Technical Pontiac bellhousing bolt patterns

Does anyone know definitively what the differences are between the '55-'60 Pontiac bellhousing bolt pattern and the '61-'63 pattern? Different bolt locations? (all bolts or just one or two) Different locating dowel locations? There's no starter issue, from '55-'63 they all bolt to the bellhousing.
My research so far has found Wilcap, Speed Gems, etc are not helpful with dimensions these days. They just say it doesn't fit. Modern adapters use a flywheel spacer, a non-Pontiac flexplate, and a Mopar starter. They are also on the plus side of a grand, if available at all.
I have access to an adapter to put a Chevy automatic trans onto a '61-'63 389 Pontiac engine for a reasonable price but my engine is a '59 389. Unfortunately, the kit is not local, or I would just borrow it or make a template and compare.
Things changed at the bellhousing surface in '61, and I want to know what. Thanks in advance.

 

Bell Flange: 1955-1960, 1961-1979. The pattern is completely different between the 2.
As you know, starter mounts vary, other variations are minor.
If you can find an adapter for the later Olds & Buick engine, they also fit the '61+ pattern

 

I thought that the major change to the familiar BOP pattern came in late ‘64-‘65 along with the change in the intake manifold water passages.

 

I think that the actual bolt pattern is the same from 1961 on up, but the '61- '64 full size blocks had no provision for the starter, so the cast iron bell housing with the starter mount had to be used. 1964 A body engines used the block mounted starter and aluminum bell housing.

 

I guess the simple solution would be to buy the engine he has for sale that matches the adapter he’s selling. Lol. Hey, it’s only money, right?
I’m just trying to think ahead. My truck has a ‘59 389 with a Jetaway. It’s been sitting indoors for 50 years. The engine seems to run ok but I’m not getting my hopes up that I’ll be that lucky with the poor old Jetaway. I keep picturing chunks of clutch and band material falling into the pan.
I just got the brakes back in action, I plan to finally drive it when I get home from the March Meet.
I have a fresh Chevy TH400 sitting under my workbench, if I can make this adapter work it will be the easiest (and cheapest) solution if the Jetaway dies.

 

And this is why I asked the question. Lakewood lists their scattershield as ‘64- ‘81, Wikipedia “List of GM bellhousing bolt patterns” says, post ‘65. I know it changed after ‘60, but it didn’t become BOP until ‘65. So what was the difference in ‘61-‘63? I believe it was a crank to block surface change. If so, the adapter I’m looking at will bolt up but I will need to look at the crank flange adapter and flex plate location and maybe make some changes.

 

61-63 have provision for a bellhousing mount starter and the engines use 7/16 bolt to mount the bell.
The newer bell will line up but no place for a starter.

 

This isn’t exactly the information you are looking for but some details may help. I just installed a BOP TH350 on a ‘62 389 with the bell housing starter. I used the adapter plate from Butler Performance #P01004-kit and the crank to flex plate spacer BPI-1024. The flex plate I used was a stock ‘69 Pontiac. The spacer adapts the 2.5625” crank O.D. to the 2.75” flex plate I.D. Everything mounted up nice and the starter positioning is perfect.

In addition to the adapter, the crank pilot hole will not accept a stock TH350 or TH400 torque converter. The pilot hole on the pre-64 Pontiac crank is 1.625” and the newer converter snout is around 1.75”. The pilot hole depth in the crank also needs to be made a little bit deeper. I didn’t pull my crank for machining so I had the TC snout machined down to size by the rebuilder and I manually made the shoulder deeper in the crank to allow the TC to seat fully.

 

^^^^ That was quite a bit of work considering you were able to buy the adapter plate and the crank piece. Good info about the crank hole and the converter snout. Thanks.

 

I dont know if all, but our 64 catalina had pad and holes for block mount starter.
It did have bell mount starter with rotohydramatic.
I did not check if there was threafs in those starter bolt holes but im sure there was holes.

 

Essentially all (99.99%) 64 Pontiac blocks are drilled & tapped for the starter mount. Doesn't matter whether full size B bodies or midsize A bodies.

