3 Speed Bolt out Swap for M20
 

Hey all - I've looked and searched alot, but havn't found the answer to the following.

Last week I blew out the stock 3speed in my '71 long bed.

The truck has a 350 in it and I am looking for a bolt in replacement 4speed (M20).


My question is, what do i need to get? Input shaft splines? output shaft?

What else do I need to know? What do I need to order?

Thanks!!!!

James

Re: 3 Speed Bolt out Swap for M20
 

here is a pic of the truck i am working with http://www.kingsrook.com/jmaes/photo.jpg

Re: 3 Speed Bolt out Swap for M20
 

You have a 10 spline input and 27 spline output. A Saginaw 4 speed would be a direct replacment, but if your Sag 3 speed didn't die from natural causes, you will very likely grenade the Sag 4 speed too.

This means you want something stronger, like a Muncie or T-10. If you don't want to replace your clutch disc or driveshaft yoke, I think that pretty much limits you to a Muncie from about 63-70. Later ones than that were 26 spline input and 32 spline output.

As for the T-10 and ST-10, I think that spline combination might be kind of scarce. The early ones (up to 63) were 10/16 and the later ones (after 74) were 26/32. These are GM original equipment units. I think the 10/27 combination might have been used for aftermarket, but I'm not real certain about the non-OEM stuff.

Ray

Re: 3 Speed Bolt out Swap for M20
 

Ray - thank you for the info. Just what I was looking for.

I agree about the Saginaw - the 350 is putting out around 325 and it ate up that 3 speed with very little abuse.

As for changing out the slip yoke and / or clutch pack, i am fine with either or both, - its just a matter of not knowing what I don't know right now.

I was looking at putting in a rebuilt M20, but most everything I come across is 26In / 32Out - (ie this) and I am unsure what changes are needed to handle that.

I guess to put it plainly - if i wanted to put in a new / rebuilt M20 (or M22) and I was currently running the Sig 3 speed, what all parts am I looking at?

• The new trans (lets assume is a 26/32)
• New shifter (Hurst universal?)
• New cluch pack (11 or 10 inche?) with the 26 spline inner gear / hub
• New 32 spline slip yoke
• New or shortend drive shaft?
• others??

Re: 3 Speed Bolt out Swap for M20
 

Shifter sits farther back on the tranny too. Might need to notch your seat or use bucket seats.
And get a good Hurst shifter. The universal ones won't handle a lot of crap.

Re: 3 Speed Bolt out Swap for M20
 

I put an M20 in my 68 behind the 6 cylinder. Hurst shifter works great. I do have a mid 90's Chevy split bench in it. I had to shorten the drive shaft and get a longer speedo cable.

Re: 3 Speed Bolt out Swap for M20
 

You pretty much got it all right there. The 32 spline transmissions are slightly longer than the 27 spline. I would say put the new yoke on your present shaft and try to install it. As long as you have at least 3/4" before bottoming out the yoke (at ride height) you should be ok. If anything less than that, you need to get the driveshaft shortened.

If you can't find a truck-specific shifter, Hurst makes one for 55-57 Chev cars that will clear the truck bench quite nicely.

Ray

Re: 3 Speed Bolt out Swap for M20
 

thoughts on the drive shaft? Will i be okay with the stock on or have to get it shortened?

Re: 3 Speed Bolt out Swap for M20
 

Well, that's what I don't know. I suggest that you try it unaltered first. Just swap the yokes and see now much free travel it has. If there is at least 3/4" I would say you are good.

Ray

Re: 3 Speed Bolt out Swap for M20
 

I'm not sure about the driveshaft, but you will love the M20. I have one in my 67 Camaro with a 3.73 gear and it has took all of the abuse my 406 can give. :chevy: I highly recommend that you have the flywheel and pressure plate balanced. When I converted my car to a 4 speed I didn't have it balanced. When I pulled my 350 and put the 406 in, I had it balanced and the difference was amazing. I had a vibration through the shifter that was pretty bad over 3500 rpm before, and now that its balanced, the vibration is completely gone. Well worth the money in my opinion.

Re: 3 Speed Bolt out Swap for M20
 

Well right now I have a line on a factory 64-65 GM Muncie M20 wide ratio with Hurst high performance Competition Plus unit.

http://www.kingsrook.com/jmaes/m20.PNG


Its a 10/27 spline unit, so hopefully will be a direct bolt in and I might only have to deal with a shortened drive shaft (and there is a great machine shop here locally that does them).

Fingers crossed!

Re: 3 Speed Bolt out Swap for M20
 

Well - good news and bad. I didn't get the M20 listed above - but I did get a brand new rebuilt from www.KajunJon.com.

Tonight I pulled the drive shaft and 3 speed out (again) and will hopefully have the new M20 shipped out tomorrow.

http://www.kingsrook.com/jmaes/72tranny/1.JPG
http://www.kingsrook.com/jmaes/72tranny/2.JPG
http://www.kingsrook.com/jmaes/72tranny/3.JPG

Re: 3 Speed Bolt out Swap for M20
 

perhaps the worst movie - ever - but here is the old 3 speed with the side cover off - best I can tell while driving it - its a bad shaft bearing, but...

