acording to my vin number my truck was ordered with a 391 engine but the seller told me it was a 351. How can i tell for sure? thanks

 

Looking at it should tell you.
If you have an F500 or greater truck from this era, you may have the FT engine, it is similar to the FE engine and looks completely different than the 335 series 351M that you probably have.
Hit up the guys in the FE, or FT forum, and you will find out several differences, and why most stay away from the 391.

 

giving my question a bump

 

No FE engines after 1976, given your truck is a 1978 it has a 351M.

Josh

 

A 391 is an FT motor, the 351M was a standard motor from 77-82 in light duty trucks. The 351M is a member of the 335 series of motors. The FT motor is visually very similar to the FE motors. My understanding is that FT motors were made for medium and heavy trucks/busses and continued in production later than 76 when the production of FE motors ceased. Look at the angle that the spark plugs screw into the heads. A 351M, the plugs would be all the same angle. The FT motor would have the rear two spark plugs angled forward, an the front two plug angled back on each head. Count valve cover bolts, the 351M would have 8, the FT motor 6. The thermostat housing on 335 series motors bolts directly to the front of the block and not into the intake manifold. FT motors would be similar looking to FE motors, with the thermostat housing as part of a wide intake manifold that forms the inner third of the valve cover seal. How many valve cover bolts do you have?

 

FE/ FT engines have 5 bolts per valve cover.

And although an FT "might" look like an FE they are very, very different.

Josh

 

Thru 1980, the 4th digit of the VIN is the engine code.

1977/79 F100/350 VIN engine codes:

B = 300 I-6 // F = 302 2V // H = 351M 2V // J = 460 4V // S = 400 2V.
---------------------------------------------------------------------
FT engines (330 2V M/D / 330 2V H/D // 361 2V // 391 4V): 1964/78 F500 and larger trucks, with one exception.

1973/78 F350's made for U-Haul by Ford have 330 Medium Duty FT engines.

 

I should have known better about 5 bolts on FE/FT valve cover, I have a 390. This inaccuracy aside, I believe that the visual cues that I quantified are accurate. I know that many things between the FT and FE motors are different. The context of what I was trying to say is that the motors are visually similar. I'm not proposing that while looking at an engine, that I could see that a crank snout is larger or smaller by hundreds of an inch or how the heat passages inside an intake manifold are routed. I wasn't proposing that the engines parts on FE or FT are freely interchangeable. I have read about people that swap pieces between FE/FT engines knowing what works and what needs to be machined and what won't interchange. The features I identified such as the intake manifold overlapping the valve covers, and the spark plug angles on a FE/FT vs a 335 motor's visual uniqueness such as the thermostat bolting directly to the block, would help answer the original question 351 vs 391. It is easy to say that it's a 351M and I'm betting it is. These trucks are old and subject to 30-40 years of engine swaps so I wouldn't rule out a less than obvious anomoly (who knows what somebody could have special ordered). I've been a bit lax with some info such as the stroke of a 390 being 3.75" instead of more accurately 3.78". My intentions when I post is to help answer questions asked based on my actual experience or research I've done from books or this website. I appreciate re-education when I am wrong. I used to be pretty involved with my "old brown truck" back in the late 90s using this forum to help me learn about what I had/have and accomplishing two different engine swaps. I've been dormant from working on my truck until recently. Now I have a 79 4X4, my first 4X4 and I've now have a 351M that I'm learning about. I also have delt with two different 302 engines in dentside trucks. My perspective is not as a machinist, a master mechanic or a Parts expert. I'm just a guy that has owned several dentsides, a bumpside, an old 70 econoline, and has swapped pieces. My first 390 required that I remove the heads, install an intake manifold, cam, lifters, rocker arm shaft. I've swapped engines, transmissions, and 9" pumpkin. I made the classic mistake of rebuilding heads with a weak low end causing broken rings and a broken piston skirt. I've had a 302 thrust bearing go bad. I've been successful in changeing power steering gear, steering column, swapping 3 different kinds of transmissions into my 76. Perhaps this only makes me dangerous.

 

No need to write a book... it's pretty obvious the engine isn't a 391.

Josh

 

looks like i have a 351M. What does the M stand for? thanks

 

looks like i have a 351M. What does the M stand for? thanks

 

Doesn't stand for anything actually.

Basically Ford had to distinguish it from the 351W (Windsor) and 351C (Cleveland)

Some call it a Modified, but it isn't a modified anything.

Some say it stands for Michigan, but in fact some blocks were cast in Michigan, but they were also cast in Cleveland. Would that make it a 351MC?

Josh

 

I like to call it modified, it is basically a modified 400.

 

I've heard M for Midland TX, or M for Michigan, heard it debunked as innaccurate Heard M for Modified as in a cross between a Cleveland (same heads) with a Windsor (taller deck than cleveland and same larger crank journals as the Windsor). The block is a 400 block with a shorter stroke crank. The reality is that it's different than a 351 Windsor and different than a 351 Cleveland, and you got to list it somehow to distinquish it so that you don't buy parts for a completely different engine. 351M is what everybody uses to identify the motor. I believe I read somewhere that Windsor engine blocks have been cast in the Cleveland foundry. Just names to identify different designs. This discussion reminds me of the great debate of what FE is suppose to stand for.

 

No idea how Midland ever got stuck with it. The most common explanation I've heard for FE is Ford Edsel. Oops, did I just fuel the fire?