I have a Gen5 454 (1992 w/casting number 10114182) and am looking at a set of square port heads from a 1985 400 hp Merc Cyclone. I believe that makes mine a Gen5 and the heads Gen4. Are these compatable?

The quick answer is NO...
Whats the casting # of the heads you want to use?
6272990,14096188,??? 118cc,sq port heads
either one wont work...so I'm told
Dan

DENNIS MOORE'S BOOK ON H.P. MARINE ENGINES COVERS IT ALL! Part #'s, Gaskets, What you can and cannot use with what. That $15 book will save you thousands. Never knew how much my well meaning street rod guys were were costing me till I got into it. (and I was in the marine business for over 25 yrs.) GET THE BOOK ! --- Jer

I know it is possible to run Gen5 heads on a MarkIV motor (because I'm doing it now!!!). But, I think the water passages are an issue. To run my combo I have to add a small hole for a water passage. BUT I do not believe you can run 4 heads on a 5 motor.

What JpClear said!

Mike
Different combos dude
5 heads can be made to work on a 4 block with the right gasket setup
4 heads wont work without leaking on a 5 block..But will on a 6 block
Something to do with the water passages not lining up and leaking, Ive heard this more then once.
BTW check your casting #'s...5 heads need to be converted to be able to adjust the valves(as I know yours are)..did you have them converted?
Other then that....hows life?
Dan
BTW
Planning a trip to FL this winter?

Yo! Dan! How be you?

Ill buy the book, in the meantime be a sport and look it up for me. I just requested the casting numbers from the seller. Thanx, Chris

PS. Anyone got a line on a set of Gen5 heads ?

I screw up trying to give you 2nd hand advice and it costs you down time & $. I could try to fax you the important pages but I'm not very computer literate. Better you get the info. first hand and decide how to deal with it yourself. That way I don't have to feel bad cause someone took my halfa$$ed interpertratation. But I know the real stuff is out there and in print. You want- You can e-mail me direct or send a PM and I'll give you my home phone; you can call me and I'll read you whatever you want, verbatum. --- Jer

DanB, yes my heads are all set (been running this combo for 3 years now, hope it works!!!!! LOL!!!). As far as Floriduh goes, trying to work it out to go to the worlds in November. Buddy lives in West Palm, and we're thinking of getting a room on the keys somewhere (possibly in Key West, not sure yet) and going to the races on Saturday. But, it all depends on how much money I have saved by about Oct. 1st. Will knw more about it later.