I am about to buy a 78 F-250 4X4 with a 351 M and I have seen a lot of bad postings about the engine. Can some one give me hope or atleast words of wisdom?

 

I have had two and havent had any problem yet.

 

The 351m and 400 are excellent truck motors...lots of tourque but lower hp. People just don't like them because of the low hp rating, and there weight. I sold my last 351 M a year ago with 325K on it, sure I put in bearings and rings around 200K but it outlasted my 7.3.

 

They're great motors, but they suffer from low oil pressure at high mileage. I personally would not buy one for a daily driver with much over 125K on it. They will last way past that, but they'll be running with only a few pounds of oil pressure at hot idle, and it gets worse as they get older. That's fine for some people but it's not fine for me. My 400 (same engine) has 150K on it, and it has so low oil pressure that it won't even bump the needle off of zero at hot idle. It will barely push 25 pounds at WOT. It has just enough to keep the lifters from ticking too loud. I don't drive it much outside the yard. HV oil pumps will only give you about 20% more pressure; not much. You will find countless people here with similar stories. The block has documented deficiencies in its oiling system design.

There are many nice people out there driving their 351M\400 around as a daily driver boasting that it has 175K on it and that they are such great motors, but they don't realize that they are risking bearing failure and throwing a rod because they're romping their trucks around with barely any oil pressure in them, because they disconnected the oil light when it started flickering at stop lights. Do whatever you like, but if this is a vehicle you're wanting to use to get back and forth to work without spending a lot of money to fix up and it's got a lot of miles on it, I'd either keep looking, or keep some cash behind to put towards a rebuild eventually.

 

This is a truck that I will eventually re-build, but it will be a gradual thing as I am just about to finish college. In other words it won't be my daily driver just a toy that I would like to think of as somewhat reliable. A unique truck.

 

If that's the case then, I think you'll be happy with it. When you rebuild it, you may want to consider using a crank and pistons from a 400, because that's the only difference between the two motors, so you'll get almost 50 extra cubes. There is a respectable aftermarket niche for these motors as well.

Welcome to the site, by the way.

 

Welcome aggie, I have the 351M easly in my 77 and it is a nice low end grunt engine, I am planning on swapping to a 460 at some point and time, and having a 351M/400 is the easiest way! I just drove mine down to Fresno and back and it just humms right along. You aren't going to win any races, but with my quad of 800+ lbs in the back, it will idle all day long in 3rd creeping around the fire roads up here. I say get it and enjoy it and NEVER get rid of it!

 

So far my 351M has never let me down (knock on wood!). It has less than 40 k on it since a rebuild in 95 when it got a high lift cam. Still runs very smooth and has plenty of power and lots of oil pressure.

It's true, my truck isn't going to beat a fat kid on a Huffy in the 1/4 mile, but IMO, that's not what a 4x4 3/4 ton is for anyway. In low range and in the granny gear it could probably pull my house of the foundation though. I hauled a ton of railroad ties last weekend and the motor didn't even know we had a load on.

I have heard, and believe the stories--these motors DO have a lot of problems, especially when they get some miles on them. Someday I'll likely swap something else in if the motor starts to take a crap. For now though, since my DD is a 2003 Ranger, the 79 doesn't have to run unless I want to haul/tow something big, or just drive it for the fun of it. If that's how you plan to use the truck you're looking at, as a "spare", then what's the difference?

Now, go get that truck!

 

I agree with everyone on here so far.

I've got a 1978 f250 that originally had a 351m in it. The engine ran fine but had a slight knock in it. My dad found a good deal on a super low mileage 400 so he swapped them. The 351m had less than 50,000 miles on it. This 400 we put in has a lot less, I'm not sure how much less though. Anyways, I can't remember how much oil pressure the 351m made, but the 400 will push the gauge about 3/4 of the way when it's cold, but five minutes after starting it it'll drop down to like an 1/8th or less... But the engine doesn't make any noise and has been very reliable for us. I drove it about a year and a half as a daily driver (back when gas was much cheaper) and I never once thought it would break down on me!

Interesting note: I recently bought another 1978 f250 custom with a 351m in it. I think mileage is like between 100,000 and 150,000 miles (I can't remember). I do know the top end was redone not too long ago. But this thing produces gobs of oil pressure!!! I'm starting to think there's something wrong with the wiring or the gauge!

Basically these engines can be built to give big blocks a run for their money, however, don't get me wrong there's no replacement for displacement! (I've been waiting to use that phrase!). They just don't have a very good oiling system. And they've got a bad rap for being a "smog engine" They got a lot of potential but epa smog regulations that came into effect shortly after these engines were designed really cut their potential because these engines weren't designed with those regulations in mind. So these engines never got a good chance to unleash their potential.

Go take a look in the "335 small block" forum in the engine section to get a lot of good info on these engines.

Welcome to FTE, lots of good folks and info here!

 

The best thing you can do is use the 351M as it is and take you time building a good 460. After you swap out the 351M you will never look back....unless you want to see the grill of a Chebbie.

 

I've only had my beast (79" F250 crewcab, 351m/4spd), for a couple of weeks, but I'll add my humble opinion to the mix. The 351m isn't the fastest horse out of the gate, but it's a strong motor and great for hauling. A few days ago I picked up a full load of aluminum framed doube paned windows. My best estimate would be about 700 lbs of weight in the bed. That engine didn't even feel it. Based on my "newbie" experience with this truck, along with what I've learned here at this forum (this place is a WEALTH of knowledge), I think the 351m is a workhorse motor. You won't win any races with it, but it will haul and tow pretty much anything you want.

 

Yep good motors just the oil pressure thing. My 400 runs about 5-10 psi at hot idle in gear and about 25 psi doin about 55. If you rebuild the 351 go with 400 crank and rods to get the stroke.

 

Howdy Ag! Class of 82 here. As has been stated, there is nothing wrong with the 351M. I had the engine in my 77 rebuilt at 315,000 miles. For the 100,000+ miles prior to the rebuild, it had low oil pressure.

As has also been stated, the only difference between the 351M and 400 is crankshaft and pistons (connecting rods are the same). If you choose, when you rebuild it, you can get the 400 pistons and crank and build a 400.

 

I love my 351 m its awsome i did have to put a high voulme oil pump thow to make the oil psi were i would like it but i love my truck and its motor its awsome it rocks

 

My 351m had low oil pressure when I bought it now it has none, its on a stand on my garage. I am pretty happy with it other than the oil pressure problems