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My son and I have been working on an 83 f100 long bed. This is his daily driver and we’ve done alot to the truck in the last two years. Mostly cosmetic.
we are not mechanically inclined but learning as we go and have a good local shop that does most of the hard stuff.
here’s the big question
the 300 inline 6 has now developed a big issue. Coolant in the oil and low compression. Guessing bad head gasket. (Hopefully not a cracked block)
like most young men, He would like more some more power and given it’s a manual 4 so trans (with overdrive) he needs some more for the interstate.
we found a solid 351mod at a good price and the shop will swap them out. We plan to rebuild the i6 together And learn together and he can keep it or sell it.
Up to him
here’s my question:
we have the 351mod and the correct bell housing for manual trans. We will upgrade radiator and exhaust
and likely add headers
my lack of knowledge showing with this:
the proper flywheel is now stumping us.
the truck has the original transmission (new clutch recently)
what flywheel do we need?
are they transmission specific? What tooth pattern fits?
Looking for your knowledge, Not ridicule for not knowing the answer.
This board has been a wealth of knowledge and helped us learn a lot.
appreciate the insight
Last edited by MercuryPromoVP; 03-10-2024 at 11:55 PM.
The 351m will require lots of mods that you'll have to undo to go back to the six. And if you spend money to polish that turd it's down the drain in the end.
If you must you need the bell, flywheel, clutch and starter from a 351m or 400. The clutch disc will probably work but check the spline count and depth. Check the pilot bearing size too.
The transmission to bell should be the same Ford "butterfly " rectangle.
Thanks for the fast response.
gotta get him back on the road so we will make this swap understanding that mounts will have to change along with radiators, exhaust etc.
we have the right manual trans bell housing
Juat need to know which flywheel setup to buy
351m/400. You can find them through as auto parts outfits on line. Ditto clutch. 400 bigger than 351m so beware of that. Do you have the clutch fork and linkage?
When he first started driving, (before the i6 started struggling) we had a new clutch put in it,
will the clutch fork need to be replaced with the swap to the 351?
Is your son, and/or you, prepared for the extreme expense of doing an engine swap? These things typically take on a life of their own, and the cubic dollars tend to go out the window on all sorts of little nickel and dime things exponentially. Moreso especially if you're paying someone else for the 'pleasure'. Will he be happy with spending double what he is now on gas to operate it? The 4OD trans is not especially robust and I'm not sure if it was ever offered with the 351M. Are you sure it will stand up to the bigger engine and the more powerful right foot the kid will now have? If it's his daily driver, what is he doing for transportation while it's down? You could have it up and running in a matter of days fixing or replacing the 6 vs the work involved with the swap, and at a fraction of the money. These would be my words of wisdom and insight, fwiw.
It's all about priorities, I suppose. I would agree with 85e150 the smarter course of action would be to stick with the 6. But if you're intent on the swap, stick around and document your trials and tribulations with your story here for the rest of us. We'll be interested in watching it transpire, and will help you out along the way.
Thanks for this response.
this group is sensible and knowledgable and its info we don’t possess.
He’s 17 and working his *** off in between football practice and workouts. He’s been borrowing his grandads truck while we prepare to as you say pay someone for the pleasure of making this swap
pls know one of the main reasons we went the route of an old truck is there have been days And months And years of time with my son In the barn working on his truck and learning together.
im in the music business and had no one that taught me about trucks or mechanical etc.
So we’ve enjoyed it all together.
The good and the bad and the expensive.
He’s saved his money and he wants more than the i6.
he understand that the gas bill goes up etc.
he’s prepared for that. He will ultimately buy another truck or car as a daily that makes more sense financially. But he will have this truck the rest of his life and hopefully he’ll teach his kids what he and I didn’t know when we started the project.
Last edited by MercuryPromoVP; 03-10-2024 at 11:56 PM.
The iron 4OD will last if driven easily. The aluminum will be ok if driven gently. 351m is barely ahead of the 300 power wise.
