Gen I and II engines used three different lengths of head bolts.
If you need to refresh your 350 HO engine, Chevy Performance offers a complete gasket kit (PN 19201171) that includes head gaskets, oil pan, intake, fuel pump, timing chain cover, rear main seal, and valve cover gaskets. (Photo Courtesy Chevrolet Performance)
CHAPTER 3
ROTATING ASSEMBLIES
Often when you look at a crank counterweight to check the casting number, only the last four digits of it are there. Check it against the crank casting number list by looking at the last four digits of those numbers. Casting dates are also found on crankshafts.
The 265, 283, and some 302 cranks use a 3-inch stroke with small journals. The 1968 and 1969 302 cranks have a 3-inch stroke with medium journals.
The short-lived 1975 to 1976 Gen I 262-ci engines had a cast crank stroke of 3.10 inches with medium journals.
The 307 and 327 engines used crank strokes of 3.25 inches. All 307 cranks are medium journal cranks, while 327 cranks were made in both small and medium journals. You can use a 350 medium journal four-bolt block, then use a rebalanced 307 cast medium journal crank to build a four-bolt 327-ci engine.
The 267, 305, and 350 cranks used a 3.48-inch stroke length with medium journals.
The 400 engines that were made from 1970 to 1980 used larger-diameter-crank main journals (2.65 inches) and the same rod journal size as medium journal cranks (2.10 inches) but with a crank stroke of 3.75 inches. These cranks require external balancing by using specific vibration dampers, flywheels, or flexplates for the 400 engine. This is also true if you build a popular non-factory combination: a 400 crank with the main journals cut down to 2.45 inches in a 350 block. You must use the externally balanced 400 damper and 400 flywheel or flexplate. It is possible to internally balance a 400 crank by having an experienced machine shop add heavy metal slugs to the counterweights. Do not use an externally balanced 400 damper, flywheel, or flexplate on an internally balanced engine. I bring this up because on several occasions, folks have shown up at my shop with a running 350 small-block that had a huge vibration or a newly rebuilt 350 that had broken some rods or suffered some other major disaster. When we checked the engine, I noticed that the 350 engine had a 400 vibration damper or a 400 flywheel or flexplate. The owners didn’t know the difference, and they had unknowingly pulled a tired 400 engine out of the vehicle, replaced it with a rebuilt 350, and used the 400 damper and flexplate on the new internally balanced 350 engine. It doesn’t work for long.
Crankshafts have casting numbers and casting dates so you can learn more about what you have or are looking at.
A forged 350 steel crank (casting number 3941182) has casting numbers on the front counterweight. However, only three digits (182) are discernable here.
In 1986, Chevrolet introduced the one-piece rear main oil seal and made related changes in cranks, oil pans, gaskets, and blocks. The medium journal cranks, used in one-piece seal blocks, are all externally balanced and require the use of matching 1986-or-later flywheels and flexplates. The flange on the rear of the new-style cranks was changed to fit the one-piece seals, and the bolt pattern for flywheels and flexplates was reduced to a 3-inch diameter from the 3.58-inch bolt pattern used on two-piece seal cranks from 1955 through 1985.
Cranks were balanced with whatever size engine and related rotating components it received at the factory. A 307 crank is not balanced the same as a medium journal 327 crank, even though they physically interchange with each other and have the same stroke. If you mix and interchange rotating parts from different engines, have the rotating assembly rebalanced.
Some cast, 2.45-inch main journal, 3.48-inch stroke, two-piece cranks have the same casting number (3932442), but are balanced differently. Cast cranks used on 267, 305, and 350 motors should not be interchanged from one engine size to another because of possible balancing problems, even though they have the same stroke and physically interchange in medium journal blocks.
Two-Piece Seal Cranks
Since Chevrolet switched to a one-piece seal more than 30 years ago, it does not offer a new crankshaft for the original two-piece seal blocks. Following are some of the more popular crankshafts that were available to help in your search or identification of used or NOS cranks that you may come across.
PN 3932444 is a nodular cast-iron 350 Gen I crank with a 3.48-inch stroke. This crank is used with a two-piece rear main oil seal and has 2.45/2.10-inch journals.
PN 24502460 is a raw 5140 alloy forging used to build large journal cranks of various journal sizes and strokes. This crank is no longer available from Chevrolet.
PN 3941180/casting number 1182 is a 1053 forged steel crank with a 3.48-inch stroke and 2.45/2.10-inch journals.
PN 10051168 is a 4340 alloy, raw, non-twist forging with 3.20- to 4.0-inch strokes possible. It comes with 2.900-inch unmachined journals that can be cut to fit 400 small-block 2.65-inch main journals.
PN 3951527 is a cast 400 crank, 3.75-inch stroke, 2.65/2.10-inch journals in ductile iron. It requires external balancing by using an appropriate 400 front damper and 400 flywheel or flexplate.
Some crank surface treatments, such as nitriding, only penetrate the surface of the metal a few thousandths of an inch. If the crank journals are later cut or reground, the treated surface may be cut or ground off and the crank does have to be retreated. Some aftermarket crank makers use a hardening treatment that penetrates 0.010 to 0.015 inch and is still there after a reduction in journal diameter of 0.010 inch.
One-Piece Rear Seal Cranks
All 1986-and-later one-piece rear main oil seal cranks are externally balanced, have medium-size journals, and have a smaller bolt circle (3.0 inches) pattern on the rear crank flange. This requires the use of matching late-model flywheels and flexplates, which have mounting bolt-hole patterns that match the cranks for one-piece rear main seal engines. Flywheels and flexplates for two-piece rear main seal cranks do not interchange with one-piece seal cranks.
This is the one-piece rear main seal crank, which first came into use in 1986 on all production engines. Compare the rear flange area to the earlier Gen I two-piece rear main seal crank.
Shown are the rear flange and journals of a Gen I crank that uses a two-piece rear main seal.
PN 14088526 is a nodular cast-iron crankshaft for use with a one-piece rear main seal. This is a 3.48-inch stroke crank with 2.45- and 2.10-inch journals.
PN 14096036 is a 1053 alloy forged crank with 2.45/2.10-inch journals and 3.48-inch stroke. The original ZZ1 and ZZ2 350 HO crate engines received this crank, while the ZZ3 and ZZ4 engines used PN 14088533. Both are discontinued.
PN 14088533 is a 1053 alloy forged crank with 2.45/2.10-inch journals and 3.48-inch stroke. It requires a one-piece rear main oil seal. This crank was used in the ZZ3 and ZZ4 crate engines and on the ZZ3 short-block partial assembly.
PN 12670965 is also a 1053 forged steel crank that is used in ZZ5 and ZZ6 engines as well as in post–November 1998 ZZ4 engines. It must be used with rod PN 10108688 and piston PN 10159436.
PN 12489436 is a forged 4340 steel crank used to produce 383 ci with a 3.800-inch stroke. Mains are standard 350 size and 2.100-inch rod journals.