Get­ting to know Chevro­let’s most legendary four-speed trans­mis­sion

Chevy High Performance - - Contents - TEXT: Jim Smart | PHO­TOS: the Author AND Sum­mit Rac­ing Equip­ment

Get­ting to know Chevro­let’s most legendary four-speed trans­mis­sion

Few things ex­cite like the sound and feel of an old-fash­ioned Chevro­let mus­cle car and that fa­mil­iar whine of a clas­sic Muncie four-speed trans­mis­sion go­ing through the gears. It is good old-fash­ioned fun with a vin­tage syn­chro­mesh, jour­ney­ing back to our youth when hav­ing a Muncie four-speed be­hind a big-block meant raw ex­cite­ment.

Ac­cord­ing to Paul Can­gialosi, Muncie trans­mis­sion his­to­rian, tech­ni­cian, and author of Muncie 4-Speed Trans­mis­sions: How to re­build and mod­ify, the Muncie four-speed trans­mis­sion has its roots back to 1935 and the U.S. patent num­ber 3,088,336 along with an en­gi­neer named James W. Fo­drea. This patent num­ber, ac­cord­ing to Can­gialosi, can be found cast into most Muncie four-speed main cases. Fo­drea’s legacy is far-reach­ing, ex­tend­ing into pop­u­lar trans­mis­sions around to this day, yet most have no idea who he was.

Ac­cord­ing to Can­gialosi, two com­pa­nies, Borg and Beck and Warner Gear, merged in 1928 to form BorgWarner. The T-85 three-speed trans­mis­sion was one re­sult of that merger. The T-10 four-speed was an evo­lu­tion of the T-85 three-speed trans­mis­sion, with both of these trans­mis­sions be­ing quite sim­i­lar in ap­pear­ance. If you’ve ever looked at a BorgWarner T-10 and a Muncie four-speed and got­ten them mixed up, you are not alone be­cause the Muncie (M20, M21, and M22) is a di­rect de­scen­dant of the T-10 ac­cord­ing to Can­gialosi. The Sag­i­naw four-speed box is quite sim­i­lar to both the T-10 and Muncie four-speed trans­mis­sions. How­ever, nei­ther com­pares to the Muncie in terms of strength.

That the Muncie has much in com­mon with the T-10 is no accident.

It was a mat­ter of eco­nom­ics when Chevro­let needed a four-speed man­ual trans­mis­sion for the Corvette in the mid-1950s. Ac­cord­ing to Can­gialosi, GM took the basic T-10 de­sign and beefed it up to con­ceive the Muncie M20 and M21 four-speed trans­mis­sions, which ar­rived in 1963.

GM’s goal with the Muncie four­speed was to pro­duce a bet­ter shift em­ploy­ing larger syn­chro cones. He tells us both the M20 wide-ra­tio and M21 close-ra­tio trans­mis­sions were first of­fered in 1963. The M22 Rock Crusher would come later to ac­com­mo­date the heavy twist of Chevro­let’s big-blocks in the mid-1960s.

The M20 and M21 trans­mis­sions pro­duced from 1963-’74 are easy to iden­tify in both wide- and close-ra­tio units. The M22 to come later in 1967’74 is also a close-ra­tio unit, but much stronger than the M21, with a higher torque ca­pac­ity.

Be­cause the Muncie has so much in com­mon with the T-10 and even

sim­i­lar Sag­i­naw boxes, it is easy to get this guy mixed up with nonMun­cie units. Al­though the Muncie’s blood­line goes way back, this legendary gear­box en­tered ser­vice for the Chevro­let Di­vi­sion in 1963. In fact, 1963 is a stand­alone year be­cause changes in this box came al­most im­me­di­ately for 1964, be­gin­ning with the front bear­ing re­tainer, which was alu­minum for 1963 and be­came ca­st­iron in 1964. The bear­ing bore size was also one year only with a #6207style in­put shaft bear­ing, ac­cord­ing to Can­gialosi.

The early pro­duc­tion Muncie four­speed case is the small-bore unit and can be iden­ti­fied by the GM cast­ing num­ber 3831704 (see Muncie M20/ M21/M22 Main Case Iden­ti­fi­ca­tion chart). The large-bore Muncie came into pro­duc­tion in 1964-’65 and be­came the stan­dard from then on through the end of pro­duc­tion in

1974. What makes the small-bore and large-bore case Mun­cies dif­fer­ent is the in­put shaft bear­ing size and first gear, which rode on the main­shaft void of a bush­ing for 1963. For 1964 and be­yond, it rode on a bush­ing be­tween it and the main­shaft.

Can­gialosi ex­plains there were two basic main­shafts pro­duced for the M20, M21, and M22 trans­mis­sions (see In­put Shaft Iden­ti­fi­ca­tion chart). From 1963-’70, the Muncie was fit­ted with a 27-spline main­shaft and must have the cor­re­spond­ing slip yoke. For 1971-’74, a larger di­am­e­ter, 32-spline main­shaft was em­ployed along with the cor­re­spond­ing yoke. Be­cause this main­shaft is larger, you’re also go­ing to need larger bush­ings and seals. Keep this in mind when you’re search­ing for a Muncie trans­mis­sion.

