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Small Block Chevy Crankshafts
Small block chevy crankshaft The 400 engines that were made from 1970 to ’80
used larger diameter crank main journals (2.65") and
the same rod journal size as medium journal cranks
(2.10"), but with a crank stroke of 3.76". These cranks
require external balancing by using specific vibration
dampers, flywheels or flexplates in the 400 motor.
This is also true if you build a popular non-factory
combination: a 400 crank with the main journals cut
down to 2.45" 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 motor. I bring this up because on a number of occasions, folks have shown up at my shop
with a running 350 smallblock 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 motor, 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 motor out of the vehicle, replaced it with a
rebuilt 350, and used the 400 damper and flexplate on the new internally balanced 350 motor. It
doesn’t work for long.
A forged 350 steel crankshaft Rear flange of a crankshaft for use with a two piece rear main seal
A forged 350 steel crank (Casting
#-3941182) has casting numbers on the
front counterweight. However, only two digits
(41) are discernable here.
Shown are the rear flange and journals of a
Gen I crank that uses a two-piece rear main
seal.
In 1986, Chevy 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" diameter from the 3.58" bolt pattern used on two-piece
seal cranks from ’55 through ’85.

Cranks were balanced with whatever size motor 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" main journal,
3.48" 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 will physically interchange in medium journal blocks.
The 1997 and later Gen. III cranks should be easy to spot once they become more available. The
main thrust bearing is on the middle #-3 crank journal, instead of on the #-5 rear journal as it is on the
Gen. I and II cranks.
Rear of crankshaft for use with a one piece rear main
This is the one-piece rear main
seal crank, which first came into
use in 1986 on all production
motors. Compare the rear flange
area to the earlier Gen. I two-piece
rear main seal crank.
(PN-3932444) is a nodular cast-iron 350 Gen. I crank with a 3.48" stroke. This crank is used with a
two-piece rear main oil seal and has 2.45"/2.10" journals.

(PN-366280) 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 Chevy; it has been replaced by a forged raw crank
(PN-24502460). Use with a two-piece seal.

(PN-3941180) (Casting #-1182) is a 1053 forged steel crank, with a 3.48" stroke and 2.45"/2.10"
journals, not nitrided. This crank is used with a two-piece seal.
(PN-3941184) This is the same as (PN-3941180), but nitrided; 1182 casting number.
(PN-10185100) Same as (PN-3941184), but it is a raw forging of S38 alloy and it can be machined for
3.46" to 3.5" stroke length.

(PN-10051168) is a 4340 alloy, raw, non-twist forging, 3.20" to 4.0" strokes possible. It comes with
2.900" unmachined journals that can be cut to fit 400 smallblock 2.65" main journals. Use with a
two-piece seal.

(PN-3951527) is a cast 400 crank, 3.75" stroke, 2.65"/2.10" journals, ductile iron. Use with a two-piece
oil seal. It requires external balancing by using an appropriate 400 front damper and 400 flywheel or
flexplate.
(PN-24502460) is a raw forging of 4340 alloy steel that is similar to the raw forging (PN-10051168),
but it has additional material on the crank snout that can be machined to a larger diameter for use
with bigblock Chevy vibration dampers.This crank uses the two-piece rear seal.
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 will have to be retreated. Some aftermarket crank makers use a
hardening treatment that penetrates .010" to .015" and will still be there after a reduction in journal
diameter of .010".
A cast crankshaft with its thin casting line Wide parting serface found on forged crankshafts
A cast crank can be distinguished from a
forged crank by its thinner parting line.
A thick parting line is seen on this 350 forged
crank.
One-Piece Rear Seal Cranks
All 1986 and later one-piece rear main oil seal cranks are externally balanced, have medium sized
journals and have a smaller bolt circle (3.0") 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.

Gen. III engines use cast cranks with rolled fillets and have a different firing order. These cranks have
2.558" main and 2.1" rod journals. The thrust is on the #-3 main bearing. In addition, the Gen. III uses
a front-mounted gerotor oil pump that is driven by a gear on the Gen. III crank snout. Consequently,
these Gen. III cranks are not interchangeable with previous smallblocks.
(PN-14088527) is a nodular cast-iron crankshaft for use with a one-piece rear main seal. This is a
3.48" stroke crank with 2.45" and 2.10" journals.

