I've been puzzling over something and I was wondering if I might be able to get some input on here.

My question branches off of this thread: https://www.ford-trucks.com/forums/1...e-dilemma.html Which, summed up, that I have a '79 F150 with a replacement 400 installed and the original 351M sitting to the side. I'm wondering if I should rebuild the 351 that is supposed to have 150k on it or just run the 400 with unknown mileage and hope for the best.

From what I've been able to glean from the other thread and elsewhere is that the 400 is preferable to a 351M. Is that also the opinion over here? If this is the case, can the 351 converted to a 400 since the difference in the parts seems to "only" be the crank, pistons, rods and balancer?

I'm a gunsmith not a mechanic. The only engine I've overhauled is a Briggs and Stratton in jr. high school.

I don't have a lot of space, time or money for this project so I want to keep the "hassle" down to a min. "Hassle" being swapping the 351 back in as-is, rebuilding the 400 then swapping back. The much more attractive solution, if the 400 is the better way to go, is to make the 351 into a 400.

I've learned a ton, in several respects, on this forum in the short time I've been on here and am looking forward to learning so much more.

 

I would drive it like it is for now while building the 351 into a 400. As far as I know the crank and pistons are the only difference between the two. I know the rods are the same and someone can correct me if I'm wrong but I think the balancer is the same? Aftermarket ones are listed as the same at summit racing anyway? Good luck.

 

If the 400 that is currently fitted is running OK leave it and Rebuild the 351 into a 400, I would suggest the 400 was a really good price or the 351m is buggered.
Pistons and crank are different but the rods are the same. Have a look at Tim Meyer's website and he does a whole rotating assembly amongst other things. I would also see if you can get a hold of Tom Monroes book "rebuilding ford engines-351C, 351m-400 and 429-460". Have a good read of that and see what you are up for, if you are anyway mechanically minded you will be able to work out if you are up to the task. I am a fitter turner who always wanted to be a gunsmith so i'm the other way around although i did have a friend who rebuilt engines in a previous life and he helped me on a couple of tricky things. If you are changing crank and pistons you are up for at least the block getting machined so those checks not so bad as long as the block is alright. Heads would be the next one so you will need to determine what you want out of the truck ie 4v-2v, valves cam etc. What sort of intake manifold/carby is on the 400?
If these are an upgrade over standard you could save some coin and swap these over to the rebuilt engine maybe. Same goes for the heads. Depends on what condition the heads on the 351m are like. Any cam with lift over 0.500 starts to compromise factory springs, so a K kit with Cam, springs, retainers and timing set may be the go.
All depends on the budget, ultimate use of the truck and your ability (saves money so you can afford better go-faster bits).
Even consider factory 2BBL cast manifold and fit the biggest 2 barrel Holley on it and that would be ideal for low down tq.
4v heads with a big cam and valves and big carby good to give a Mustang a fright at high speed, but not so good down low. Lot's of things to think about and how far do you want to go?

 

Thanks guys!

I thought I had understood that there were just a very few differences between the 2 engines.

As I said in the other forum, I just need this as short distance driver and occasional hauler. My budget is extremely limited right now. I can probably do the the crank and pistons if it doesn't cost too much. Someone on the other thread mentioned the timing on the cams were retarded in '79 and get cams with straight timing. I think it was also mentioned that if the cams aren't replaced there's a timing set that will compensate. I don't know if I can get any more high speed then this right now.

Wouldn't anything else be a "top end" modification? As far as I know both engines are totally stock, they at least appear to be. If I can just rebuild it with the stock top end maybe I could come back to it sometime later.

BTW, is there any way I can determine approximately how much wear is on the 400? Pull the heads and see if it looks like it's in decent condition or something? As far as I know it was just a take out of another truck for a big block upgrade and might have 350k on it. On the other hand, it might also only have 35k, I have no Idea. It was only in the truck for about a year before it was parked. If it is in reasonably good condition maybe I'll just sell the 351 and just use the 400 as is.

 

Budget and quality usually don't go together. I can pretty much guarantee your old block will need align honed. I'd say for every 10 blocks I have ever torn down 8 of them need align honed, particularly the later blocks. Pistons are a problem unless you get Tim Meyer's zero deck pistons you will have done nothing to increase it from it's pathetic starting point. Stock pistons have huge dishes and are way too far below the deck to begin with for milling to do any good but either way deck milling needs to be done with any motor being rebuilt to get it squared up and flat. Once pistons are changed out balancing is a must do. The cranks oil holes should also be chamfered. Rods should be resized with a new set of bolts. Bearing clearances should always be checked with a good quality dial bore gauge reading in .0001", not plastigage. Once you do all that then you can move on to the top end. Cam shafts should always be degreed in but if you can't do it then get a good quality double roller, set it at zero and hope for the best.

 

You can always pull the heads and see how much of a lip you have on the top of the cylinder. This is the part the rings never get to and will be pretty close to what it was last bored to. This will only sting you a top end gasket set/kit. If there is none and it blows no smoke and goes ok I would probably leave the 400 alone and maybe look at turning the 351m into a project OR you could flick it (dont expect to get much for it) and put the money into something else. There are multi keyway timing sets to help but honestly a relatively good cam and lifters (without going stupid) arent that expensive and if you have determined that there is no lip in the bore etc and your heads are ok, you could do that yourself over a weekend easy as long as there isn't any other stupid problems. Get the timing set any way.
I would suggest having a friend around who has some experience who wouldnt mind sharing his knowledge for a couple of beers in the case you've bought for the occasion LOL Beer economy at it's most pure.
Just dont start to early and make sure the complicated stuff is done before you start Ha Ha. I have tried to do this exact same thing as yourself but with a top end and it ended up having to rebuild the engine not many miles but about 18mths later. As said before does the 400 go ok? is so leave it and start on the 351m. Once you've done that up the way you want, slot that in and sell the 400 instead. Just remember everything you do or buy within reason can be changed over to the running vehicle to keep it going in the event of a failure. Mark A, the advice for the dial gauge is fine, but that is getting into shop tools and the majority of people wont have access to that or even know how to use it. Probably a good thing, but If I had a block machined and assembled the engine myself (allowing for the fact it isnt a really hi-po engine) the plastigauge is there to find a fault. If there is a problem you take it back to the shop. Most of what you have suggested is what the shop does and a great idea but if the shop is worth their salt they should be doing it anyway. Good info for GOVTMOD though.

 

mark a. & crsmiffy,

Thanks for the input. I'll keep that stuff in mind.

I'll see how it runs and then pull the head(s) and take a look.

I just might have the resources in the beer, gun, trigger job & ammo exchange to handle the stuff that I can't handle myself.

 

Ha ha. Good stuff thats the spirit

 

Actually no joke. My minimum for the "well, I've got to give you something" is a 6 pack of Samuel Adams Boston Lager.

 

If it runs Ok leave it alone and save your money for a rebuild on your other motor . If you are worried about how long the motor will last , do a non evasive compression check .

First thing you need to do it take the 351 apart . It could be a basket case , they didn't pull it out just for fun .

Its your time and money , but for my two cents , I would not worry about expensive zero deck pistons , fancy cams , etc . A stock rebuild with maybe a high volume ( not high pressure ) oil pump for a smooth running dependable motor . It is easy to spend money on a rebuild .

If you have extra money and the truck is a toy then have a blast .

 

I've had some interesting developments in the past few days that may solve my dilemma.

I'll let you know when I see how things pan out.