Put an Old Trans Behind Your New Gen III V-8

Note the 3/8-inch bolt hole is counterbored to use shouldered bolts like the one in the photo. The bolt shoulder acts as a locator to accurately position the pressure plate on the flywheel. Never use fully threaded bolts to attach the pressure plate on traditional clutch applications. The metric bolt pattern uses locating pins, therefore the bolt holes in the flywheel are fully threaded.

Note the 3/8-inch bolt hole is counterbored to use shouldered bolts like the one in the ph

Gen I Automatics Behind Gen III/IV Engines
The same mismatch between the 0.400-inch-shorter crank flange position relative to the bellhousing is what makes matching up a typical TH350/TH400/200-4R or early 700-R4 to a Gen III/IV engine a bit different. The beauty of this swap is most of the mismatch situations can be solved with the use of a simple steel adapter. GM actually faced an identical situation when bolting the 4L80E automatics behind 5.3L and 6.0L engines in larger trucks. The 4L80E automatic is basically a Gen I-style automatic, which required an adapter. To compensate, GM created a simple steel spacer that bolts on the Gen III/IV crank flange and combines with a flat flexplate. All this information is published in the GM Performance Parts LS1 engine swapping performance pamphlet. The adapter and flat flexplate, along with longer metric bolts, are intended to be used together and will allow you to bolt any Powerglide, TH350, TH400, 700-R4, or 200-4R automatic to a Gen III/IV version engine. The part numbers for these pieces can be found in the parts list at the end of this story.

Here is a look at our 5.3L engine with the custom McLeod flywheel, clutch, and pressure plate assembly enclosed in a Keisler bellhousing that mounts a Keisler-modified Tremec TKO-600 five-speed trans, which will eventually find its way into our red '67 RS Camaro.

Here is a look at our 5.3L engine with the custom McLeod flywheel, clutch, and pressure pl

Unfortunately, there are also a few minor details that need to be addressed. For example, the flat flexplate used with the spacer is designed for a 300mm (11.8-inch) bolt pattern, while most Gen I transmission torque converters use an 11.7-inch bolt pattern. The difference between these two bolt patterns ends up being roughly around 1/8 inch. As you can see in the photo, the 300mm bolt pattern holes can be carefully elongated with a file or die grinder to accommodate the smaller Gen I converter bolt pattern.

We dug a little deeper and discovered TCI makes a similar package that offers an SFI-approved performance flexplate and includes the adapter and bolts, all for an excellent price through SDPC. This flexplate bolts up to the Gen III/IV crankshaft and also features a dual converter bolt pattern that will accommodate both the early and metric torque converter bolt patterns, so no modifications are necessary. Just bolt it up and go. It doesn't get any easier than that.

If you intend to use a steel scattershield with a block plate, be aware that the Gen III/IV oil pan extends slightly rearward of the bellhousing flange. This necessitates cutting a clearance slot in the block plate as shown. If this relief is not machined into the block plate, the bellhousing/block plate assembly will not bolt up flush to the engine, causing all kinds of problems.

If you intend to use a steel scattershield with a block plate, be aware that the Gen III/I

By Jeff Smith

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