(PN-14096036) is a 1053 alloy forged crank with 2.45"/2.10" journals, 3.48" stroke. Requires a
one-piece rear main oil seal. The ZZZ, ZZ1 and ZZ2 350 HO crate motors received this crank.
(PN-14088533) is a 1053 alloy forged crank with 2.45"/2.10" journals, 3.48" stroke. Requires
one-piece rear main oil seal. This crank is used on the ZZ3 and ZZ4 crate motors and on the ZZ3
short-block partial assembly.
Used Cranks
When you are looking at a used smallblock crank, determine if it is a cast or forged crank. You can
expect to pay more for a forged one. The cast crank has a thin parting line on its throw arms. The
forged crank has a much thicker or wider parting line.

Check the casting number and casting dates to determine what stroke it is. Does it use a one-piece or
two-piece rear main oil seal? Measure the rod and main journals. Has the crank been previously cut
down? Machine shops that recondition cranks usually stamp the amount they have cut the crank on
the front crank throw arm. If your measurements show that the crank journals have been previously
cut more than 0.010", walk away. You can find another one.
Are the threaded holes in the rear flange stripped? Has the drilled hole for the pilot bearing at the
rear of the crank been elongated or damaged? Has the thrust surface on the crank been worn or
damaged? How about the threaded hole in the crank front snout? Some early crank snouts are not
drilled or tapped for a vibration damper retention bolt. You can get the snout tapped and threaded for
the bolt at a machine shop. Also, some early cranks that were used in automatic transmission cars did
not have a hole drilled in the rear of the crank for a manual transmission pilot bushing.

Remember, when you get the crank to the machine shop, ask the crew to clean it, checked for cracks
and straightness, clean out the threaded holes and oil passages, and possibly have it ground and
micro-polished. If any of the journals have been damaged, you may be able to have the journal
welded or chromed, then cut back to acceptable size. However, you need to compare the cost of fixing
a damaged crank to the cost of buying an undamaged one. At some point, it’s cheaper and easier to
find an undamaged used crank or buy a new one.
Cast Versus Forged Cranks
Chevy stock cranks are either nodular cast-iron or forged steel. To figure out which yours is, check
the parting line, which is left from the molds when the crank is poured. A cast crank has a thin parting
line, while a forged crank line is thicker or wider. Tap a crank lightly with something hard, and a cast
crank will emit a thudding noise. A forged crank rings like a bell; it is a quite distinctive sound. Of
course, you can also check the casting number.

Some people who build high-performance motors believe that they must have a forged crank and that
cast cranks just don’t cut the mustard. They would rather shell out the extra money for a forged crank.

The truth of this belief depends on your situation. Up to a point, cast cranks are fine. In a short-
duration, bracket drag race engine, 450 genuine horsepower is around the upper limit for a
smallblock cast crank. Almost any street application, short of insanity, can get along just fine with a
cast crank. At horsepower levels higher than 450, or if you are planning a nitrous or high-boost
blower or turbo applications, move to a forged steel crank. Good used cast cranks are more
numerous and less expensive than cast cranks. You can take the money you saved and spend it on
something else. Just be sure the crank is magnafluxed and properly prepped and use a high-quality
vibration damper (especially with a cast crank). Good things will happen.
These days, a large number of forged cranks are available from aftermarket suppliers for a large
range of applications, stroke lengths, steel alloys (most use a very strong 4340 alloy) and total
weights. These aftermarket cranks can also be prepared in various ways.
measuring a crankshaft with micrometers
Check the crank journal
diameters to see if the crank has
been previously cut and to
determine the size of the
bearings you need.
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This has been a sample page from

Chevrolet Small Block Parts Interchange Manual Chevrolet Small Block
Parts Interchange Manual
by Ed Staffel
Beginning with the earliest small block and carrying through the
very latest "Gen III" models, CHEVROLET SMALL BLOCK PARTS
INTERCHANGE MANUAL provides complete factory parts
interchange information, allowing the hot rodder to custom-build
his own high performance version of the famous Chevy "Mouse"
motor from off-the-shelf parts. Includes factory numbers, casting
marks, production histories, suppliers, component performance
capabilities, etc.

Only 2 Left in Stock, Order Soon!


This is a great book and one that any
enthusiast will love!
View Sample Pages
1)
Engine Blocks
2) Crankshafts
3) Oil & Lubrication System
4) Timing Chains & Covers
5) Cylinder Heads
6) Intake Manifolds
7) Ignition Systems
8) Gaskets
9) Exhaust Manifolds
Condition: NEW
Softbound
8.5 x 11-inches
144 Pages
300 B/W Photos
Item: SA55P
Price: $22.95
Click here to buy now!


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