Muncie Gear Box

[Aussie Mike]

Hi Guys,

I'm in the process of getting all the parts together while I wait for my kit to arrive. Years ago I sold a Muncie box to a friend but he hasn't used it. I'm going to run an LS1 Chev and thought I might buy the box back to use behind it.

The box has a corse spline input shaft and the original Vette shifter with lift up reverse gate. The shifter is neat and kind of like the original cobra with the reverse gate. Am I in for some trouble with this box? ie reliability, shifting, ratios etc? Will it hold up to the 350HP from the LS1 with maybe more to come?

Cheers

[Jeff Frigo]

Mike:

A Muncie is to Chevy as is Toploader to Ford. These were used in all of the muscle cars during the sixties and early senventies. They were used with LS6 big blocks, the most powerful production engine GM ever built. There is a wide ratio, and a close ratio. Unless you are racing, the wide ratio is better for the street. 350 HP is no problem. You must pay attention to the flywheel, clutch, and bellhousing to get everything to work right.



Jeff

[Aussie Mike]

Thanks Jeff,

I did a bit of ringing around and found out the standard clutch behind the Corvette LS1 uses a 26 spline input shaft. Hays make a clutch and flywheel setup to suit the coarse spline box to the late Chev but that will cost me about AU$1700. That makes a AU$3500 T56 gearbox look a lot more attractive since it'll bolt straight up. Also if I use the 4 speed I'll probably need to change my rear end ratio which is currently 3.45/1. So I figure I'll save my money and buy the 6 speed box or a Tremec and use the original Corvette clutch.

Cheers

[Cal Metal]

I have had experience with both. I wouldn't equate the Muncie with the toploader, though, unless you have an M-22 with the 22 degree helix gears. Muncies are strong. Regarding the M-20 (wide), M-21 (close) beware of the '63 boxes. M-22 gearing is now available and runs about $800.00 which is cheap, as good original boxes are non existent. The Muncie is nothing more than a heavy duty version of the Warner T-10, as the gear layout is essentially the same, with reverse being located in the tailshaft. I would replace the input and mainshaft bearings, as well as the brass synchro rings as a minimum. Most of these boxes that were behind hi-po engine often need 2nd and 3rd, too. None of these boxes, TL or Muncie, will hold up over a sustained period of time if used for drag racing.

[Aussie Mike]

Thanks Cal Metal,

I figure it's going to be an M20 or M21. Is there a way to tell externally which one is which or will I need to pull the side cover off and count some teeth?

Cheers

[Cal Metal]

If the tranny is handy, give me all the numbers off the transmission and I will put it into my decipher machine and tell you what you have. It is not as simple as a Toploader because of the various years they produced them and their ongoing coding procedure changed.

[Cal Metal]

Aussie Mike:

Here might be a short cut. If this is a '69 to '74 tranny, look at the serial number which begins with P (for Muncie). It should look something like: P3D10B.
The last letter is the operative one. If it is A, you have a wide ratio; if is is B it is a close ratio; if it is C you have hit the mother lode because it is a M-22 Rockcrusher. If this is not a '69 to '74 tranny, you will need to get all the data off of it to determine the ratio. Only these years had a desigator using letters.

[Cal Metal]

Aussie Mie:

Just checking in. Any luck with those tranny codes??

[Dominik]

Mike,

I thought the input shaft with 26-spline just calls for a 26-spline clutch blade and there you go.
Bellhousing and pressure plate are the same. Maybe not for the later LS-1 engines ...

Cal,
I used a M-20 behind my very torquey big-block for 7 years under hard driving conditons, not racing, and it held together perfectly.
The only modification was a sealed bearing at the input shaft.

I hated the ratio though. Gap from 3rd to 4th was too wide.

Dominik

[Aussie Mike]

Hi Cal

My friend came over today and he brought the numbers with him.

The number on the box is 3851325 PAT 3083836
On the ectension housing 3846429
and on the side cover 385707

Hope this makes sense.

Hi Dominik

This trans has the coarse spline input shaft, only about 12 or 14 splines. The LS1 comes with a 12" clutch with the standard 26 spline center. One of the problems is I'm not sure if any one makes a 12" friction plate with the corse spline center.


Cheers

[Cal Metal]

Aussie Mike:

It is a '64-'65 box; M-20 Wide Ratio with a 2.56 first ( 1.91 second, 1.48 3rd, 1.00 4th). The input should have NO RING groove around the outside. The spline count should be 10. Tooth count on the input should be 24. If it is 26 it is a close ratio. Also, if you are looking for a M-22, go to the Gearzone and get all new gearing. It is very low cost compared to what these guys want for used boxes which are normally beat to heck and have no residual value.

Dominik:

Didn't mean to sound critical of the Muncie. Have replaced too many gears in all these, including the TL, from out and out drag racing, to feel that any of them hold up well under those conditions for any length of time.

[Specialk]

Mike,

Big Block pickup trucks have a 12" disc and coarse splines. I had a friend with a 12" clutch in a Camaro and when it came time for a replacement he used a truck disc.

Kris

[Dominik]

Cal,

I did not mean to call the Muncie the hero of the day, there is always a possibility to get something better.

I wouldn't hesitate to use a Muncie when I get a cheap one.

I also wouldn't ever use cast pistons again, having lost two engines with them.
I know what you mean - you know what I mean.

I like your information pool about Muncies (and Webers, and ...)

Dom

[Aussie Mike]

Thanks for the excellent info Cal.

Thanks Kris for the heads up on the 12" clutch plate. I'll have to dig out some specs on both clutches and see if it'll fit. I might have to seriously consider using this box now.

This forum is great and an invaluable source of good information.

Cheers

[Sloth]

Hey Mike,

My cousin used to get me bearing re-racing kits for these boxes pretty cheap. I used to run them in a string of Holdens, so if you want to replace the bearings, give me a hoy.

cheers

sloth

[Aussie Mike]

Thanks for the offer Sloth,

I'll have to see what it needs when we pull it down.

Cheers