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February 2015

Q. It seems like building a 383ci small-block Chevy by putting a 400 crankshaft in a 350 block is common but I’ve also heard about going the opposite direction by using a 350 crankshaft in a 400 block for 377 ci. Frankly, I really like that idea just because I’d like to have something different, but I don’t want to do something stupid just to prove a point. What’s your opinion?

Bill Jonas
Via email

A. Both these swaps have been around since the 400ci small-block Chevy was introduced in 1970. These engines featured four-bolt main caps for the first two years of production; from 1973 to 1980 two-bolt mains were used, all had cast crankshafts that were externally balanced.

To cram the large 4.126-inch bore size into a package that started out as 265 ci required some compromise. The bores had to be siamesed, which meant that water did not flow all the way around the cylinders. To prevent hot spots, steam holes were put in the decks, head gaskets, and heads. Nonetheless 400s developed a reputation for overheating, however that was often the result of using gaskets or heads that lacked the necessary steam holes.
Another difference found in 400s is the main bearing diameter as these cranks have 2.65-inch main bearing journals as opposed to the 2.45-inch found in a 350. Connecting rod journals of the 400 and 350 were 2.10 inches, however the 400s used a shorter rod that was 5.565 inches long, as opposed to the 5.7 rod found in the 350.

We can’t say for sure who was the first to come up with the combination but someone figured out that by cutting down the main bearing journals of a 400 crank and stuffing it into a 350 block bored 0.030-inch oversize, 383 ci was the result. Initially the most common way to accomplish this was to use 400 rods and a piston with the 350 wrist pin placement. Other than turning down the crank journals the only other concern was crankshaft clearance—a problem usually corrected with some minor grinding on the block. (However it’s always wise to check clearances during assembly as parts do vary.)

It didn’t take long for the 383 combo to gain popularity but there were those who found fault with the rod ratio produced when using the short 400 connecting rod. Rod ratio is the relationship between the stroke and the length of the rod and basically it indicates how severe the angle of the rod becomes during one revolution of the crankshaft (a 400 rod results in a 1.48:1 ratio while a 350 rod provides a ratio of 1.64:1). The theory was a 5.7-inch or even longer rod would put less side load on the pistons and cylinder walls, reducing friction and allowing higher engine speeds. Of course the advantages of longer rods came at a cost. To keep the slugs from poking out of the block the wrist pins must be moved up, which requires a custom set of pistons.

The other combination you mentioned is a 377, which can be accomplished two ways: by using a 400 crank in a standard bore 350 block, or a 350 crank in a 0.030-over 400 block. The later became popular among those who favored the larger bore, shorter stroke, oversquare configuration. Additional reasons often given for this combination was the ability to use a steel 350 crank as opposed to the cast 400 ‘shaft and a larger bore to provide space for larger, unshrouded valves.

While the 377 combination based on the 400 block is still popular among some engine builders, the issue has been the same as with the 383—the diameter of the crankshaft journals. In this case the smaller 350 crank rattles around in the 400 main bores. The two common cures are thicker main bearings made for this particular swap, or precision bearing spacers that allow the use of standard 350 main bearings.

Initially the 377 and 383 combinations became popular because factory parts could be used to build an affordable stroker, but today there are more and better choices. New cranks (some for less than $200), rods, and even complete kits to build a 383 are readily available and even Chevrolet Performance offers these engines in crate motor form (which is somewhat remarkable since Chevrolet never built a 383). As for building a 377 the biggest challenge is finding a good 400 block, everything else is easy to source.

There are a host of factors involved when building either engine and as much as we dislike generalities a 383 will often produce more torque but won’t twist the tach needle like a 377. The trick in either case to carefully consider all the components involved, including the valvetrain, induction, and ignition system. The best part about a big-inch small-block is that, as the old saying goes, there’s no replacement for displacement.