Clearing The Confusion Concerning GM Gen III Crankshafts - Engine Builder Magazine
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Clearing The Confusion Concerning GM Gen III Crankshafts


Both Doug Anderson and I have written a number of articles and columns
about these engines, but for some reason I continue to hear about the
difficulties. So it’s time to go back to the basics, start from the
beginning and review the old and add some new things that have come

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The first engine in the Gen III family was introduced in 1997 in the
LS1 Corvette. This crankshaft, with casting number 1255216 (or “216” as
most of us call it) is easily identified by the 24.5 mm gun-hole
drilled through the middle of all mains except the first one. This hole
was drilled all the way through from the rear so there is a soft plug
at the rear of the crank that, if not installed, will cause a
horrendous oil leak in the rear (Figure 1).
This crank is a standalone due to the hole down the center that is
there to equalize crankcase pressure between the main bearing web bulk
heads in this application.


The second crank is the one used for the 4.8L engine that came in
two flavors – automatic and standard transmission.  The standard
transmission crankshaft (c/n 1225312) has a wide rear flange 1.250? and
the automatic transmission crankshaft (c/n 12553482) is a narrow .857?.
The wide/narrow crank scenario happened in ’99-’00; after that they all
became narrow flange (Figure 2).

The early manual crankshaft engines are few and far between and finding
crank cores is a tough nut to crack. Obviously that became an issue for
GM as well because there was an adaptor kit designed to convert an
automatic crank to a manual crank (Figure 3).  These kits are available from both GM and aftermarket providers, so trying to find cores becomes a non-issue.


Because the Gen III block says “4.8 and 5.3” on it you may have
gathered the 4.8L and 5.3L engines use the same crank. That’s a good
guess – but it’s not correct. The two used different cranks and rods
due to stroke differences and each had different casting numbers.

How do you tell the difference? When they were installed in a long
block, it might have been challenging, but here’s a visual “new thing”
that I’ve recently become aware of. Look at the counterweights of each
crank. The interior counter weight outer edges are “as cast” on the
4.8L, while they are machined on the 5.3L cranks (see Figure 4).


The 5.3L crankshaft has the same casting number (c
12552216) as the
5.7L crank but is not gun-drilled so there is very little other
identification than what you see in Figure 4 with the “as cast” and
machined counterweights. Because it was never a manual transmission
engine, it will always have a narrow flange crank. By the way, you will
not be able to use this crank for a 5.7L engine, especially without the
gun drilled hole through the mains.  

The 6.0L crank has the same stroke (92mm) as the 5.3L and 5.7L cranks.
In 1999-2000 this crankshaft was identified with c
12552215 and had
only a wide flange. Then in 2001-’04 the 6.0L went to narrow flange
only with c
1255216, the same as the 5.3L and 5.7L.


The piston weights are different enough for the 5.3L, 5.7L and 6.0L
that you would not think that any of them would interchange. However
word on the street is that 6.0L and 5.3L cranks will interchange and
that there have been no complaints. So I am surmising that they are
similar to the 305 and 350 situation: most of the time you will be OK
but there may be those few times that it could bite you. I will leave
the interchange decision purely up to you. I can tell you for a fact
that there are “216” cranks that are being rifle drilled and used as
the 5.7L crank. I am making the assumption that they are being
rebalanced but have not been able to verify it.


There is another crank that has come on the scene recently that is
known as a “218.” Reports are that it is the same as the “216” except
in the reluctor ring area. I have not seen it yet but I do know that it
has the new design reluctor ring on it. You will need to keep an eye
out for this one as well.

Roy Berndt has decades of machine shop experience.
He is the EDS Data Acquisition Contractor for the Production Engine
Remanufacturers Association (PERA), and Program Manager for PROFormance
Powertrain Products, a PER in Springfield, MO. You can reach Roy at [email protected].
Figure 1 On the left you can see the rifle-drilled hole through the center of the LS1, 5.7L crankshaft. On the right, the soft plug is installed in the rear of the crankshaft to seal off crankcase oil. This is the only Gen III crankshaft that has these features.Figure 2 Wide and narrow flanges were found in the Gen III cranks in 1999-2000. The narrow one was used with the automatic and the wide with the manual transmissions.Figure 3 Narrow flange automatic transmission Gen III cranks can be made into wide manual transmission by the use of a spacer adaptor and longer bolts.Figure 4 The crank counterweights on the left are 
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