This is what the 1999 iron LQ4 6.0L engine crankshaft flange looks like. Note that it protrudes from the rear of the engine block by 0.400-inch. This is contrary to all other Gen III LS engines where the crankshaft flange is even with the rear of the block. (Image/Jeff Smith)

Will a 2002 Chevy transmission work behind a ‘99 Chevy motor? Is wiring or computer an issue? I work at a junkyard and the interchange says one year only. This is for a rat rod build. Thanks. — S.S.

Jeff Smith: We will have to make several assumptions to answer your question.

The quick answer to your question is — it depends!

Our first assumption is that we are talking about a 1999 LS engine and not a small-block Chevy. However, there are many situations in this changeover year where the small-block was used in several GM heavy-duty vehicles. So we’ll deal with both applications but for the benefit of future questions for this column, more detailed information in the question creates a better answer.

First we’ll deal with the LS (Gen III) engines. In 1999 GM used three different displacement truck engines: a 4.8L (293 c.i.d.), a 5.3L (325 c.i.d.) and a 6.0L (366 c.i.d.). The 4.8L and 5.3L engines used a crankshaft flange that was essentially flush with the block bellhousing flange which is identical to the Corvette and Camaro 5.7L all-aluminum LS1 and LS6 engines in that year.

The iron-block 6.0L LQ4 truck engine found in the larger 2500 series trucks and some vans used a 0.400-inch longer crankshaft flange that protruded from the engine’s rear cover. GM used this different crank flange to adapt the 4L80E four-speed automatic transmission that had earlier been designed for use behind big-block Chevys.

The longer crankshaft flange on the early 6.0L engines were employed to essentially duplicate the crankshaft flange position of the earlier small- and big-block Chevy engines that the LS replaced. By doing this, GM employed a flat flex plate and this allowed the 4L80E’s torque converter hub to nestle directly in the extended 6.0L’s crank flange and using a flat 6.0L flex plate.

We will also assume that the 2002 automatic you refer to is a 2002 LS version 4L60E transmission used in Camaros, Corvettes, and also light duty pickups. If so, then that trans was designed for engines using the traditional LS crank flange that was essentially flush with the bellhousing flange. There is insufficient room for this transmission’s torque converter if you attempt to bolt this trans to a 1999 LQ4. On the other hand, if you are using a 1999 4.8L or 5.3L truck engine for this rat rod, then that 2002 4L60E automatic will bolt up using the stock LS engine’s flex plate and everything will work.

Now, if the engine you are using is a traditional small-block Chevy that was still available in certain Chevrolet and GM vehicles like Suburbans and trucks, then this is an entirely different ball game. This engine will require an earlier overdrive 4L60E transmission designed for a small-block Chevy. If your transmission is a 2002, then likely it is designed for an LS engine. One way to tell is to look at the bellhousing flange. A small-block Chevy bellhousing bolt pattern has six attaching bolts that are laid out symmetrically with three on each side and no bolt hole at the 12 o’clock position.

The LS version bellhousing bolt pattern is skewed with four attaching bolts on the driver side with a bolt at the 12 o’clock position and only two bolts on the passenger side. This identifies the transmission as intended for an LS engine. It gets even stranger with the Gen V LS engines with yet another change to this pattern, but we’ll leave that alone for right now.

This is the LS bellhousing mounting flange pattern. Note the bolt at the 12 o’clock position and the four bolt holes on the driver (left) side with only two mounting holes on the right. (Image/Jeff Smith)

So with a small-block 1999 engine you will need a 4L60E intended to be used with a small-block Chevy. These transmissions were used in several Chevrolet cars and trucks between 1993 and 1998.

In all of the above applications, the transmission will also require a stand-alone controller since these transmissions are electronically controlled. There are several companies that offer these controllers and they range in price from around $600 to over $1,000.

Brands like B&M, Chevrolet Performance, FiTech, MSD Atomic, Painless, and TCI all offer the controllers you’ll need.

You didn’t mention what induction system you will be running, but if it’s going into a rat rod, then likely it’s a carburetor. You will also need a separate ignition box to run the spark side of this engine. MSD now offers a universal spark box that will operate with either the early 24x crank position sensor wheels or the later Gen IV 58x wheels.

Author: Jeff Smith

Jeff Smith has had a passion for cars since he began working at his grandfather's gas station at the age 10. After graduating from Iowa State University with a journalism degree in 1978, he combined his two passions: cars and writing. Smith began writing for Car Craft magazine in 1979 and became editor in 1984. In 1987, he assumed the role of editor for Hot Rod magazine before returning to his first love of writing technical stories. Since 2003, Jeff has held various positions at Car Craft (including editor), has written books on small block Chevy performance, and even cultivated an impressive collection of 1965 and 1966 Chevelles. Now he serves as a regular contributor to OnAllCylinders.