Can­gialosi tells us all M20 and M21 trans­mis­sions were pro­duced with

both 10- and 26-spline in­put shafts. The 26-spline in­put shaft came into pro­duc­tion later in the 1970s for the M20 and M21.

What you will find in your search for the right Muncie four-speed is con­flict­ing in­for­ma­tion from dif­fer­ent sources. Be­fore you here are the ba­sics of Muncie M20, M21, and M22 four-speed trans­mis­sions. Glean the ba­sics and use them in your re­search. Ex­pect to find var­i­ous com­bi­na­tions out there be­cause these high­per­for­mance four-speed trans­mis­sions have been thrashed, trashed, and re­built through the decades. You’re go­ing to find var­i­ous com­bi­na­tions of main cases, tail­shaft hous­ings, and side cov­ers cou­pled with vari­a­tions in­side of each case. Pure dumb luck will lead you to a com­pletely un­mo­lested Muncie.

One more way to iden­tify the

Muncie is via stamped codes in the case, which in­di­cate when the trans­mis­sion was built at Muncie

(see How To Read Muncie Build Date Codes chart). Ac­cord­ing to Can­gialosi, the date was based on model year, not nec­es­sar­ily the cal­en­dar year.

Of course, re­stor­ers and re­builders will tend to re-stamp the date code to suit a par­tic­u­lar restora­tion. This can add confusion to what you’ve found. Muncie trans­mis­sions also tend to get per­for­mance im­prove­ments such as a gear ra­tio change and stronger in­ter­nal parts. This is when you have to re­move the side cover to see what’s in­side. Ex­am­ine the in­put and out­put shafts to see if you’ve found the box you want.

And fi­nally, if forced to choose be­tween a Muncie or the BorgWarner T-10, it is sug­gested you choose the Muncie due to its brute strength.

The Muncie was orig­i­nally a stronger al­ter­na­tive to the T-10 and Super T-10 be­cause it could take the torque. If you find an M22, you have the ul­ti­mate Muncie de­signed for the high torque of a big-block that will bolt to your small­block. CHP

The clas­sic Muncie four-speed trans­mis­sion in all its many forms gen­er­ally looks like this (this one pro­vided by Ana­heim Gear), though ap­pli­ca­tions will vary in ap­pear­ance. Cases are ei­ther small-bear­ing (1963) or large-bear­ing (1964-’74). The cases of­fer a broad range of in­ter­change­abil­ity. Muncie trans­mis­sions from 1971-’74 have the larger 32-spline out­put shaft.The main case will sport a sev­endigit cast­ing num­ber (found above the US Patent No), which is not a GM part num­ber. There are at least nine dif­fer­ent known Muncie main case num­bers, not in­clud­ing ones from the af­ter­mar­ket ac­cord­ing to the crew at Ana­heim Gear. Author and tech­ni­cian Paul Can­gialosi tells us the orig­i­nal U.S. patent for the Muncie four-speed was filed on Novem­ber 29, 1957, and approved May 7, 1963. Main cases man­u­fac­tured prior to May 7, 1963, will say Patent Pend­ing. Any­thing af­ter that time pe­riod will say US Patent No 3088336.

If you ex­am­ine the 1966-’70 wide-ra­tio M20 in­put shaft, there are two grooves cut amid the 10 splines. The 1963-’65 M20 in­put shaft will not have grooves. And keep in mind that not all will sport these grooves. From 1971 through the end of pro­duc­tion in 1974, the M20 got the 26-spline in­put shaft.

This is the M21 10-spline in­put shaft with a sin­gle groove and the ap­pli­ca­ble 27/22/19/17-tooth coun­ter­shaft.

The guys at Ana­heim Gear tell us there are seven to­tal Muncie in­put shaft options. Shown here is the M20 10-spline wide-ra­tio in­put shaft and counter gear. There is also a fine-tooth 26-spline in­put shaft em­ployed from 1971-’74.

The Muncie four-speed, like the T-10, is a side­load­ing trans­mis­sion. Re­move the side cover and forks for a look at the geartrain. Re­moval of the front bear­ing cover is where dis­as­sem­bly be­gins. This frees up the geartrain.

From left to right are the three basic Muncie in­put shaft/coun­ter­shaft com­bi­na­tions. On the left is the M20 coun­ter­shaft and 10-spline in­put shaft. In the mid­dle is the M21 coun­ter­shaft and in­put shaft. On the far right is the M22 26-spline in­put shaft. In the 1970s, vir­tu­ally all Muncie four-speeds had 26-spline in­put shafts.

Here’s a typ­i­cal Muncie gearset con­sist­ing of First, Sec­ond, and Third gear.

The M22 Rock Crusher 26-spline in­put shaft sports a wider gear for greater torque ca­pac­ity. Just be­cause you’ve found a 26-spline in­put shaft does not mean you’ve found an M22 in­put shaft, which is made of a stronger al­loy. The stan­dard 10-spline in­put shaft found its way into new Muncie M20 and M21 trans­mis­sions from 1971-’74.

Side-by-side are the two coun­ter­shaft sup­port sizes, which are 7/8-inch (left) and 1.000-inch (right).

On the left is the M20 coun­ter­shaft, which is eas­ily iden­ti­fied by the larger 29-tooth gear. On the right is the M22 coun­ter­shaft with 27-teeth. When you have the two side-by-side it be­comes ob­vi­ous which is which.

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