Wooly15

1968 472 Heads

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I recently burned an exhaust valve in my 68 deville with a 472 and am in the process of sourcing parts. I found a gentleman that has cadillac heads with a 1609423 casting number. This traces to a 77-79 425, but he claims they will work. I cannot find anything that backs that up. Any experts out there?

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Good morning , Wooly15. I am not quite the expert you need , but I sure can get you started , and then hand you off to the real experts. First of all , no ,they are not directly interchangeable. They will bolt up , BUT : Early big block Cads , 472s and 500s , all high compression '70 and earlier and early low compression , '71 thru maybe '73 or  '74 or so , had low volume combustion chamber heads. It has been about 35 years , but I think 78 cc. Later heads, continuing all the way thru the 425s and I think even the 368s went to 127 cc combustion chambers , if my memory serves. Now this MIGHT just provide an opportunity. 10:1 compression Cadillacs really don't like modern , relatively low octane gasoline. Yes they are great at elevation , Denver and higher , but you really can't step on it at sea level without detonation. Some guys don't believe it which eliminates them from the gang of experts , and others retard the timing a little. Retarding the timing runs hotter , and too much too often is hard on exhaust valves. Hmmmmm......... . Hmmmmm ..... ? Did you say your engine was recently rebuilt ? Burned an exhaust valve already ? Hmmmmm......... ??? Do you know all specs on the rebuild ? Should have decompressed it a little , and cammed and curved it accordingly if done in the last 40 years or so. But again , some guys don't believe it. O.K. , what's done is done. Perhaps this MIGHT be the time to set all this straight. I don't know , but maybe switching to 127 cc heads could by slim chance give you modern 92 octane compatibility. Never run less than the highest octane pump gas in ANY of these engines if you are running at sea level.

 

You might be wise to make MTS your next stop. THEY are the experts , and will certainly know what I am trying to get at here.

https://cadillachighperformance.com/        Talk all this over with them , and also discuss different cam profiles , maybe longer duration if you have to run 78 cc heads , and also installing a properly re-curved HEI distributor.  I hope I have been of some help to you , Wooly'. I am INTENSLY curious to see how you get on with all this. Please , PLEASE , keep us in the loop !     Best of luck to you , and I hope you are back on the road soon , running better than ever.     Your forum friend ,    - Cadillac Carl 

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Thanks for the response! You say they will bolt on but are not directly interchangeable. What modifications would I have to make? 

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It is the change of combustion chamber size. I don't know what the overal drop in compression ratio would be. You COULD luck out. Let's say , for example , that you would lose a point to a point and a half by going to 127 cc chamber heads. BINGO ! You are now at 8.5 , 8.75 , maybe 9 : 1 compression. You would be automatically able to use all your throttle , any time , at sea level. I have owned more big block Cads than I can remember. Literally. I have driven them to the max extent of their capability at a huge variation of altitudes (low octane needed for max performance at altitude) , and 10 : 1 472s on gasoline up to 110 octane. Detonation is REAL. Even the 8.5 big blocks need their 92 octane. Even though the octane deniers resist paying a few pennies more for the good stuff , they sometimes admit that they do occasionally hear the nasty pinging which is technically SEVERE detonation. Detonation occurs before you can hear it.

 

  That is why I said this may be an opportunity. Verify my old memories by checking with MTS , and see if there is a build they can help you with to make sure your '68 runs right and stays right. I once bored out a low compression 500 .050 over (514 cubes) , did head (the old 78cc) and port work on it , put 7 : 1 forged Pistons in it for use all over Mexico on the old 70 something octane junk you had to run in those days down south of the border. At the time I charged ahead with these mods , I did not realize that I could have easily decompressed it by changing out to 127 cc heads. AND , it would have been reversible. I have some faint recollection that such a change would have dropped me from 8.5 to around 7 ! I am certainly not sure of this number , but I could be spot on. I do clearly remember wishing I had known of this. Should have done my homework , right ? Take advantage of my expensive lesson , Wooly. As I say , I am no expert. It is just that this is something unusual I have dealt with , so I was extremely happy to see the possibility of some good coming from my specific experience. I love AACA , our forums , and all the good helpful people who hang out here. I learn so much , and every now and again I get the chance to give back. Some times it is hard to figure out exactly what a person needs to hear. Can be hard to tell what experience level each other has. A phone call might be in order. You , or for that matter any of our fine friends here , may call me. I am on the West Coast , a little hard to catch at times. Might take a couple or three try's. Or leave messages.  408-621-8261 , or 206-790-6912 .     - Carl 

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Just to close this thread, I ended up not using the 425 heads, but just sending mine off for a rebuild. 

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