A Slight Alteration

Summit Racing’s Cool Adjustable Transmission Mount

By Jeff Smith   –   Photography by the Author

It’s no secret that the LS engine is probably the hottest idea in engine swapping since the introduction of the small-block Chevy in 1955. The LS has found its way into just about every GM car on the planet. Part of the engine’s popularity is based on the widespread use of custom motor mounts that move the engine into non-stock positions. Often this is necessary for oil pan or header clearance or perhaps to clear components like the factory accessory drive.

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001 Summit Racing adjustable trans mount
This Summit Racing adjustable trans mount comes with a ¼-inch steel flat plate that bolts to the transmission. The U-shaped slide portion is also steel and fitted with a pair of 7/16-inch-diameter studs welded to the body. The large cross bolt is 12 mm. All fasteners employ Nyloc nuts.

When the engine is relocated this also affects the position of the rear trans mount to the crossmember. Generally, this requires either modification to the crossmember or sometimes building or buying a custom crossmember to complete the connection to the transmission.

Recently Summit Racing created one of those simple yet highly effective products that may save engine swappers the hassle of building a custom crossmember. The solution is a simple GM trans mount with an integrated 2½-inch adjustable slot. The mount is built from ¼-inch-thick steel that is nicely powdercoated satin black and uses a large 12mm lateral bolt encased with a polyurethane mount. Adjustment is accomplished by positioning the mount in the proper place within slots cut in the mount.

002 disassembled the mount for a better view of the components
Here we’ve disassembled the mount for a better view of the components. We’re not sure why the large through bolt is metric–perhaps because it fits tighter through the polyurethane mount. If this is annoying, you could probably squeeze a ½-inch-diameter bolt 3½ inches long in its place.

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We measured our mount and it appears we might be able to get a touch more than 2½ inches of adjustment out of this device. We bolted this unit to a Muncie four-speed just to see how it would fit. We also played around with relocating the U-shaped mount 180 degrees to get a little more adjustability out of it while leaving the flat portion of the mount in its original location on the tailshaft. There may be applications where this might not be possible as we did not check this on all the different manual and automatic transmissions.

003 adjustable mount all the way back
Here we’ve slid the adjustable mount all the way back (Photo A) and then all the way forward (Photo B) with a range just slightly more than 2½ inches. As you can see, the shortest adjustment doesn’t line up with the factory mount but you could easily flip the slide and adjust it to align properly because the studs are offset relative to the slide. Photo C shows the mount in the middle of its adjustment range.

004 all the way forward 005 mount in the middle of its adjustment range

We’ve run into situations where even stock components create a tolerance stack up that won’t allow the rear trans mount to bolt up and we’ve had to perform minor elongation of holes to line everything up. This mount would solve all of those issues, but this adjustable mount is really aimed at engine swapping where the engine has been moved relative to the “stock” location. Because of the way the mount is designed, it also pivots fore and aft slightly to accommodate the inevitable slope created by the engine and transmission placement. This removes any binding from the mount and reduces stress on the transmission case.

006 flipped the slide mount 180 degrees
In this orientation, we kept the base (the part that bolts to the transmission) in a fixed position and flipped the slide mount 180 degrees. This creates a greater range of adjustment.

007 greater range of adjustmentAnother advantage is that this mount fits a broad range of GM manual and automatic transmissions because our favorite general has retained a consistent bolt spacing for the transmission mount that dates back to the first aluminum Powerglides in the early ’60s all the way through to the 4L60E and 4L80E transmissions. This is a 3 ¾-inch bolt pattern. There appear to be some TH400 transmissions with a slightly wider 4¼ inches but the Summit mount accommodates that slight change with elongated mounting holes.

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This Summit Racing adjustable mount will, of course, also work with the older small- and big-block Chevys but it’s really aimed at those engine swappers who tend to move LS engines within the chassis for various reasons. Here’s one case where a “universal” part really does work solving a multitude of minor alignment headaches.

008 bolted the trans mount to a Muncie four speed
Here we’ve bolted the trans mount to a Muncie four-speed to show how it mounts to the transmission. Again, you can vary the position of the mount by switching the U-shaped cover of the mount 180 degrees to get maximum coverage.

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Trans Length Chart: GM Trans Mount Length Chart

These specs are the distance from the bellhousing face on the transmission to the centerline of the rear transmission mount. These distances are not exact numbers as our research has revealed multiple lengths for each transmission that we’ve not listed here because we can’t verify their accuracy. It’s best to measure your specific transmission to determine its actual rear trans mount position. But with these numbers you can at least get an idea of the relative differences in trans mount position.

Transmission Distance (inches)
Muncie 4-speed 20¼
Super T-10 20½
TH350 20 3/8
TH400 24 5/16
200-4R 27
700-R4 22½
4L60E (early) 24 1/3
4L60E (LS) 26
4L80E 30 5/8


Summit Racing
(800) 230-3030

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