I am new to this forum and I have a couple of ?'s about my motor. How do you know the differance between the 351 or 400, I own a 1980 F-350 Dump Truck. It overheated and I think it blew a head gasket on the exhaust side. It has white smoke coming out the exhaust, but no water in the oil or radiator. The engine tag reads 351/400M, I assume it is a 400 since it is a 1 ton, but I could be wrong. I would like to repair it since the truck is restored and I use it alot.
If the heads are warped or cracked I have a local machine shop that can rebuild them, but if they are fatal, what heads will work on it?

Thanks Wayne

 

The 351m/400 sticker on the valve cover says on there somewhere which it is. Look a little closer. It doesn't matter as far as any head work goes, everything up top interchanges between the two.

 

Yeah, the sticker says:
Engine Family: 351M/400
Displacement: 400 cid (6.6 liters) OR 351 cid (5.8 liters)

If nothing else can be seen, there is no other way to tell by looking. Everything is identical on the block, including bore size.

Other heads that will work on these engines with stock intake are: 351 Cleveland 2V, other 351M or 400, or Australian 302C quench heads. Junkyards are still full of these motors with exception to the latter heads, of course! If you want a pick-me-up in power, the Aussies will be a treat. Increases compression ratio, even with stock pistons.

By the way, there is only one 351M. The 400 has no letters after it, and was the first one built (1971), preceeding the 351M by 4 years. Some refer to these two engines as 'M-blocks', as a general reference.

 

Thanks guys, the v/c sticker is long gone. I just wanted to make sure there was not something simple I was missing.

Wayne

 

Well the good news is, if you are pulling the heads to do gaskets, you can easily measure the stroke of the piston when at BDC up to the top of the cylinder. 4 inches and you know you have a 400. Half an inch shorter is a 351.

 

clevelen or modified ? I know they are 1" taller deck for the 400.

 

351M and 400 are the same dimensions 100%, including deck height. The intake manifold is therefore wider than the 351 Cleveland. Does that answer your question?

All they did to the 400 to make it a 351M was chank the crank to a shorter throw and put in taller pistons to make up the difference.
Here are some excellent resources for you to study:

https://www.ford-trucks.com/article/...00_Engine.html

and:

http://www.projectbronco.com/Technic...e_build_up.htm

and:

http://www.projectbronco.com/Technic...the_rumors.htm

and finally:
http://www.fordcobraengines.com/Modified%20History.htm

 

i have discovered 9just yesterday) that the harmonic balancer differ from the 351M and the 400.
The 351 has a counterweight and a small convoluted ring, and the 400 had the same counterweight, but the convoluted rinh is almost twice as wide, so it covers the counterweight fully up.
in other words: if you "see" your couterweights when rotating the crank, its a 351M, if not (and the ring is approx. a inch wide) its most likely to be a 400.
Hope that helps.
No guearantees naturally, but it's a step in the right direction of finding out.

 

Yes, this is in one of the links I posted today:

"The 351M uses a different harmonic balancer than the 400 -- It is best to buy a new 400 harmonic balancer to go along with a 400 crankshaft. You can use a 351M balancer, however, you will have to rebalance the 400 crankshaft by adjusting the counter weights to bring the assembly into balance. Also, the timing marks on the 351M balancer will be nearly useless on a 400 crank -- I know because the engine shop used the 351M balancer on my engine. I had to set my timing at 20+ degrees to get the engine properly timed."

credit to: projectbronco.com

So basically, buy yourself a new balancer for a 400 and be done with it.

 

if i'm not mistaking, that's not what he's asked...

 

His question has already been answered (what heads will work on it). These other posts are just extra information for anyone with a 335 series engine who wants to know more about the 351M and 400.