Matched 3.73 ring and pinion for the GM 8.875 inch 12-bolt car rear axle. Fits 3.07-3.73 series carriers; Richmond EXCel 12BC373T.
The Richmond EXCel 12BC373T is a 3.73 ring and pinion for the GM 8.875 inch 12-bolt passenger car rear, the axle builders want under a 1967-1972 Camaro, a 1964-1972 Chevelle or El Camino, a 1969-1972 Nova or a big Impala. Owners shop for it for two reasons. Either the original gears are worn and whining after fifty years and several engine swaps, or the car still has a tall 2.73, 3.08 or 3.31 highway ratio that makes a built small-block or big-block feel lazy below 40 mph. A 3.73 is the middle ground: real launch without the freeway screaming of a 4.10 or 4.56.
The 12-bolt name refers to the twelve bolts that hold the 8.875 inch ring gear to the carrier. In this set the ring gear has 41 teeth and the pinion has 11, which works out to 3.727 to 1, sold as 3.73. The pinion has a 30 spline yoke end and a 1.625 inch shaft, and Richmond cuts both gears from 8620 steel and laps them together, so they install only as a matched pair. Behind a three-speed automatic like a TH350 or TH400, a 3.73 keeps the engine in its torque band leaving a light; with an overdrive swap such as a 700R4 or 4L60E it brings cruise rpm back down while first gear stays strong.
Many of these cars left the factory with a 10-bolt, and many have been swapped since, so identify the axle before anything else. The car 12-bolt has an irregular cover with a bulge at the top and differs from both the 8.5 10-bolt and the truck 12-bolt; our 10-bolt vs 12-bolt differences guide covers the tells. Then check your current ratio by counting the ring and pinion teeth or using the method in the axle ratio and code guide. Carrier series is the critical check on this axle, explained below.
Richmond lists 12BC373T for the GM 8.875 inch 12-bolt car rear with standard rotation, and notes that it requires a 3.07 to 3.73 ratio carrier. If your car runs 3.07, 3.31, 3.36, 3.55 or 3.73 gears now, your carrier takes this set. If it currently has 3.90 or deeper gears, it has the other carrier series, and the ring gear will not sit at the right height without a carrier swap; our carrier break guide explains why. Richmond's application list covers the 1967-1972 Camaro and Firebird, 1964-1973 Chevelle, 1965-1972 El Camino, Impala and Caprice, 1965-1968 Chevy II, 1969-1972 Nova and 1970-1972 Monte Carlo, plus a long list of 1965-1972 Buick, Oldsmobile and Pontiac models. Many of those B-O-P cars came with their own 12-bolt axles that take different gears, so confirm you have the Chevrolet 8.875 car rear by the cover and ring gear before ordering, or send us your year, model and VIN. It does not fit the truck 12-bolt or the 9.5 inch 12-bolt. Compare the 3.90 set for the same axle if you want more launch, and keep in mind it falls on the other side of the carrier break.
This is a setup job. Replace the bearings and races with the Timken 12-bolt car master bearing kit or the Yukon YK GM12P overhaul kit, which add a new crush sleeve, pinion seal and shims. Revolution's published 12-bolt car starting numbers are a 0.030 in. pinion shim, 14-19 in-lb pinion bearing preload, 0.006-0.010 in. backlash, 55 ft-lb on the ring gear bolts and 60 ft-lb on the bearing caps; read the contact pattern with marking compound and adjust depth from there. Fill with 75W-90 gear oil, with friction modifier on Positraction units. Drive the first miles gently with a few heat cycles, then change the oil, following our ring and pinion break-in procedure. While the axle is apart, the Dura Grip posi for 30 spline 12-bolt cars is the traction upgrade that matches this carrier range.
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Doing this repair right usually means replacing these at the same time. Same transmission, ships together.
Only if it is the 3.07 to 3.73 series carrier. If your 12-bolt currently has 3.07 through 3.73 gears, it will fit. If it has 3.90 or deeper gears, you need a carrier for the 3.73-and-numerically-lower series.
No. The car and truck 12-bolts share a ring gear size but use different gears and housings. This set is for the 8.875 inch passenger car 12-bolt only.
Richmond lists a 41 tooth ring gear and an 11 tooth pinion for a 3.73 ratio, a 30 spline pinion, a 1.625 inch pinion shaft and 12 ring gear bolts, made from 8620 steel.
All carrier and pinion bearings and races, the crush sleeve, the pinion seal and the ring gear bolts. A master bearing or overhaul kit covers these, and it is the easiest way to keep new gears quiet.
Drive 15 to 20 easy miles, let the axle cool fully, repeat a few times, avoid hard launches and towing for the first several hundred miles, then change the gear oil.
Yes. With a 700R4, 200-4R or 4L60E, a 3.73 gives strong first gear launch and a reasonable cruise rpm in overdrive, which is why it is the common pick for those swaps.
