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Clearing The Confusion Concerning GM Gen III Crankshafts

I am not sure why but there continue to be struggles with the identification of the Gen III engine crankshafts that have been the staple small block V8 engine since 1999 for GM trucks and many passenger vehicle applications.

By Roy Berndt
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 along.

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\n 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\n 12552215 and had only a wide flange. Then in 2001-’04 the 6.0L went to narrow flange only with c\n 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 rberndt@enginebuildermag.com.