is a car transmission and I"m wondering will the car bellhousing and clutch fork work in my pickup? Would it be compatible with a fluid clutch? If not am I better off just using mechanical linkage? I know that I will probably need to change my gear ratio, get a different driveline yoke to fit the m21 output shaft, but is there anything else major that I'm not seeing? Thanks in advance.

-Jerry

 

[HotRodPC]

You just might be surprised at how well this swap goes. Yes the bellhousing is fine. The bellhousing will determine which type clutch you use. If you have a fluid bellhousing, then that's what you go with, if it's manual clutch bellhousing, then go with the z bar linkage. Oh wait, so you have the 9 inch output Squarebody Th3500. DAMN !!! If it were a passenger car 6in tailshaft, you woudln't even have to mod the driveshaft. The Th350 and M21 both have 27 spline output shafts so you'll use the same yoke and u joint. The bellhousing and trans is the same length as the passenger car Th350. So you will need to add 3 inches to your driveshaft somehow. I have a feeling, if you found a driveshaft out of a 3 speed column shift truck, it would be the correct driveshaft.

 

[lowblackc10]

Awesome! Thanks for the input but I should have specified that I have the 26 spline input and 32 spline output, I'm not 100% on the casting numbers and measurements but i will get it on the lift on the next few days and measure things up. With the age of it I would guess its probably manual linkage. I had to buy a used drive shaft from a local junkyard and it seems he had one that was a few inches longer, I will have to try to see if its still around.

 

[lowblackc10]

So I got my truck on the lift today and measured the length from the transmission mount to the end of the tailstock. I have an H-pipe in my way from getting an exact measurement but it seems that they are both the same length. I thought it should be since I pulled the autotragic out of a '79 cutlass. Hopefully tomorrow I can get to my uncles junkyard to get a yoke, I'm not a fan of the cold but winter time is wrenching time.

 

[HotRodPC]

If you have a passenger car Th350 in the truck with the 6in tailshaft already, then yes, the M21 will bolt right in and use the same driveshaft? Why get a different yoke? It's the going to be the same. The output shaft on an M21 and a Th350 are 27 spline.

 

[lowblackc10]

The reason I have to change the yoke is that my m21 is a 1970-74 and has the 26 input and 32 output spline shaft, same as the m22. I was under the impression that it was a m22 until I looked the numbers up a years or so ago. Most m21's did come with the 27 spline output like you say though, Im not 100% on the history of the trans since it was rescued from a dirty aluminum pile 25ish years ago by my father.

 

[HotRodPC]

IF it's a 32spline, then it is a M22, and 26 spline input? That too should mean M22. M21 is usually a course spline input like 10 or so, M22 is fine spline looking like an output shaft. Sounds to me you have a Rock Crusher. So you need a yoke off a TH400. Need to get a passenger car Th400 yoke so it has the right size U-Joint, or use a Special Conversion Joint.

 

[lowblackc10]

Okay, that's good to know because there is a 1 ton 3+3 with a th400 at my uncles, but if that is different, maybe my local yard will have something. Thanks again for all the input and saving me a trip.

 

[Driver4r]

O.O did someone say rockcrusher??? Alot of $$ there.
I wanna do a straight cut gear conversion in 1-5 in a t56

 

[HotRodPC]

Okay, that's good to know because there is a 1 ton 3+3 with a th400 at my uncles, but if that is different, maybe my local yard will have something. Thanks again for all the input and saving me a trip.

NO, you can't use that Yoke off a GM Truck, unless it's a V6 1/2 ton truck in the mid to late 80's with a Slip Yoke Th400. The 3/4 and 1 ton trucks use a threaded output shaft where the yoke bolts to the trans. It's not only a bolt on yoke, but it's the bigger U Joint also, so there's 2 reasons why a 3/4 1 ton truck yoke WILL NOT work. So you need a Passenger car Th400 yoke which should also be the right size u-joint for a 1/2 ton truck. Find one an older Pontiac, Olds, Cad, like late 60's early to mid 70's and likely with a Big Block. An old 74 Caprice with a 402 for example will have the yoke you want, or a boat car like an old Olds Eighty Eight or Pontiac BonerVille.

 

[zieg85]

Keep in mind the M21 has a 2.20 first gear and with highway gears typically in a mid 80's half ton you may need to slip the clutch some to get it rolling and forget any trailer pulling... My $.02

 

[HotRodPC]

Yeah, Lower gear would be more desireable, BUT hey, It's a Muncie. Just rev it up half throttle and side step the clutch and you'll still take off like a rocket. If you have any horsepower, you might break a U-Joint, twist your driveshaft, or break a ring and pinion, but it's a Muncie. I'd rock it just because. The positive side to that 2.20 first gear is, the rest of the gears are pretty close ratio. I say

 

[lowblackc10]

Thanks for the input guys, I have put some thought into it and I decided to keep the th350 for the time being and maybe save the Muncie for a hotrod project in the future. Between the tall first gear and what are probably 3.08's in the rear I just don't want to have to run it like that, being my daily driver.

 

[HotRodPC]

Thanks for the input guys, I have put some thought into it and I decided to keep the th350 for the time being and maybe save the Muncie for a hotrod project in the future. Between the tall first gear and what are probably 3.08's in the rear I just don't want to have to run it like that, being my daily driver.

Ehhh, I woudln't let the tall first gear keep me from an M21. Fun as hell to drive. A tall first gear in a manual is nothing like a tall first gear in an automatic. 2 reasons being, you don't have the hp loss in a manaul trans, and you have unlimited stall with a clutch and no torque converter. Once you get your clutch usage down on the take offs, it's 2nd nature and then you have a close ratio once you're rolling. You can get by with many things running a manual trans. Like taller gears, tall first gear, oversize overcarbed motors etc. You can make them work just fine.

 

[lowblackc10]

Well that's a pretty good point. My main issue is pedals but I'll do some more looking around at local yards. Its been kind of tough trying to find any with this winter we've had in in cny, and a standard pickup is proving to be hard to find around here. Other than that and a yoke I should have everything to do the swap.