Yeah, that's kind of what I was thinking, easily and gently, right up until that 6K clutch dumping, abbreviated smoky burnout as the cluster gears fall out the bottom. Yeah, I was 17 once, a long time ago.
Thanks for this response.
this group is sensible and knowledgable and its info we don’t possess.
He’s 17 and working his *** off in between football practice and workouts. He’s been borrowing his grandads truck while we prepare to as you say pay someone for the pleasure of making this swap
pls know one of the main reasons we went the route of an old truck is there have been days And months And years of time with my son In the barn working on his truck and learning together.
im in the music business and had no one that taught me about trucks or mechanical etc.
So we’ve enjoyed it all together.
The good and the bad and the expensive.
He’s saved his money and he wants more than the i6.
he understand that the gas bill goes up etc.
he’s prepared for that. He will ultimately buy another truck or car as a daily that makes more sense financially. But he will have this truck the rest of his life and hopefully he’ll teach his kids what he and I didn’t know when we started the project.
Dad, good on you for taking the time with your kid and giving him a healthy outlet for learning and keeping the lad out of trouble. This kind of encouragement seems to becoming more and more rare in today's society. The truck looks great and I wish you both the best of luck with the project. Like I said, please keep up posted on your progress.
I would of found a EFI 300 six as it is a drop in, no swapping or finding of parts.
I bet with a good running 300, read not worn out, he would be happy with that.
Use the EFI exh manifolds with the factory log intake as the EFI flow better and run 1994 F150 exh pipes as they are bigger than the 80's and it is a bolt in.
Or you can step up to headers and a v4 intake and carb as they say that helps in power.
If you did not want to run a carb with either the factory log intake or after market v4 could look at Holley's Sniper EFI.
They make a kit that bolts to the factory log intake.
Oh your new clutch is most likely a 10" with the six as mine was and the v8 needs a 11".
So a new clutch and pressure plate will be needed.
I would be looking for a different trans as I bet it will not last long, them 3sp OD were junk to start with.
Look for a M5OD out of 87 up 150 / 250 trucks that ran 300 / 302 / 351 motors.
Only thing is the really newer ones may not have a speedo hole for the cable so ook for that when looking.
It has over drive and shifts more like a car than the T18 / T19 / NP435 granny first truck transmissions.
I love my 81 F100 flare side with the 300 and NP435. I added over drive to it but it is no longer made.
I run the 300 log intake and carb with EFI EXH manifolds and factory 94 F150 pipes, Thrush welded muffler less cat and it sounds pretty good.
I avg 15 MPG combo driving and that is not really trying to get good MPG. That is over 5 years and 28+K miles.
I have seen 17 MPG at times just dont remember what I did to get that
The other thing is when I go to cars & coffee or shows everyone has a v8 only real man have straight sixes
My .02
Dave ----
Fuzz Face,
thanks for taking the time to lend all these thoughts.
i believe you are correct. The new clutch that was installed about a year ago was a 10”.
so we will weigh the pro’s and cons of a new clutch and plate vs maybe looking at a more appropriate used transmission. Not something we want to do right now but something that we should at least explore I guess.
Back to original question. Can anyone tell me about the needed flywheel for the 351mod?
I know there are different tooth patters etc.
which flywheel do we need if we stick with the original manual trans?
Sorry to say I dont know as the 351m (400) were only used in think just the 80 year truck and dont even know if used in the F100?
You may want to check the <79 truck area as the 351M / 400 was used in them year trucks so they may know.
Also there is a motor area down on the main page and they break it down on motor size so the answer may be in there?
Or ask in both places what you got and want to do as I am sure someone at least in the truck area would know.
Yea on the trans did not think you would do it now unless it falls in your lap but something to look into down the road.
When I was re-building my 81 F100 flare side I checked out all different year truck to see what others were doing and what might work on mine.
I knew I wanted to stay with the 300 for MPG and granny first for pulling my car trailer but anything else was open.
That is why the EFI EXH manifolds and large 94 pipes for power and over drive splitter for MPG (not that it helped much) but able to split gears for pulling.
03-10-2024, 10:44 PM
