425 to 500 Conversion Flywheel Question

[acaciaavenue]

I just got my car back from the rebuilders after putting a 500 in place of my old 425. There seems to be some sort of rattling that occurs when the engine starts up for about 30 seconds to 1 minute and then goes away. It sounds like it's coming from the flywheel area, but when the mechanics put it on a lift, by the time it gets raised, the sound has gone away.

The shop is standing behind their work to resolve it, but I told them I would check my personal resource of Caddy experts and get back to them.

It is my impression that the 425 flywheel from my old engine is being utilized on this 500.

Has anyone ran into this problem or could suggest how to proceed?

[Nashalac]

POSSIBLY a cracked flex plate.

[DaveM]

when I got my motor a 1970 472 the flywheel or flexplate had three big cut outs in it..so wile at the junk yard I got a flexplate off a 1979 425.. It did not have any cutouts and was stronger than the first.. so if yours has no holes chances are better its not the flex plate.. but it could be the bolts loose or the torque convertor...A convertor can make some pinging or rattle sounds when they start to come apart... but when they do that, they go out of balance , and you can sometime feel that out of balance.. so get in the car , put it up on the lift and then start it over and over until someone below pinpoints where its coming from..

[acaciaavenue]

Thanks guys for the input - anyone else is welcome to chime in. I think the converter is okay, as it's brand new with the engine.

I'd like to add that this flywheel is the 425 flywheel - is there any structural difference between that one and a 500 flywheel? The shop mentioned that the swap wasn't always exact (no particular instances were mentioned) but they did have some issues. Also, the place that rebuilt the engine is an extremely reputable performance shop - and I trust their judgement as I have done lots of work with them over the years, so I don't think this is an error of sloppy mechanic work.

[Terrible One]

I'm sure the one for the 500 is a bit beefier, but I wouldn't know.

Take the starter off and turn it over to check each bolt and for cracks. Pop the inspection plate and look through that.

[STDog]

I'd like to add that this flywheel is the 425 flywheel - is there any structural difference between that one and a 500 flywheel?

None. There were 2 flexplates used at different times. One has utout fo weight reduction and the othere doesn't. The '72 472 I just took out had the cutouts, and the 425 I put in didn't. My '70 472 had the solid plate. I don't know what determined which one was used.

General consesus is the solid one is stronger, but I know 35 year old cars with the cutout version still going without issue.

The shop mentioned that the swap wasn't always exact (no particular instances were mentioned) but they did have some issues.

I'd be curious to know the details on that. What they did chnge. The might have done something odd, due to inexperience with these engines.

The oil pump probably needed swapped, since the 425 has a diffeent angle. And the oil pan/pickup combo would be swapped too. And last, the exhaust would be swapped, or a RWD 472/500 exhaust used. The FWD exhause was different, but only on one side (driver side? I think the FWD exhaust both exit the rear instead of the driver's side exiting the middle)