This simple illustration shows...
This simple illustration shows the differences in position of the Gen I crankshaft flange versus the Gen III/IV flange in relation to the bellhousing mounting surface.
Given all this, the quickest way to adapt an early four- or five-speed manual trans to a Gen III/IV engine is to make up this 0.400-inch difference in crankshaft flange position. McLeod Clutches, which was recently purchased by B&M, has solved this problem with a 168-tooth flywheel that will bolt up to the Gen III/IV crankshaft and extends the flywheel face by exactly 0.400 inch. This places the clutch and pressure plate surface at the correct height so you can either bolt on a new clutch and pressure plate assembly or use a Gen I clutch and pressure plate assembly you already have. This custom McLeod flywheel is drilled to accept the traditional 11-inch Borg & Beck/diaphragm-style pressure plate bolt pattern using original 3/8-inch shouldered pressure plate bolts.
The Keisler aluminum bellhousing...
The Keisler aluminum bellhousing employs both the Gen I and III/IV bolt patterns so it can be used on either engine. Of course, an 11-inch Gen I small- or big-block bellhousing can also be used as long as you're willing to sacrifice one mounting bolt hole. Standard small-block 101/2-inch (153-tooth) bellhousings will bolt up to a Gen III/IV engine but won't clear the 168-tooth flywheel.
Before we bolt on the bellhousing, there is one other crucial step that must not be overlooked. It should be common knowledge that a pilot bushing positioned in the end of the crankshaft supports the transmission input shaft, preventing deflection when the clutch is disengaged. In our trans-swapping effort, remember that while the depth of our bellhousing is the same as with a Gen I engine, the crankshaft flange is 0.400 inch farther forward. Since the input shaft length on our traditional four-speed or five-speed transmission has not changed, we also have to move the position of the pilot bushing rearward to compensate for the discrepancy. McLeod has again come to the rescue with an adapter pilot bushing that sits in the larger recess in the crank and therefore supports the input shaft in the proper location. If you don't pay attention and incorrectly place a stock pilot bushing in the original spot in the Gen III/IV crank, the transmission input shaft will not be supported and the input shaft bearing will fail almost immediately. We also discovered that GM makes a special LS2/LS7 pilot bearing that will adequately support the shorter input shaft.
This brings us to the bellhousing. Because all Gen III/IV engines use a 168-tooth flywheel and flexplate diameter, not all small-block Chevy bellhousings will work when it comes to bolting on an older four- or five-speed manual trans. This larger flywheel requires the rare 11-inch-style bellhousing. To use the larger flywheel, you'll either have to dig up a used 11-inch bellhousing or scattershield, or you can order a brand-new aluminum factory-style bellhousing from Keisler that will not only accommodate the larger flywheel but is drilled for the Gen I and Gen III/IV bolt patterns. One thing we discovered is the Keisler bellhousing is cast 0.200 inch deeper than a standard GM bellhousing. The standard GM bellhousing depth is 6.290 inches from the engine bellhousing face to the transmission mounting face, while the Keisler specs out at 6.490 inches. This affects input shaft placement relative to the crank as well as the position of the clutch disc. Another option is using a scattershield from Quick Times that also combines the Gen III/IV bolt pattern with the standard Gen I-style bell-housing depth.
Even using the custom offset...
Even using the custom offset McLeod flywheel, this custom McLeod pilot bushing adapter is necessary to properly support the input shaft.
We're not quite there yet. We prefer to keep everything simple by retaining the original mechanical clutch linkage, avoiding the added expense of a hydraulic system. The problem is Gen III/IV engines do not have a provision in the block for the engine-mounted ball stud pivot for the clutch linkage Z-bar. While discussing this with Scoggin-Dickey Parts Center's (SDPC) Nickey Fowler, he mentioned SDPC makes an adapter bracket originally designed for the Gen V/VI factory Rat blocks that also did not come with this ball stud provision. This SDPC bracket bolts to the lower two bellhousing bolts on the driver side and places the ball stud in exactly the right position for the Z-bar. This is the best scenario for adapting a Gen I-style manual trans behind a Gen III/IV engine that is easy and not overly expensive.