Originally Posted by 77&79F250 (Post 14211866)

When the 351 Cleveland was discontinued after the 1974 model year, Ford needed another engine in that size range, since production of the 351 Windsor was not sufficient and the 390 FE was also being retired.

To replace the 390, Ford took the 400 engine's tall-deck block and de-stroked it with the shorter-throw crankshaft from the 351 Windsor, and taller pistons, to produce a 351 cubic inch (5.8 L) engine whose components were largely compatible with the 400. 351M block castings were modified to prevent cracks in the lifter area; additionally, the 351M casting contains X marks cast next to each lifter bore.

351M production began in 1975 in the Michigan Casting Center, and continued until mid-year 1978, when manufacture was transferred to the Cleveland Foundry/Cleveland Casting Plant.

The M-block, as it became known, was the last push rod V8 block designed by Ford. The M-block also shares some elements with the Windsor engine family: bore spacing, cylinder head bolt-patterns and crankshaft journal dimensions.


There exists debate as to what Ford intended the "M" designation of the 351M to refer to. Some claim the "M" stands for “Modified” - due to the derived components from both the "Cleveland" (block, heads) and "Windsor" (crankshaft) components - though others claim that the "M" refers to the Michigan Casting Center, where the 351M began production.

Likewise, Ford's use of the 400 block in the creation of the 351M engine has resulted in the 400 mistakenly being referred to as the "400M" or "400 Modified," despite having been the design basis from which the "modified" 351M was derived. Additionally, while the 351M and 400 motors are both based on the Cleveland block, Ford's official name for the block contains no additional designations - the proper nomenclature is simply "400."

History of the Ford 351M

M-Block 351M/400 Parts Reference

351M/400 Performance Build Up

Rumors & Myths

Originally Posted by NumberDummy (Post 14212026)

M referred to Modified from day one. The heads used on these smogged up turds (1975/78, FoMoCo changed 'em in 1979) were very prone to cracking.

Most of the time this occurred after the 2 year or 24,000 mile warranty period ran out, which p!ssed off many owners, as Ford refused to do anything about it.

Because new heads didn't come with valves, usually rebuilt heads with valves were installed to replace the cracked turds.

The Ford dealer parts & labor cost to the customers back then was roughly $700.00, which included the heads core charge, since the POS heads were no good.

700 bucks was a lotta money back then. From circa 1975 thru 1982, I passed out well over 400 sets of these heads to mechanics to replace the turds.

One parts guy at one dealership, now multiply this by 7,000, the number of US/Canadian FoMoCo dealers back then.

IMO, the 351M (and 1975/78 400) belong in the same FoMoCo scrap heap as the awful 3.8L "miracle" V6, A4LD & AOD.

Originally Posted by trozei (Post 14213080)

Just gonna but in here and point out that there is no such thing as a 400M. Nice truck though! Love it.

Originally Posted by spikinfool05 (Post 14212883)

ya thats a LOT of weight for that pickup. not the safest way to haul a backhoe but guess it gets the job done.

Originally Posted by Pickupmanx2 (Post 14215651)

LOVE it!! Always like seeing old trucks still working and doing what they were made to do!

Not to hijack, but here is my 79 F250 workin!

Originally Posted by spikinfool05 (Post 14214869)

i wouldnt even haul it with a new 6.7. for what you can buy a old day cab semi and flatbed for why spend the money. thats why i sold my diesel.

Originally Posted by david schmidt (Post 14216038)

Yes sir!! Don't get me wrong, I love a clean restored truck, but its good to keep 'em working. That is quite the payload!