EZ-SPEC Form

Your INPUT SPLINE and CLUTCH HOUSING
Your Maindrive (Input)
General Motors muscle used 10 and 26 spline clutches and we supply maindrives (inputs) for both. if you have a choice, prefer the fine spline as it's stronger. Our optional 26 spline includes a torsional cushion to moderate shock spikes and support for an oilseal to keep 90 weight oil off your clutch or floor.
Your Clutch Housing
General Motors performance four-speeds were delivered behind clutch housings with two different pilot bores --- 4.190 inches in 1963, and 4.680 inches otherwise. The first was a bad idea you don't want, but you might benefit from an option GM didn't offer, one of their 'truck' housings with the 5.125 inch bore --- it accepts larger clutches. We support both of the larger sizes.

Your OUTPUT SPLINE
General Motors used 16, 27 and 32 spline mainshaft (output) splines during the muscle car era. The first was hopelessly overloaded by even small V8s and we do not offer it. If you're replacing an early T10 or a three-speed with 16 splines, step up to the 27. The large 32 spline mainshaft arrived in 1970 and with it the driveshaft length, shifter mounting location and shifter support hardware changed. Unless your duty requirement demands the larger mainshaft use the spline appropriate to your car year and model. The 27 spline mainshaft is almost always adequate for street use.
Your Mainshaft

Your SPEEDOMETER DRIVE
The gear that drives the speedometer is assembled on the mainshaft (output) and needs tailoring to your axle ratio and tire diameter. We'll do the math for you (no entry required).
Your Speedometer Drive Ratio Target
Pencil Gear Teeth
Pencil Gear Age #
Circle Gear Teeth
Circle Gear Age #
Your Speedometer Drive As Recommended

Your DUTY LEVEL
We offer three duty levels --- our 'Standard Duty' M20 for mild street applications, M22-based 'Heavy Duty' gearboxes for tuned street and street/strip combos, and an Auto Gear exclusive 'Severe Duty' platform we call an 'M23' for the meanest street motors and dedicated competition chassis. We'll make the recommendation for you (no entry required).
Standard Duty
Heavy Duty
Severe Duty

Your LOW GEAR
Back in the day you could order almost anything that would bolt together and not every car was intended for the street; 'Numbers matching' doesn't always mean street drivable. For general use --- starting a car with a mild street motor and typical tires on a hill or parking garage ramp without slipping the clutch --- General Motors aimed for a total reduction of 9:1 (transmission first gear multiplied by the axle ratio). We'll use that and your application profile as a starting point and make the recommendation for you (no entry required).
Your Low Gear Target
Your Low Gear Recommendation

Your GEARSET
Your engine has a preferred rev band and, to stay within it, you might aim for similar rev changes across shifts; this is called 'geometric' gear spacing. However, road and circle track racers build their gearsets 'top-down' with wider first/second spacing and dragsters build 'bottom-up' with wider third/fourths. We'll use our experience and your application profile to select the gearset for you (no entry required).
Your Gearset

Your LOW GEAR SUPPORT
Muncie four-speeds have a removable steel sleeve supporting low gear. Our gearboxes offer an improved version of General Motor's basic design or a roller bearing alternative. We recommend the bearing for all but cars seeing significant strip time --- it's mechanically more efficient. Dragsters want our improved sleeve because it puts more surface under the shock when launching the car (flattened rollers don't roll). We'll make the recommendation for you (no entry required).
Steel Sleeve
Roller Bearing

Your SURFACE FINISH
Racers looking for an edge 'superfinish' the gears and wearing parts of their transmissions to reduce friction and accelerate break-in. Our option covers ten items: the maindrive (input), mainshaft gears, countershaft cluster, sliding clutches, shift forks and mainshaft (output).
Superfinish
- Yes
- No
Email
This field is for validation purposes and should be left unchanged.

First, we'll need to know about your ride: