Dumb question?
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Junior User
Joined: May 2002
Posts: 65
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From: Plymouth USA
Dumb question?
I hope this doesn't sound too stupid. I am working on a 1977 Ford 750 Series water truck. The people I work for just bought this truck and were told by the previous owner that the engine is a 391. The engine, to me, looks identical to the 390 in my '74 F250. It even has a backwards 105 cast under the left head. My questions are: What is the difference between a 391 and a 390? Are parts interchangeable between the two?
Senior User
Joined: Oct 2001
Posts: 479
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From: Miami U.S.A.
Dumb question?
The parts from one motor to the next are somewhat interchangeable. But, there are some difference in them. I odn;t know what all of them are. I would suggest looking on this site for a book called "How to rebuild Big Block Fords". It will help to inform you of the differences between the two motors. There are almost no external differences between the two. The changes are internal. The intakes can be swapped but the 391 uses a different sized distributor. So the hole in the intake is a different size. If you use a FE intake you will need a FE distributor. This will give you some idea how many differences can come into play between the two Engines. Good luck with the fix up job.
Scotty
Scotty
Post Fiend
Joined: Feb 2001
Posts: 6,624
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From: Missouri
Dumb question?
One way to tell is by measuring the diameter of the front snout on the crank. The 391 snout will be bigger than the FE. Also most of the time (not always) the FT engines will have a shiny plate on the side of the engine with all the information.
Elder User
Joined: Jan 2002
Posts: 593
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From: Texas panhandle
Dumb question?
Wrong, Scotty. The 391 intake will fit a 390 but the ports are all different. The same with the exhaust manifolds. They exit in the center, pointing down, the exhaust crossover is bigger on the FT motor and it has 2 more bolts (one on either side of the crossover passage) in the ex. manifold. The 391 will have a forged steel crank with about a 2 in. diameter snout. It probably will have a cast iron front cover and the engine is mounted on a plate that bolts to the cover. The flywheel housing should have mounting ears on it also. The flywheel is larger that a 390 and is drilled for a 13 in. clutch. It also has a counterweight cast in the backside. This flywheel was used on 428 industrials, also. You can put a 390 crank in the 391 block and use car/pu heads on it and make a pu motor out of it. Some urban legends say that they (391s) can be bored to 428 but becareful with that one. Some guys grind the crank snout down to car/pu dimensions and have a steel 390/427 crank. 
The 391 and the 390 share a lot but there are a lot of differences, too.
The 391 and the 390 share a lot but there are a lot of differences, too.
Post Fiend
Joined: Feb 2001
Posts: 6,624
Likes: 8
From: Missouri
Dumb question?
The dizzy hole in the intake is the same for both. Its the hole inside the block that will need to have a bushing installed for the FE dizzy. This can still be bought at your local ford dealer I believe.
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Junior User
Joined: May 2002
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From: Plymouth USA
Dumb question?
> DHH, lots of interchangeablity here, but not everything.
>What are you trying to do ? Is the 391 running or is it a no
>starter ? DF
DF, the 391 is running fine at this time. I am just trying to verify that it is actually a 391 and not a 390. I will be working on this truck for as long as I have this job, so the more I can learn, the better off I'll be. Was the 391 ever used in the F-Series trucks or is it just a "big" truck motor? Thanks for all of your and everyone else's help.
>What are you trying to do ? Is the 391 running or is it a no
>starter ? DF
DF, the 391 is running fine at this time. I am just trying to verify that it is actually a 391 and not a 390. I will be working on this truck for as long as I have this job, so the more I can learn, the better off I'll be. Was the 391 ever used in the F-Series trucks or is it just a "big" truck motor? Thanks for all of your and everyone else's help.
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