3 Broken Driveplates / Flexplates 1964 Cadillac DeVille Convertible

Hi All from the UK,

My family have had this car from new, bought in London!

This was a very reliable car driven all over Europe and even China. But then it was interfered with and I have been dealing problems ever since. Most of the drive train was rebuilt including engine and gearbox.

But given lots of time and money it has been running quite well and has done 3000 miles since the rebuild...

BUT it has just broken it 3rd Drive Plate!! Hairline crack in the central section.

Does anyone have any ideas why this might be?

Thanks in Advance Hugh

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Hello Hugh, are you sure the TH400 flexplate is being installed, not the Hydramatic flexplate? Center bore is 1/8" bigger on the Hydramatic plate, mounting holes line up though. First thing comes to mind...

thanks Robert....i will check on the last 2

but the original was definately the right one, which was also broken

Check you shop manual. 

The 4-speed Hydra-Matic is very different from a 3-speed Turbo Hydra-Matic. Don't think many parts interchange.

MrMechanic will most likely have the correct one for you. His site is in Swedish but call him and explain the problem. There is not one thing he doesn't know about these gearboxes. He can also rebuild yours. If the part you are looking for is not on the website it will most likely be in his warehouse. Was the case when I needed a new flex-plate last year for my Hydra-Matic And as you are still part of Europe you wont have to pay customs and so on.

I also think that the same gear-box was used in RR. At least there is a 3-speed Turbo Hydra-Matic in my -72 RR. So you can try Flying Spares or other RR part sellers.

thanks, very useful breakdown picture... will have a chat with him.

RR man looks after my car there are differences and not interchhangeable.

I think the issue could be the engine rather than gearbox....

There is a 1/8 inch difference --in the end of the engines crankshaft --that meets up with the drive plate on some 429 engines depending on which of the trans was being used with a given engine
You can only install --the trans -that belong with the correct engines crankshaft end --when doing a trans replacement
If you install the wrong drive plate ( or trans ) --the 1/8th difference in the center engine crankshaft end - will cause the drive plate to break as it is not fitting correctly --at its center -and will flex and break

Do you have a replacement trans in the car ?
Possibly the wrong one installed for your end crankshaft demention --in the past ?


You need to match the size of the 429 - crankshaft end -to the trans that belongs with it to have them work as intended
The 64 turbo trans will require a spacer to be installed behind the 429 with the correct cranksaft demension for that trans To work correctly


thanks Tony & Ginny

engine and tranny are originals to the car.

I think the spacer could be the reason....since rebuild there has been 'no' spacer (must have been one there before it was interferred with)

I hear you can buy 'new' drive plates (rather than 54 year old originals)....does anyone know where?

the 'spacer' does anyone have details or an illustration/pic of it and any pointers where it can be sourced?


Quality of stuff today is not great. 54 years old might be better than new. Like me. . Or not. 

called bjorn said he doesnt speak english, emailed him, now answer yet

There has to be the adaptor ring. the transmission (Turbo Hydromatic) will not mount up to the engine without it.



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