425 heads and 500 Cadillac

Matt Ledbetter

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Does anyone have a valid reason why it isn't a good idea to use 425 heads on a 500 give some proof that we all can see.most people only go by what the vendors selling parts tell them without seeing for themselves
 

Roy Heisenberg

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Howdy there, Matt:

I have seen this done before, and it is almost the same dance as putting 350 heads on a 400 SBChevy.

Here is the way I remember it... The trick is to just look at the extra holes on the 425 head gaskets, and get out your drill, and make sure the new 472/500 gaskets wind up with the same steam-holes in the correct places. Pretty sure we used the blue Fel-Pro head gaskets, but there may be many options to make the 425 heads work

Here is another cool reason to try it.... The 425 ports have a bit less volume, and this can give your big block Caddy gobs of smooth, low-end grunt !! Moving the peak torque lower can make the tires smoke harder, and that can be lots of fun!

On another note, if the 500 had the larger, open chamber heads with huge cc numbers, and the 425 heads have smaller cc combustion chambers, you could wind up with more compression. This sounds like a huge win/win, and smaller ports with higher compression have their “tire-shredding” place in the world of big block Caddy monsters.

Bottom line - It can be done, I have seen it work on a 472, and I say go for it !!!

Enjoy the torque,
Roy of the 472 Jeep
 

Matt Ledbetter

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I have done it a few times it work well when you are replacing large chambered heads makes a more torquey street engine on a budget.
 

Roy Heisenberg

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Good point !

Next set of 425 heads I see at pink-N-pull are gonna get a new home in my Garage. I need a backup set of heads, and wonder how far one can shave these heads ... can I shave .125 off and run them ?
 

Matt Ledbetter

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If you knock that much off you will have all kinds of alignment problems,intake,push rods,not to mention your compression will be a little on the high side the heads I cc'd and 500 late block and Pistons came out to 9.2 to 1 combustion chamber is larger than is claimed. That was 423 castings.
 

Roy Heisenberg

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Good to know, Matt. I’ll limit the cut to about .040” and see if my machinist can angle mill the intake gasket surface.

Thanks,
Roy
 

Matt Ledbetter

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You can mill .040 flat on head surface and you don't have to mill intake side. It will also preload lifters better with stock rockets.
 

Cad Man

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Dont machine the intake flange. ..,
No reason too.. it will cause gasket issues as well.. just match ports
 

Roy Heisenberg

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Ok... just gonna match the ports.

Thanks,
Bro. Roy
 

gary kosier

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I keep reading about the intake ports being smaller on the 425, but the EFI intake manifold is the same part for both motors. I find it hard
to believe Cadillac would have done that if they didn't match up. I gave away the only 425 ever had, so I have no basis for comparison.

Gary Kosier
 

kzhurley

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Ive seen the carb 425 intake and if you put a 472/500 valley pan gasket on it, it's like 1/8 smaller port all the way around
Maybe just the efi intakes must have been the same
 

Matt Ledbetter

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I have never cc the ports on them but they are really close to the same thing
 

kzhurley

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If be very curious to see them compared. There was a significant visual difference in the port opening area on the intake between the 425 intake and 500 pan anyways.
 

Matt Ledbetter

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I went out and checked a 423 casting 425 head against a 493 smoger 500 head with a intake gasket and saw no difference in size port entrance. Maybe people are getting 368 heads it has been many years sentence I have see a 368 head off.
 

Roy Heisenberg

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Correct. 368 heads have very small ports.
 
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