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How to Put a Manual Transmission behind a Cadillac V-8

Make ours a 425, 472, or 500-cube torque monster!

The 425, 472, and 500 Cadillac engines of the late 1960s and '70s were luxury items meant to provide tons of gooey torque to get those land yachts moving, while still delivering as smooth a driving experience as possible. Part of delivering that velvety driving experience was equipping the vehicles these engines came in with automatic transmissions. Because none of these big-cube torque monsters were equipped from the factory with a manual transmission, many people don't consider them as a viable engine to stab in front of a manual. Thankfully, these Caddy engines share the same bellhousing bolt pattern as Buick/Olds/Pontiac V-8s, which at one point or another were all used in vehicle platforms that included a manual transmission option from the factory. For the 1970 400hp/550lb-ft Eldorado 500 I started installing in my '53 Willys flatfender and then later actually mounted in my 1985 Ramcharger, at one point in time I had worked out how to mate this engine to both an SM420 and SM465 four-speed transmission. I'll run down the specs on both, but here are some general things to consider when swapping a manual transmission behind a Cadillac 425, 472, or 500 engine.

Bellhousing Bolt Pattern for Cadillac 425, 472, and 500 V-8

Cadillac V-8s from the late '60s and '70s share the same bolt pattern as Buick, Olds, and Pontiac V-8s.

The starter motor on these Caddy engines is on the passenger side, which is the same as Buick V-8s. Oldsmobile and Pontiac V-8s have their starters on the driver side. Although the bellhousing bolt pattern is the same for all Buick/Olds/Pontiac/Cadillac V-8 engines from the V-8 highwater era—generally mid-to-late '60s through '80s—if you're using one of the factory BOP bellhousing, you'll want the Buick so the starter pocket on the bellhousing clears the starter, though in a pinch you could use a Oldsmobile or Pontiac bellhousing and just cut the bell to clear the starter. There were also some factory GM bellhousings that had starter pockets on both sides, and some aftermarket bellhousings are dual pocket as well.

Bellhousing Bolt Pattern and Bore

A BOP Lakewood SFI bellhousing allowed us to mate an SM420 to this 1070 Cadillac 500 engine.

Most standard GM car and pre-1967 truck manual transmissions use the standard GM bellhousing with a retainer bore of 4.686 inches. This includes the venerable SM420 transmission, so if the SM420 is your transmission of choice, you'll have the option of using a relatively common factory Buick car or aftermarket manual transmission bellhousing. I used a Lakewood SFI bellhousing for a BOP application to bolt an SM420 to the 500 when I was mocking it up to mount in my flatfender. That worked out just fine.

The SM465, however, was only offered behind 1968-1991 GM pickups and SUVs using a Chevy-pattern bellhousing. Unlike the SM420, the SM465 uses a much larger 5.120-inch retainer bore, so you can't just bolt a factory or aftermarket Buick bellhousing behind your Caddy engine, plunk an SM465 on, and be on your merry way.

Back when I was working out my system to mate an SM465 to install the 500 in my Ramcharger, the advice I got from Advance Adapters was to machine the transmission bearing retainer down to 5.00 and then increase the bore of the bellhousing to 5.00, allowing a few thousands of difference between the two so they go together without needing a press. In the end, I did neither, selecting instead to use a thin BOP-to-Chevy adapter plate. I bolted the adapter to the Caddy engine and then bolted a GM pickup SM465 bellhousing to the plate.

Pilot Shaft Depth

Cadillac 425, 472, and 500 engines don't have the crank drilled for a manual transmission pilot tip.
If you drill your Caddy crank by hand for a pilot tip, make sure you have your drill centered in the crank and don't let the bit walk off-center.
Christian Hazel trimmed the pilot tip and nipped back a bit of the input shaft splines to get this SM465 to mate to a Cadillac 500 engine.

Having never been designed for a manual transmission, the Cadillac 425, 472, and 500 crankshafts are not drilled to accept a manual transmission pilot shaft. Depending on the bellhousing you're using and the pilot shaft length, chances are the pilot shaft will bottom on the crankshaft before the transmission fully seats to the bellhousing. The solution is to either cut the tip of the pilot shaft, assuming you've got enough room before the input shaft splines bottom on the pilot bushing, or drill the crankshaft. Drilling the crankshaft is definitely the more ideal of the two options. If you're rebuilding your engine and it's going to the machine shop, having the shop drill the crank while it's being worked on is the surest way to guarantee a successful conversion. If you do decide to drill the crank by hand, take a lot of time and precaution to ensure you're drilling absolutely perpendicular to the face of the crankshaft. The last thing you want is an off-centered hole nearly an inch deep in the back of your crank. When I was mocking my SM420 to the Caddy engine, I never actually went through with either of these scenarios, ditching the plans to run the 500 in that vehicle before I mated the tranny firmly to the bellhousing. On the Ramcharger, however, I wound up cutting a bit of the SM465 pilot shaft tip off and used an angle grinder to nip the transmission input splines back, so they didn't rub against the pilot bushing. In this configuration, the SM465 bolted smoothly to the back of the 500, and the tranny could easily be spun by hand without any resistance.

Flywheel

You can get an aftermarket flywheel from several aftermarket sources. The billet steel flywheel I used on my 500 used a 166-tooth ring gear to work with a factory-spec Cadillac style starter. Stock 425, 472, and 500 engines are neutral balanced.

Pilot Bushing

The bushing on the far left is the custom machined unit Advance Adapters built for this conversion, machining down a bronze bushing with a 0.590-inch pilot tip to fit inside the Cadillac 500 crankshaft.
The Advance Adapters bronze bushing was the same diameter as the Caddy crank bore, so before final assembly I stippled the bushing and used some green Loctite.

Cadillac crankshafts aren't machined for a pilot bushing or pilot bearing. I tried working out a solution with a few off-the-shelf pilot bearings, but I didn't find any that had the correct outside diameter to fit inside the Caddy crankshaft while still fitting the 0.590-inch GM transmission pilot tip. I took a careful measurement of the inside diameter of the rear of the Cadillac crankshaft and asked Advance Adapters to machine a bronze bushing to the correct outside diameter while having the right. I forget the exact measurements off the top of my head, but the bushing easily slipped in and out of the crank, meaning I should have asked them to build it a few thousands bigger so it would be a light press fit inside the crank. As a solution, I used a steel punch to stake and stipple the outside of the bronze bushing before final assembly. I doubled up with a bit of green Loctite, which worked perfectly.

An early shot inside Christian Hazel's garage sometime around 2001 showing the SM420 transmission, Cadillac 500, and Spicer 18 being prepped to come together as a unit.
The 1970 Cadillac 500 being dropped into the engine bay of Christian Hazel's 1985 Dodge Ramcharger.
The Cadillac 500 in Christian Hazel's Ramcharger just before engine mounts were fabbed up. The wider Caddy engine block required the passenger-side header to be dented for frame clearance, but otherwise it was a good fit; well worth the easy 400hp/550lb-ft.
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Sources:

Advance Adapters
800/350-2223
advanceadapters.com

CAD Company
505/823-9340
cad500parts.com