Eclectic Motorworks
eclecticmotorworks.com
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.
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.
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.
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.
We test-fitted our adapter to the Miata gearbox, traced the gearbox mount, and made a few notes on our pattern.
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.
We tapped the holes with 5/16-24 threads to accept bolts for the mount.
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.
There is no such thing as too many test fittings. We checked to make sure our adapter fit the gearbox one more time.
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.
We bolted the angle iron pieces to the original factory mounting locations.
We then tack-welded the center of the crossmember together.
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.
We clamped some scrap angle iron on both sides before tack-welding so our center section wouldn’t move.
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.
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.
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.
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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