We learned this lesson only after the clutch wouldn’t release properly on our small-block and Muncie four-speed El Camino. We had previously installed a new small-block Chevy engine and used an 11-inch factory aluminum bellhousing from an early Chevy pickup. We needed the larger, 1-inch bellhousing to clear the larger 11-inch flywheel and clutch assembly. After we removed the trans, bellhousing, and clutch assembly, we discovered the pilot bushing was nearly wiped out. This seemed odd since it had been in place for less than a thousand miles.
This set us on a course of investigation that was eventually resolved when we mentioned the problem to our friend and muscle car resto artist Frank Saenz. When we mentioned that we were using the truck bellhousing, he informed us that those ‘70s-era 11-inch truck bellhousings used a larger input shaft bearing support opening compared to a typical Muncie or Super T-10 style passenger car bellhousing. Sure enough, when we measured our housing, we found the opening was substantially larger at 5.125-inches instead of the passenger car spec of 4.68-inch.
Once we compared the truck bellhousing to a stock passenger car bellhousing, we could actually see that the truck version opening was clearly larger. The reason our four-speed did not shift properly was because the larger input opening in the truck bellhousing caused a huge misalignment between the transmission and the crankshaft which in turn placed the pilot bushing in a bind. This is why the pilot busing wore out so quickly.
Before we realized the input support hole was larger, we checked alignment of the truck bellhousing and it was only slightly out of spec at around 0.009-inch of run-out. This was puzzling until once we realized that the opening was oversized. That explained why the clutch wouldn’t release properly because the input shaft was misaligned.
Our solution was to use a new Lakewood aluminum (non-SFI) bellhousing (PN LK4000) that is actually a very nice piece. It checked out well within spec so it did not require custom aligning with offset bushing like other aftermarket bellhousings we’ve checked. Another solution that would also work would be an adapter ring that would take up the difference in the diameter for the truck bellhousing. Advance Adapters sells this ring under PN 716078 available through Summit Racing.
With the new bellhousing in place, the clutch now releases properly and the new pilot bushing will last a very long time. This was a simple fix once we realized the error of our ways.