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Project Corvette front differential mount replacement

Too many nights have we looked up at the floorboards in Project Corvette, scrubbing or bench-pressing heavy parts back in place. Many of these projects have consumed an inordinate amount of time and effort, sometimes stretching weeks to accomplish.

Replacing a C3 Corvette front differential mount is fairly easy, once the driveshaft is out. Watch out for the brake line though.
Photos by author, Aaron Ahlstrom

So it’s refreshing to see something that can actually be fixed relatively easily. We spotted the worn, original front differential mount while doing some suspension work and it seemed like a straightforward procedure.

Besides, we needed a quick-fix to restore our sanity. These so-called little triumphs are sometimes all that keep a restorer from going nuts when their knee-deep in multiple, more-extensive projects.

Here we can honestly say this tutorial is a true bolt-in, a quick-fix by C3 (’68-’82) repair standards.

It is highly recommended to remove the driveshaft in order to do this swap. We've heard stories from online friends and forum members insisting it can be done with the ‘shaft in place, however, we can’t confirm this. Since our TH400 and driveshaft were out being rebuilt, we had room to maneuver. With the ‘shaft in the way we’re not so sure the bolt would be able to clear.

This project can be accomplished with regular hand tools with jackstands. We stopped periodically along the way to snap pictures, which took longer. Most would be able to install the replacement part in about an hour.

Keep in mind that later C3s (‘80-’82) utilize a revised aluminum IRS (Independent Rear Suspension). For this article, only Sharks from ’79 and earlier apply.

Follow along as we replace the worn, original front differential mount on Project Corvette.

A little about diffs
Unlike most traditional solid-axle muscle cars where the differential is retained by trailing arms and coil springs (Chevelle) or sprung by a pair of leaf springs (Camaro), the Shark is an entirely different breed. Though the C3 diff is bolted to a crossmember above it, the IRS really takes care of supporting the rear end through its trailing arms and half-shafts (think small driveshafts).

Solid-axle cars also can possess a pinion “snubber” (usually bolted above the pinion) which acts as a lever to counteract axle wind-up. This snubber is usually made of rubber (or poly) and of a predetermined height so when it arches and contacts the floorboards, the axle keeps a certain amount of pinion angle.

For our purposes, the C3 has no pinion snubber. Still, many incorrectly refer to the front differential mount as this, even though the bracket and mounts are located below the pinion and are bolted to a crossmember.

The front diff mount is important for while it does not solely support the axle, it does cushion the suspension from the shock of accelerating and regular driving.

Where to look?
The front differential mount is located out back. Ahead and just under the axle yoke, where the driveshaft connects, is a metal bracket. This bracket holds two rubber cushions (sometimes called “pucks” or “biscuits”). A bolt passes through these bushings and sandwiches them around a frame crossmember. In a simplified sense, the front differential mount acts as a centering point.

Eventually these rubber cushions deteriorate from a combination of time and the elements. All the horsepower and torque transferred through the driveline usually takes its toll on the top cushion first, as it is slightly smaller in size. Though both mount cushions were shot, Project Corvette made no audible clunks or suffered any bad handling characteristics.

Since the front suspension had been completely overhauled, the rear was thoroughly inspected. Everything checked out fine (sans diff mount) before having a professional 4-wheel alignment, so crucial with these C3s. (Surprisingly, we found the strut rod bushings and rear spring bolt bushings to be newer poly units.)

There’s no downside to replacing this little part. In fact, it may even help the ride quality.

Project Corvette sports stock, factory-style engine and transmission mounts. These rubber mounts are the preferred material for OEM vehicles because of their relative low cost and good dampening ability. Engine and transmission mounts should also be of the same type of material. It is not advisable to mix and match, say, poly motor mounts with a rubber trans unit. In addition, solid mounts should be reserved for racing, as they contribute a significant amount of vibration and noise through the chassis.

It makes sense, then, to further use rubber for the front diff mount. At least, that’s what most Stingray owners told us. And most of them endorsed the NCRS (National Corvette Restorers Society) factory-correct, softer material.

In contrast, keeping Project Corvette completely stock wasn't one of our priorities. We had previous experience using poly mounts with good results. Under normal circumstances we've yet to encounter a problem with poly, successfully using them on numerous small and big block Chevy motors and under plenty of TH350 and TH400 transmissions.

Poly construction no doubt lasts longer – better resistance to oil, dirt and heat – and will perform better when pushed to the extreme in harsher conditions (i.e. drag racing, autocross, etc).

As with everything C3 related, more research was needed to reach a decision. After conferring with numerous forums, online parts suppliers, and C3 owners, the consensus was this: Unless you are all-out racing or track-testing the curves, rubber mounts (for the motor, trans, diff) are the way to go in a Stingray.

Drop the puck
With the driveshaft out, access to the differential pinion is much improved. It also makes it easier to see what you are doing. Lighting can be an issue, especially when the vehicle is hoisted up by relatively short jackstands. A few small handheld lights were used to illuminate the working area.

Project Corvette still had its original mount. To unseat the mount, we placed a stubby box end wrench up top and a socket on the bottom. With the nut removed from below, the bolt was tapped up and out.

A flat pry bar braced against the frame crossmember separated the mount from the metal bracket. Be careful to avoid prying on the metal brake line, though.

Surprisingly, our mount popped out with hardly any resistance, probably due to the fact that the factory rubber pieces were nearly cracked in half. The upper mount, which takes the brunt of the abuse, was smashed flat like a pancake.

Installing the new, thicker unit in the lower mount was simple: leverage the pinion down with the crowbar and push the "puck" in place. Slip the top rubber piece around the metal bushing sleeve (looks like a silver top hat). Then drop in the bolt, washer, and top mount assembly from above. Fit the washer and nut underneath. Though we found differing amounts, according to our records, 65-foot/lbs was the preferred torque setting.

Corvette Central supplied the replacement parts. The reproduction kit was of good quality and the rubber cushions were identical to the factory part and fit as intended. However, we did notice the repo bolt was slightly longer. This didn't affect our project, but those looking to aim for a 100-percent concourse-correct piece may choose to clean or restore their original hardware.

For more information about Project Corvette and C3 restoration, check out the links below.

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