Swap Science: How to Make Transmission Mounts

By Carl Heideman
Sep 26, 2018 | Mazda, MG | Posted in Shop Work , Drivetrain | From the Aug. 2015 issue | Never miss an article

Photography by Carl Heideman and Alan Dalman


Those cool engine mounts we fabricated in the last installment are only part of the story. That engine swap is going to need some custom gearbox mounts, too. 

Our swap project involves fitting a 1994 Miata drivetrain into a 1967 MGB GT body. So far, we’ve planned the swap and made the engine mounts. Now it’s time to fabricate that gearbox crossmember. 

And don’t worry if you’re working with a different car and engine: The particulars may be different, but the general process will likely be similar. All right, let’s do this.

Step 1: Determine A Mounting Point


First big question: Where on the Miata gearbox did we want to attach our mounts? Miatas don’t have traditional rubber rear mounts; instead, a larger powerplant frame ties the gearbox to the differential. There’s a smaller bracket mount at the extreme tail of the gearbox, but we were concerned that using it could break the tailshaft housings under stress. We decided to make a mount adapter for the PPF connection.

Step 2: Pattern and Rough-Cut an Adapter


A few measurements and a cardboard pattern set us on our way. This rough-cut piece of 1/2-inch plate was ready to bolt to the gearbox as our adapter.

Step 3 Choose A Mount


We decided to use a traditional rubber gearbox mount. Sticking to our single-source principle, we wanted either an MGB part or a Miata part. MGBs normally use two of the mounts pictured here–the center one and the one on the right. Since we preferred a single mount, we considered a beefier engine mount from a rubber-bumper MGB or a Miata. Neither option would give us the clearance we needed, though, so we went back to the single MGB gearbox mount–the one on the right. Here’s our justification: On the stock MGB, the mounts are angled and in shear, but our installation will have the mount in compression. In short, it should hold up fine.

Step 4: Trace, Test-Fit, Adjust


We test-fitted our adapter to the Miata gearbox, traced the gearbox mount, and made a few notes on our pattern.

Step 5: Mark and Drill Holes


Happy with the fit, we got the adapter ready to accept the MGB gearbox mount. We used a transfer punch to mark the holes before drilling them.

Step 6: Thread the Holes


We tapped the holes with 5/16-24 threads to accept bolts for the mount.

Step 7: Make It Pretty


Although our adapter was functionally finished, we didn’t like the rough look. We cut away some of the excess material and radiused the corners. Nobody will see it, but we’ll know it’s there.

Step 8: Test-Fit The Adapter... Again


There is no such thing as too many test fittings. We checked to make sure our adapter fit the gearbox one more time.

Step 9: Design a Crossmember


With the adapter finished, it was time to consider a bolt-in crossmember to hold the gearbox. A little time with a tape measure, some 11/2x11/2-inch angle iron, 1-inch DOM tube, and 1/4-inch plate yielded the pieces for our crossmember.

Step 10: Bolt On The Angle Iron


We bolted the angle iron pieces to the original factory mounting locations.

Step 11: Tack-Weld The Crossmember


We then tack-welded the center of the crossmember together.

Step 12: Mark The Tubes For Placement


The scribed line showed us where we needed to place the tubes. Proper placement was important here for a couple of reasons: It set the 3-degree slope of the engine and gearbox, which is necessary for a properly functioning set of U-joints, and it ensured good clearance for the exhaust.

Step 13: Clamp The Center Section


We clamped some scrap angle iron on both sides before tack-welding so our center section wouldn’t move.

Step 14: Place The Tubes


This shows why we placed the tubes up high. Note the C-notch for the exhaust in the rear subframe crossmember. We wanted our crossmember no lower than that notch so the exhaust would have sufficient clearance.

Step 15: Alternate Ending


We were originally going to use this piece of rectangular tubing as our crossmember. This route seemed so easy at first, but we soon realized how much work it really entailed: We’d have to modify the tubing to get the gearbox at the right height and add a C-notch for the exhaust. It also weighed about a pound more than our small tube crossmember. The salient point: Sometimes we scrap a plan and change course when have to make too many compromises.

Step 16: Test-Fit and Finish Welding


After tack-welding, we did one more test fitting before the finish welding. Next time, we’ll turn to other aspects of our engine swap.

Read The Whole Swap Science Series:

  1. Swap Science: 14 Steps Before You Swap That Engine
  2. Swap Science: How To Build Custom Engine Mounts From Scratch
  3. Swap Science: How To Make Transmission Mounts
  4. Swap Science: How To Get Your Clutch Setup Just Right
  5. Swap Science: How To Make A Functional Gas Pedal
  6. Swap Science: How To Hang An Alternator
  7. Swap Science: Engine, Steering, and Exhaust Fitment
  8. Swap Science: Keeping Things Cool by Building a Cooling System
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81cpcamaro Dork
9/26/18 11:19 a.m.

Good article and good timing, as I will be doing a similar mount with my MGB V6 swap. I like the tubing vs the heavier rectangular setup.

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