What was the better of the two engines? I always thought it was the 400 but i heard they have problems with over heating cause of thin clyinder walls.

 

351M and 400 have exactly the same block. 400 just has a longer stroke crank with different psitons that raise the wrist pin to accomodate the added stroke without shoving the pistons through the head. Like any engine, they can overheat due to poor circulation, stopped up radiators, or falty thermostat.

351M and 400s do take a different thermostat than any other Ford motor, it has to be the one used as the bypass is cast into the block itself just below the thermostat and as the thermostat opens, it restricts the bypass, forcing more water to go through the thermostat. If the wrong thermostat is used, if it doesn't reach down and restrict the bypass, that will allow a certain amount of water to bypass the thermostat and radiator and just get pumped back through the block and heads hot.

I did once replace mine as a standard practice with antifreeze change one fall, used a correctly sized (diameter) one meant for a 302 / 351W as couldn't locate one for a 351m on the spur of the moment. Shge would slowly just keep warming up as you drove along, even on a cool day. I changed it to a Robert Shaw HiPer Thermostat found at Auto Zone or Advance a few days later, one intended for 351m/400s. Cured the warming feature, it stays rock solid now once warmed up, winter or summer. That's the only kind I'll use in the truck now.

I have been told that 351ms are harder on main and rod bearings, especially mains. I replaced mine in the truck from underneath at 75,000 miles when oil pressure had dropped to 4 psi on a trip. Crank looked good, front main was worst being really well worn, they looked better as I worked towards the rear. Rods weren't all that bad. Machinist of many years experience said that was normal for a 351m, he thought it was a balance issue as 400s seemed to be better about it ... but I think I read somewhere here it was related to oil feeding the cam, etc???

I also installed a HV oil pump then along with the new std bearings, and at 101,000 she maintains 70 psi even when hot. 351m doesn't use a drop of oil yet.

Gonna finish a 400 I have in the basement on the stand someday and put it in just because I have it and I figure gas mileage won't be no worser. Needs body parts replacement worse now I think.

 

The 351M and the 400 share the same block. The crankshaft is the difference.

IMO, the 400 has a better potential over the 351.

Jason

 

77 and earlier blocks had a tendency to crack in the lifter galley. 351Ms do pound the snot out of the bearings.

Are you asking because you want to choose a motor to use in your truck? If so, I would strongly suggest looking for a different engine, or do some serious searching for answers on how to build a 400 to not be a turd. The parts recipie is out there, but its expensive. Build a 400 over the 351, because its 49 free cubic inches.

 

I'd go with the 400, b/c it has more torque, and they are the same other than that.

 

Clicky linkey

 

I have two trucks i am looking at each the same but different engines. if i could i would put a 390 in it.. is the tranny not going to bolt up though it ha.... a c6 in it. 4 speed would be nice though

 

You would need a different bell housing pattern and engine mounts to put a 390 in there.

 

I would have to agree with the observation that 351M is tough on mains. I worked on quite a few back about 25 years ago when I was turning wrenches, seemed to occur at about 75000 mi. Conventional wisdom at the time was that they would live longer with HV oil pump. Don't recall seeing this so much with 400.

 

I read most of this stuff & shake my head with agreement because for the most part with these old trucks I've been there, dodn that.

but every once & a while you learn something.

My 351M creeps up in temp just as you stated.. I assumed I was due for a radiator as I have been through these hoops before with other old fords... but I think I'm going to try a thermostat first... and I'll make sure it's the right one

Thanks for posting that info on the thermostat, as simple as it might seem, I would have never given a second thought to simply tossing the correct dia stat in there & expecting it to work.

Dave

 

Yep, that's exactly what Brownie at the machine shop told me those years ago. When I took one main and one rod shell with me so he could measure to tell me size (Ford numbers on back didn't help, not even with the Ford parts man, Brownie measured the thickness of the shell over near the tang on side, and compared to a book), his first question was how many miles. I told him 75,000 and he just said "Right on schedule" and laughed. Said 400s didn't seem to do it, and I have a buddy who used to run 400s in a couple F-350 wreckers and he never had a problem with their bearings, even after well over 100,000.

Maybe the theory of a balance problem isn't so far fetched after all? I do recall mine's front main was worrn way down into copper, and that the ones closer to the rear were showing less copper as I worked my way back. All the rods were worn near abouts the same, not as bad as the mains. Of course, have that flexplate and torque converter balancing at the rear.

Flow Kooler Robertshaw Series High Performance Thermostat number BRA-333-180, about $25 now I think, it's all I'll use in her now, keep a spare.

It was my buddy I mentioned above with the wreckers that pointed out why the 351m/400 took a different thermostat number in all the books. You are welcomed, just hope it cures what ails her.

I also added a circular disc to the water pump impeller and attached with three stainless steel pop rivets, same diameter as impeller blades, to keep water on the paddles as it turns. I think it came in a kit once. Easy to transfer. Easy to make too if one was needed.

 

cool, this is all good info. other than obvious stock power advantages of the 400 over the 351, I had always thought of the 351 as the one that often gave you mileage advantages but still had adiquite power... well, now I have one. I had a 400 in my last F-150 which was a 78. I was mean to that engine, really mean. I hate to admit it but I honestly did try to blow that engine up quite a few times, over rev, float the valves.. mean, mean, mean!

She would never let go, couldn't do as much as bend a pushrod or jump the timing gear... sure she was clapped out, old & tired... but I couldn't break it. I finally rebuilt that one, damn tough old bugger deserved it.

Now I've got the 351 in a 77 & your comments on balance really mean something to me. I swear to god when I start that engine cold I can watch that damn crank shakin in the journals!! as it warms up it smooths out but I worry about the froont crank bearings.

power is way below my old 400, mileage seems no better with the 351.

I'm thinkin I'll look for a 400 cank, flex plate & H-balancer... See I have a spare 351 in a stand in the shop, might as well turn it into a 400. do I need the 400 rods or do they use the same rods?

 

My 351M is just about dead now. It knocks, it smokes, it hardly runs, heck, it has no compression in 2 cylinders that i am aware of. If I can get it running again, I plan on putting some cardboard and plastic under it, put a brick on the gas pedal, and see how long the motor runs at WOT before it dies.

 

If you end up doing that let us know how that goes i guess its a good test to see how tough it is but it sounds weak

 

And I have TWO of them now....Cool!

Don't know what I'm gonna do with 'em...

Boat anchor?

Paper weight?

Any ideas?

Anyone?

Anyone?