I’m looking for a picture, pdf, or drawing with the dimensions for the 1959-1960 Pontiac bellhousing bolt pattern. I have an adapter that fits the 1965 and later Pontiac engines to a Chevy TH400 but I’m wanting to add a couple holes to install the TH400 on a 1960 389. I’m aware of the starter issues, this adapter already has a provision for the trans mount starter. I just need the dimensions for the 1959-1960 block dowel pins and bolt holes, I intend to add those holes to this adapter with a milling machine. I already have the drawing that’s attached, it’s for the later engines, need one for the earlier 59-60 engines.
Investigated doing something similar when I had my '62 Catalina and had talked to Bendtsen's (now Speed Gems) about an adapter. If I remember correctly they told me there were differences in crankshaft flange size as well. I never made the swap since the old RotoHydramatic "Slim Jim" held up just fine as long as I had the car. FYI here's a link to the current Speed Gems setup which does show a crank hub adapter: Pontiac 2.59 Crank to Chevy Automatic — Speed Gems May be something to be aware of, and at least it's a bump for you...
Appreciate the info Terry. I’ve got a drawing for a machined crank flange adapter, just need to get it machined before installation.
You may want to use a BOP version of the TH400 with the adapter shown. The two top bolt holes match the 55-60 Pontiac V8's bell pattern. Use those with the early bell housing to derive the other holes needed.
Follow up. I took the transmission adapter that bolted to the back of the earlier 59-60 engines and set it up on a milling machine this weekend. This is the adapter that Pontiac installed to mount the early transmissions, it’s about 3” wide. Using the digital readout I recorded the XY coordinates for the block dowel pin holes and the transmission bolt holes. As d2 Willy’s stated above (and thanks), the top two holes are in the same location as the later engines, so I’ll only need to transfer the two dowel pin holes, (which measure .500”, compared to the later engines .625” dowel pins) and the four lower transmission bolt holes. Interestingly, the lower transmission bolt hole that sits above the starter mount is closer to the center of the crankshaft than the other one. Not sure why Pontiac did that but I suppose they had a reason. I plan to install the adapter on the bed of the milling machine this next weekend and drill these holes if I have time. I’ll also need to counterbore the adapter plate and install two stepped dowel pins to locate those in the TH400 transmission holes. I know I could have purchased an adapter that would do this but I would rather have the satisfaction of building my own, even if there are a few headaches along the way. One other item I’ll need to address is machining the end of the early crankshaft to a larger ID so the torque converter nose will fit. I’m in the beginning stages of rebuilding the 1960 389 so the engine and trans should be in the 63 GP by Spring of 2025. The car had not run for 35-40 years so I was surprised the old Roto-hydramatic still worked but it’s held up in the few hundred miles I’ve driven it. It’ll be off to the storage shed at some point this winter, probably never to be used again.
I appreciate your efforts to do this yourself with what you have. I just went through the same thought processes with my ‘59 Pontiac powered Stude pickup that had been stored in a warehouse for fifty plus years. I picked up a Wilcap adapter for the 61-64 engines and was hoping to modify it to fit the earlier engine too. As luck would have it, I bought a rebuilt ‘59 engine and it has the early adapter on it. So I now have both. Equally funny, after a good clean out and some new gaskets and seals, the StratoFlight Hydramatic in the truck works just fine. I’m prepared if it fails, but I bet it will still be working fine when I’m ready to upgrade the engine. Fun ain’t it?
That sounds like a neat ride. My grandfather loved Studebakers and I’ve always been a Pontiac fan, good combination! I looked for a used Wilcap adapter for a while but finally went ahead with my own plans, anxious to get it all bolted in.
Setbacks. The threaded holes for the 1960 Pontiac engine block bell housing are 7/16 x 14 tpi, whereas the 61(?) and later TH400 cases are 3/8” x 16 tpi. I’ll need to drill out the top two holes in the TH400 case to a .437 diameter (or slightly larger for clearance). Not an insurmountable problem by any measure. Also, the steel plate adapter that I already had fabricated has the nuts for mounting the chevy TH400 tack welded to the back side of the steel adapter plate (they would sit between the block and the adapter plate). Two of the nuts are hitting on the flanges of the block bell housing wall, so that’s a fairly large setback because the plate won’t sit flat against the block. The steel adapter plate isn’t thick enough to thread for the trans bolt holes, so I’ll have to start over and machine one from aluminum plate. Fortunately I have all the data saved in a Solidworks model, but I’ll need to do some more figuring on depths of bolt holes and such. Either that or break down and purchase a Speed Gems adapter while I focus on the engine. Transmission is already rebuilt so that’s done. Fork in the road.
Ohhh, your adapter is different from the ones I have. Mine are approximately 5/8” thick aluminum. Good thing is, these days you can lay out another one with the changes you need and have it cut by laser or water jet. You could even make it from thicker material if you want. I hadn’t seen one like yours until now. Here’s mine on a ‘59 389.
I am in the process of doing the exact opposite from what you are doing. I am adapting a dual range hydramatic to a BOP pattern Buick 225. In the process, I will be using two 1 inch identical adapter plates to provide for the starter drive clearance. Then, the Buick will use it's flex plate with the starter. In between that and the HydraMatic flywheel will be a 2 inch spacer/adapter for the crankshaft. In my case, I have the bellhousing and adapter bolt holes already at 7/16 diameter.
I fabricated this adapter from 5/32” thick 4130 sheet. Everything was going great. The bendix to flexplate clearance was good and all the bolt holes were lining up. That is until the welded nuts that would attach the transmission to the adapter interfered with the block bellhousing flange. I’m sure I can find a use for this adapter on a 61-63 block at some point, but for now I’m planning on machining one from aluminum very similar to yours and d2_willys.
The Edelbrock intake, AFB carbs and fuel injection distributor are the ones installed back in the ‘63-64 time frame when the engine was originally built. It supposedly has a M/T stroker crank and a roller cam. I’ll need to open it up to verify that. It was “freshened up” a few years ago. No major parts were replaced, just resealed and a valve job I was told. This engine was built and installed in a “sister truck” to mine. Carman Pisano, owner of Wedge Engineering (scattershields) built three similar Studebaker pickups at the same time. Two were Pontiac powered and one had a 413 Mopar. They were seen regularly around my hometown when I was young. Two of them survive today, the two Pontiac powered ones. Unfortunately one was updated with a more modern drivetrain.
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