 

Thanks Mark. Funny, I have it book marked.
I thought maybe I could combine it with something else to put my Muncie 4 spd behind the Pontiac. I donated my Ford 3 spd/OD to Squirrel back when he was having his Edsel adventure.

 

Dang, I finally read that enough times and drank enough coffee to figure out exactly what you were asking.
This might cause you to drink Cheap adult beverages for a couple of months but isn't a bank breaker.
I'd contact them and make sure though. https://butlerperformance.com/i-244...-1964-and-earlier-blocks-bpi-p010004-kit.html
It won't work with a Chevy transmission case though you would need a B-O-P case that has the starter bulge on both sides.

The plate is from Bendsten.

 

By the engine with the bell. Try the bell on your current engine, if it works then sell the worse of the two engines and grin.

 

^^^^ That’s what I will probably do.
The ‘59 engine has a nasty miss at idle. Pulling the plug wires one at a time indicated #2. My initial trouble shooting ruled out a flat lobe/stuck valve, the plug, plug wire and cap. I didn’t see any bubbles in the water but it seemed to get hot. (I don't have a gauge on it yet). I need to do a compression test again. It may have a blown head gasket or something.

 

Pontiac V8 Bellhousing Designs

1 Buick Nailheads used a half-circle bolt pattern. BOP used a "twin-peaks" bolt pattern.
2 Buick 3-spd trans pattern (wider at top) fit M11 Light-Duty, M13 Heavy-Duty, etc.
Std GM bolt trans pattern (wider at bottom) fit Saginaw/Muncie. Pontiac tilted trans 15° to
driver.
3 All 1955-63 starters mounted on the bellhousing. All 1965-79 engines had block-mounted starters.
In 1964, all 326/389 blocks got a drilled & tapped starter pad, but ONLY the A-bodies
(Tempest/Lemans/GTO) used a block-mounted starter and an aluminum bell; full-size Pontiacs used a
cast iron bell that mounted the starter.
4 Engine mounting tripod changed from "tricycle" to "taildragger" arrangement:
1955-1960 a mount on each side of bellhousing, and one on block nose.
1961-1979 a mount on each side of block, and one on transmission tailhousing.

GMC offered the Pontiac V8 as an option in its 1955-59 1/2-3/4 ton pickups.
Pontiac bellhousings fit 10.5" & 11" clutches, while GMC V8 bellhousings' bigger contour accommodates a 12" truck clutch. The GMC fork is a HD piece, and the fork opening is very different than Pontiac.
Pontiac bells have a top inspection hole; GMC bells don't. Top & bottom covers are scarce.

 

For the most part the information in the previous posts are correct. There is however a common set of bolt holes between 1955-60 and 61-up blocks. The two top bolt holes match each block.
I know this since I have designed and checked an adapter plate between a BOP 225 v6 and a 1955 hydramatic front bellhousing. Since the Buick V6 has block mounted starter, the adapter plate simply adapts the block and front bellhousing. The remaining block bolt and dowel holes are located on the adapter, while the remaining bolt and dowel holes for the bellhousing are also positioned on the adapter. Only one of the bellhousing bolt holes is not used.

 

Great info gtofever, thanks!
As of right now, my '59 389 is running fine and the StratoFlight is working as it should, incredible seeing as how they sat for 50+ years. The '62 389 that I bought is stuck in machine shop jail for now but I hope to spring it shortly. It will get a fresh TH400 when I decide to make the swap, but for now I'll continue to run the '59 engine and trans.

 

If I was going to swap a trans later and make/buy an adapter............I'd be looking for a decent 700R4 or 2004r so it has overdrive. Probably better takeoff and definitely better cruising manners. I think you can get shift indicators with the extra gear.

 

I have the ‘61-‘63 adapter that came with my ‘62 389 and a fresh TH400 in stock for immediate delivery. My truck has a 3.36 Posi and 8.20-15 rear tires. It should be fine for the way I use it - as a local parts chaser.
Now there is a slight chance that I might install the ’64 Corvette rear end that I have during the swap to the ‘62 389, it’s exactly what the original builder, Carman would’ve done. It is a 4.11 Posi which would definitely work better with an OD transmission.