350 to 455 swap question?? - Pontiac GTO Forum
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post #1 of 7 (permalink) Old 07-16-2019, 10:14 PM Thread Starter
 
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350 to 455 swap question??

I’ve pulled and sold the 350 that came in my 72 LeMans sport. I also bought a 455 and the stroker crank to swap in.

The car is stripped down to its frame for rust removal and coating when I read something that gave me pause.

I heard mentioned that swapping in a 455 would require moving the motor mounts. While searching I’ve also seen posts saying the two should just swap straight across, and also a post saying it depends on the year of car and motor.

Where I no longer have the old motor to look at, is there a way of looking at the engine and frame and determining if I need to move the mounts? Currently it looks like my frame has holes for two different locations. Low and kind of centered is where the mounts currently are. There’s also another set of 3 holes maybe about 4” higher and a bit farther forward.

Any help would be appreciated as doing the change now while it’s just the bare frame would be much easier.
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post #2 of 7 (permalink) Old 07-17-2019, 07:54 AM
 
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The 350 & 455 can use the same frame mount location. Most Pontiac V8 blocks are the same size, on the outside.

I think a Chevy inline six was used in some of those cars. That may be what the other set of holes is used for.
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post #3 of 7 (permalink) Old 07-17-2019, 10:34 AM Thread Starter
 
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Originally Posted by bigD View Post
The 350 & 455 can use the same frame mount location. Most Pontiac V8 blocks are the same size, on the outside.

I think a Chevy inline six was used in some of those cars. That may be what the other set of holes is used for.
That’s what I originally thought and why I didn’t look before selling the old engine. Seeing people talk about having to move the mount locations made me wonder though.

Thanks. Hopefully that’s the case.
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post #4 of 7 (permalink) Old 07-17-2019, 12:55 PM
 
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Just one other note about the 455's. They all had 5 motor mount bolt holes on each side of the block, therefore will work in '69 & earlier Pontiacs, as well as '70-up models, with both 2 & 3 bolt mounts. So did most 350 & 400 engines used in '70-'76 models. I think most '77-'79 model year blocks had only 3 holes per side, requiring 3-hole mounts, or some type of adapters.

I've also read that SOME early '70's A-bodies used 3-bolt mounts, & some used 2-bolt mounts. I'm sure there are some guys here who can probably tell you exactly which A-bodies came with which mounts on what engines.

This Year One ad seems to indicate that 455 equipped '70-'72 A-bodies came with 3-bolt mounts.

https://www.yearone.com/Product/1964-72-gto/ng243

This thread says that the original 455 cars came with 3-bolt mounts & different frame mounts. BUT, the 350/400 mounts will work just fine with a 455 engine.

http://www.pontiaczone.com/forum/showthread.php?t=21914

Last edited by bigD; 07-17-2019 at 06:37 PM.
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post #5 of 7 (permalink) Old 07-17-2019, 03:41 PM Thread Starter
 
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Just one other note about the 455's. They all had 5 motor mount bolt holes on each side of the block, therefore working in '69 & earlier Pontiacs, as well as '70-up models, with both 2 & 3 bolt mounts. So did most 350 & 400 engines used in '70-'76 models. I think most '77-'79 model year blocks had only 3 holes per side, requiring 3-hole mounts, or some type of adapters.

I've also read that SOME early '70's A-bodies used 3-bolt mounts, & some used 2-bolt mounts. I'm sure there are some guys here who can probably tell you eaxctly which A-bodies came with which mounts on what engines.

This Year One ad seems to indicate that 455 equipped '70-'72 A-bodies came with 3-bolt mounts.

https://www.yearone.com/Product/1964-72-gto/ng243
Thanks for the info. I’ll check the numbers of holes when I get home.
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post #6 of 7 (permalink) Old 07-20-2019, 11:04 AM
 
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On my 70 455 GTO the motor mounts were located forward on the frame as seen in the first two photos I took at disassembly and on my 70 LeMans 350 the motor mounts were located aft as seen in the other three photos. These are mounts that bolted to the frame and were different also. Iím not sure if the location moved for the 12 bolt vs 10 bolt rear that was used on the 455.
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post #7 of 7 (permalink) Old 07-30-2019, 08:15 PM Thread Starter
 
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Originally Posted by Slow&Easy1 View Post
On my 70 455 GTO the motor mounts were located forward on the frame as seen in the first two photos I took at disassembly and on my 70 LeMans 350 the motor mounts were located aft as seen in the other three photos. These are mounts that bolted to the frame and were different also. I’m not sure if the location moved for the 12 bolt vs 10 bolt rear that was used on the 455.
Your second set of pictures are definitely the same as my car.

Hopefully the 455 bolts up to them just fine.
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