I recently had an opportunity to bolt an aluminum aftermarket manual trans bellhousing to the back of an LS engine for part of a project. I measured the bellhousing centerline run out and was surprised to find this bellhousing substantially out of spec. It took several tests before I decided to check the vertical alignment of the transmission mounting flange to the engine. This is when I discovered that the bottom of the bellhousing was not parallel and was in fact, angled rearward at the bottom by 0.011-inch.

LS engines are designed to use the factory cast-aluminum oil pan as a structural member that is bolted to the bellhousing to create a 360-degree attachment arrangement. When I rebuilt the LS engine, I added a Holley cast pan to allow fitment into my ’66 Chevelle. It soon became clear that I had not installed the oil pan correctly, and its rearward position was forcing the bellhousing in that same direction.

The correct resolution to this problem would be to remove the oil pan and relocate it correctly with the pan’s vertical portion exactly aligned with the bellhousing flange. If the oil pan is too far forward, bolting the bellhousing to the oil pan would tend to draw the bottom of the bellhousing forward and cause misalignment.

It’s critical that the vertical portion of the LS oil pan does not extend past the engine block’s vertical bellhousing flange. If the oil pan extends rearward past the bellhousing flange, this will directly affect bellhousing alignment. This is true for both manual and automatic transmissions.

While we discovered this problem with a manual-transmission bellhousing that did not offer attachment points to the oil pan for an LS engine, it does point out that this would be an issue when using an LS version automatic transmission like a 4L60E. In this situation, an incorrectly installed oil pan would create misalignment problems with the input shaft and torque converter that could cause other problems that might surface as damage to the input shaft, torque converter flange bushing, or perhaps a leaking front pump seal.

We used a dial indicator on the face of the bellhousing flange to check vertical alignment. If the transmission mounting face is not parallel with the block, this causes a misalignment that cannot be repaired with dowel pins. The face must be within 0.005-inch of parallel.

This is a critical issue that LS engine builders must not overlook when installing the oil pan. The ideal situation would be to accurately check the position of the oil pan relative to the block. This is not easy to do but can be accomplished with a straightedge across the rear face of the oil pan and the block.

Yes, this will add to the time required to build the engine compared to a small-block Chevy for example. But this will also reward the builder with a package that will perform as expected.