7.3L block with a 6.9L cylinder head

[StevoBelievo]

I bought a 92 F350 drw, Western Hauler package. The mechanic I bought it from promised me it was a 7.3 and only needed a new head gasket, but I soon discovered it needs a lot more. In short, the engine needs rebuilt. A task I thought a friend and I could tackle.
The machine shop told me the cylinder head for the engine was for a 6.9, so I thought it must have had an engine swap.
I had the cylinders measured to know if they were ever bored out and find that it's bored out to the same specs as a 7.3.
My question is, can I put it back together as is with the thinner 6.9 cylinder head and some high end engine studs?
My concern is that if I do a rebuild and use the 6.9 head the higher compression for the 7.3 will warp the head and ruin the engine, waisting a lot of time and money.
Shops and individual mechanics I've talked to have said, "Well if it was running before it will probably be fine."
I am hoping for a more informed/knowledgeable answer.

Thanks y'all

 

[pelky350]

6.9 and 7.3 had the same compression. The difference is the diameter of the head bolt holes 6.9 being 7/16 and the 7.3 1/2 inch. 6.9 heads can't be on a 7.3 because you can't put 7/16 in a 1/2 hole and have threads holding anything. Check the serial number of the block on driver side near injection pump gear housing it will say 7.3 xxxxxx or 7.3t xxxxxx or 6.9 xxxxxx that will mean 7.3l or 7.3 factory turbo or 6.9l. Idk the shop your going to but something isn't right.

 

[david85]

I never confirmed this myself, but I was told the head bolt lengths are slightly different between the 6.9 and 7.3 due to the thickness of the boss on the head casting. Did you see this engine running with the 6.9 heads? Are the head bolts stock and unmodified?

As for compression, the late model 6.9 has slightly higher compression if I recall correctly, so no issue there.

Later 6.9s heads seem less prone to overheating and cracking (1985-1987).

6.9 prechambers are better suited to cold starting, but might result in slightly higher fuel consumption.

7.3s had a better rocker arm design, but that can be swapped over.


What kind of work does the engine need?

 

[StevoBelievo]

Pelky,
Thanks for pointing me to the serial number. I trusted the shop people; just didn't seem like any reason to lie about something like that, but should have tried to confirm myself when the answer was so weird.
Serial number shows a B6.9.
So I guess the mechanic I bought it from was a son of a... Oh squirrel!
Where was I? Ah yes, guess I don't have what's supposed to be in the truck, but now I know I'm not working with a mixed bag.

David,
Yes it was running, albeit badly.
From the machine shop I used for the heads, the boss is thicker on the 7.3 which is why they knew right away the heads were off a 6.9.
The shop I'm working with for the rebuild doesn't machine so I had to use a machine shop for the machining.

The work that needs done is as follows:
New pistons (one had time that had failed), rings, pins, and retainer clips.
Head gaskets replaced
Low end gaskets replaced,
Clutch replacement
Injector replacement
Injector lines w/ new install kit
Machine cylinder heads (the block was fine after cleaning up, less than 0.05mm gap at the worst).
I think that's the list.

 

[icanfixall]

Welcome to the forum...6.9 heads can't be installed on any 7.3 without first drilling out the 6.9 heads to fit the 7.3 head bolts. This has been done before too and it works great. Just plenty of work drilling out 34 holes bigger. You 6.9 thats bored to 7.3 can be used but my suggestion is install a set of ARP head studs. then the heads will not lift from combustion ever.

 

[IDIBRONCO]

Don't worry about the bore size, if you want to know for sure which engine you have, figure out what size your head bolts are. 6.9s use 7/16" while 7.3s use 1/2" head bolts.