Cody said:
the 351c and 351m are bigblock. the 351w is smallblock.
the best way is by the valvecover.
you have to know the bolt pattern of the valve cover to get the family of blocks then go off the numbers to find out what it is.
The quick way to tell on a big block is the valve cover is flat at top and the intake goes under the valve cover.
but to tell the diff. between the 289 and 302 not much really diff i dont think so it would have to be varified by numbers again.
It's been awhile since i went through this so i might be off alittle
Actually that is wrong, 351C, 351M,351W, and the 400M are all small blocks, with big block bell housing pattern.
To answer your question Rastus, this information below should help you out.
221/260/ Early 289
Although the 221 served duty as a reliable grocery-getter and was never known as a performance engine, the 260 and 289 engines changed how small-block Fords were viewed, first in the Falcon Sprint and later in Hi-Po Mustangs. These early engines can be identified by their unique five-bolt bellhousing pattern.
Late 289/302
The small-block that solidified Ford's reputation for performance was the Hi-Performance 289 4V. Produced from 1963-67, its distinctive idle, and the cacophony of its mechanical valvetrain coupled with ferocious rear tire shredding potential, made its presence known. However, of all the small-block engines Ford has produced, the most popular and frequently modified are the 302, or the 5.0L, small-blocks.
Introduced in 1968, the most desirable of these for street rod use are the hydraulic roller tappet engines. First seen in the '85 GT Mustangs, roller blocks have taller lifter bosses, and the cam bearing bores are bigger to accommodate the shaft's larger diameter bearing journals and base circle. But the real icing on the cake came the following year when electronic fuel injection was added. Unquestionably, for street rod applications, these are the most desirable of the Ford small-blocks.
302 Boss
One of Ford's most famous engines, the 302 Boss was a hybrid produced in '69-70 to compete in Trans-Am racing. They featured canted-valve heads (Cleveland style) solid lifter cams, stamped rockers with threaded, adjustable studs and guide plates, four-bolt mains, special rods, and forged pistons.
351W (Windsor)
Because they look virtually identical, 351W engines (so named because they are built in the Windsor engine plant) are often mistakenly identified as a 289 or 302. In fact, the 351W is based on an entirely different block that is stouter and has greater deck height than the 289/302 (289/302 is 8.206 inches while the 351 is 9.480 inches for '69-70 and 9.503 for '71-current). However, both series of engines use the same bore spacing and the bolt patterns are the same, so 289/302/351 heads will interchange. But, because the increased deck height puts the heads further apart, the 351W requires a unique intake manifold.
Other differences between these similar-looking engines are main bearing diameters (289/302s measure 2.25 inches, 351Ws are 3 inches); slightly larger rod journals (2.1232 inches compared to 2.311 inches), and the the fact that the 351W uses a longer connecting rod (5.956 inches compared to 5.1550 inches for the 289 and 5.0900 inches for the 302).
When swapping parts between these engines, the most confusion surrounds the camshafts. Camshafts will interchange, but the 351W has a different firing order (1-3-7-2-6-5-4-8) compared to the 289/302 (1-5-4-2-6-3-7-8). Compounding the situation, the '82-and-later 302 HO engines use the 351W firing order.
The 335 Series Small-Blocks
351 Cleveland
These unique engines were only produced for four years. They featured canted-valve heads that provided excellent breathing. Heads for the 2-V engines were open chamber with round ports, while the 4-V heads had a quench chamber and huge ports.
351C Cobra Jet
Appearing in 1971 and carrying through to 1972, these engines were an improved Cleveland design with four-bolt main caps. Heads were the 4V open chamber design.
351C-Boss
Also debuting in 1971, these engines had four-bolt mains, 4V heads with quench chambers, and Boss 302-style valvetrain components including solid lifter cams. For 1972 open chamber heads were used and the name was changed to 351C H.O.).
351M/400
Clevelands and 351M (Modified) and 400 engines are similar in appearance because they all use canted-valve heads, but there are some significant differences in the blocks. The deck heights of the 351M and 400 are 1.100 inches greater than the Cleveland's, and while the heads will interchange, the intake manifolds are different. In addition, the main bearing journals are larger, the engine mounts are unique, and the bellhousing bolt pattern is the same as the Lima series