Re: 3 Speed Bolt out Swap for M20
 

Def. get a competition plus hurst and immediately upgrade the bushings that come with it to the bronze sintered type (the plastic crap will not last and makes the shifter sloppy) the bronze will tighten it up nicely. you may also need to tweek the throws a little to shorten them but this is a great shifter to play with. In a former life I had a m22 wth the comp plus and a centerforce clutch behind a health 355 with a tunnel ram and drive shafts and rear yokes became a problem after a while- but I was young and it was fun, out one night spend a week making enough money to buy parts and go play again.

Re: 3 Speed Bolt out Swap for M20
 

Several years ago I replaced the non synchro first 3 speed in my 66 with an M20 which was a 10/27 spline. I think it was a 69 transmission. I bought a Hurst competition Plus shifter that was supposed to work for trucks, but the handle had to be cut and rewelded at the base to clear the bench. Even now it is up against the bottom of the seat in 2nd and 4th. Obviously the 3 speed floor shifter hole in the floor isn't going to line up with the 4 speed as the shifter is on the rear of the tranny. If I recall correctly, driveshaft was real close. See if you can get a 27 spline yoke that is about the same length as your 3 speed yoke.

Re: 3 Speed Bolt out Swap for M20
 

What kind of shifter is that on the Saginaw? I like the way you can move it along the mounting bracket.

Ray

Re: 3 Speed Bolt out Swap for M20
 

Looks like mine Ray. Mr. Gasket or Hurst Indy. You can move them on the bracket or drill new holes for optimum positioning.
Sorry. Don't mean to highjack JJAMES.

Re: 3 Speed Bolt out Swap for M20
 

Guy i work with had a 69, 396 LB. Put the saginaw in it, and used the 3 on the tree column shifter. Had a rod beside the seat he used for reverse. It was really neat. I remember he got it stuck between 2 and reverse one day.

Re: 3 Speed Bolt out Swap for M20
 

That's pretty neat. Thank you.

Ray

Re: 3 Speed Bolt out Swap for M20
 

So if you get the 63-70 Muncie, the 10/27, is it a direct swap for the original 3 onthe tree? Or will I need to get a different bell housing or driveshaft lenght? Thanks for any input on this, for I will be tackling this next spring, but getting the parts together now.

Re: 3 Speed Bolt out Swap for M20
 

If your bellhousing is 67 or earlier, just bolt the Muncie on. 68 or later will need a spacer ring like this.
http://dennymac.com/mod.php?mod=user...103&page_id=54

Your driveshaft will work as-is if your 3 speed has a 7 bolt side cover. If it has a 4 bolt cover, measure from the bellhousing face to the rear seal. You are hoping for ~ 21". If it is closer to ~ 16", you will need to shorten the shaft. There is also a 9 bolt cover, but they are EXTREMELY scarce. If you have one of those it isn't practical to modify the stock shaft to work with the Muncie, so you will need to get a replacement.

Ray

Re: 3 Speed Bolt out Swap for M20
 

I just crawled underneath and it is a 4 bolt. I did a measure and she is about 21 inches so I guess I am in luck, however I will still need either that spacer ring, or a 67 bell housing, as my truck is a 70. Thanks for the info. Muchly appreciated.

Re: 3 Speed Bolt out Swap for M20
 

Saskgreen, the good news is, you may not have to buy a spacer ring.

All the 4 bolt transmissions I have seen had the small size bearing retainer, and they wouldn't have come stock in a 70, so I think you are looking at a transmission swap done by the PO. If he did his homework, he would have used a spacer ring, or else swapped an earlier bellhousing. Either way, you are good to go with the Muncie.

Worst case, the PO ignored the center hole mismatch and just bolted the early transmission to the 70 bellhousing without a spacer ring. In that case it would still run, but not for very long. Is the 3 speed dead now, or does it still drive ok?

Ray

Re: 3 Speed Bolt out Swap for M20
 

Actually the 3 speed runs and shifts just fine. I am switching it out because I am putting a 350 in it that will be pushing 375-400 horse, and wanted a tranny that will take it. I have heard nothing but good things about the muncie. But this is good news, perhaps at one point in the trucks life the original 3 speed failed and they put a different one in, who really knows lol.

Is there any numbers on the bell housing that I could look for that would indicate the earlier bell housing?

I had a someone drive the truck who has had many of these old trucks, and he said it shifts like a dream haha worse comes to worse I will have to switch out the bell housing I guess.

Re: 3 Speed Bolt out Swap for M20
 

3815891 should be a good casting number for the small hole, but I believe there are others that will work. Unfortunately, the number will be covered when the transmission is bolted on. Once you get the transmission off, it will be just as easy to measure the hole directly.

Ray