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Gen III GM Small Block Engine, LS1 Motor for Cars, Trucks


The first generation small block was revised to create the second generation LT1/LT4 that was used for some applications from ’92 to ’97, but the results didn’t satisfy the people at GM Powertrain, so they started all over in ’91 and designed a brand new small block.

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It’s officially called the “Gen III” motor, but it’s usually known as the “LS1” because that’s what it was called when it was originally installed in the ’97 Corvette. This new engine family had the same bore spacing as the original small block, but that’s the only thing that stayed the same. The Gen III engine was smaller and lighter, it made more horsepower and torque per cubic inch, created fewer emissions and got better fuel mileage than the Chevy 350 it replaced. It was designed to be built in multiple displacements from day one so it could be used in a wide variety of cars and trucks later on. GM also made sure that the overall size and shape of the “package” would fit in a FWD application, too, so it should be no surprise that the Pontiac Grand Prix GXP is available with the 5.3L this year.

The Gen III motor that was originally installed in the ’97 Corvette as the LS1 was a 345/5.7L version that made 345 hp and 350 ft.lbs. of torque. After two years of successful experience with the 5.7L in the Corvette, Camaro and Firebird, GM had enough confidence in the design to go ahead and replace the first generation 305 and 350 with the new 4.8L/5.3L/6.0L engines in all of their pickup trucks. Today, the Gen III motors are used in the Corvette, the GTO and the FWD Pontiac GXP along with all of the GM trucks and vans. And, the LS1 is winning at the drag strip, just like the original small block did back in the ’50s and ’60s, so history repeats itself all over again.


The GM engineers did their homework when they designed these engines, so they haven’t had to make a lot of changes, but there are four engines, each with a different combination of parts, so there are still three rods, four blocks, six cranks and seven heads to keep track of so far. Putting the wrong parts in the wrong engine will cause problems, so it’s important to know exactly what goes where unless you want to do it over again. With that in mind, let’s take a look at how these engines all fit together.



GM has used five different blocks for the Gen III family. There are iron blocks for the 4.8L/5.3L and 6.0L along with aluminum blocks for the 5.3L, 5.7L and 6.0L.

There’s only one internal difference in the blocks that rebuilders need to keep in mind. The OD of the cam bores was changed for model year ’04, so the later engines require a different set of cam bearings, even though the cam and everything else stayed the same.

All the head bolts have blind holes, so rebuilders need to make sure there’s no debris or oil in them before torquing the head bolts down, because the hydraulic pressure will split the block wide open if there’s anything left in the hole.


1999 – ’04 – 4.8L Trucks

The 4.8L block was made of cast iron and carried a 12551358 casting number. This block was used from ’99-’04 for all the 4.8L motors.

1999 -’04 – 5.3L Trucks



The 5.3L engines had a longer stroke than the 4.8L, but they had the same bore so they shared a common block. All of the ’99-’03 engines and most of the ’04s came with the 12551358 cast iron block, but there were a few that had an aluminum block.

The ’04 Chevy SSR truck, Chevy Trailblazer EXT, GMC Envoy XL, and Buick Rainier all came with an aluminum block that was either a 12566910 or a 12571048 casting.

The cast iron blocks have thick walls so they can be bored up from the 4.8L/5.3L standard bore of 3.779? to 3.893? to make them into a 5.7L standard bore block if the additional weight of the iron block isn’t an issue, but the aluminum blocks shouldn’t be bored over .010? according to GM.

1997 -’04 – 5.7L Cars



All of the 5.7L blocks are aluminum castings and they’re all very similar, but there are a couple of subtle differences between the early ones and the late ones.


The 1997-’98 blocks with the 12550592 casting had two holes for the oil galleys that were flush with the back of the block itself, so the right lifter galley was fed by a shallow crossover in the back cover. These blocks also had a 24.5 mm hole drilled straight through the main webs to allow better “bay-to-bay breathing.”

GM discovered during dyno testing that there was too much pressure in the crankcase due to the amount
of air and oil vapor that was trapped between the main caps, so they drilled a hole through all five bulkheads to allow the engine to “breathe” from one bay to another. That eliminated the pressure build-up that was causing some internal problems.

GM used several castings for the LS1 motors from ’99-’04, but they’re all pretty much the same so it doesn’t really make a lot of difference which one you use. However, there are a couple of changes that make each of them slightly different.

The right front corner of the block was reinforced in ’99 and there was a deep oil slot added to the back of the block to allow more oil to flow to the right hand lifter galley. Look for a 12559378 or a 12560626 casting.


The 1256118 casting was originally designed for the high performance LS6 that was introduced in ’01, but it showed up in some LS1 applications in ’01 and ’02 and it was used for all the Corvette motors in ’03 and ’04. The only difference between it and the previous block was the addition of two cast slots in each of the three center mains. They replaced the holes that were drilled through the bulkheads on all the other ’97-’01 blocks.

We’re pretty sure that the 12561168 casting was used for all the Corvettes in ’04, but it may not have been used for the ’04 GTO. Some people say the GTO had a special casting with different mounting pads for the motor mounts and power steering pump, so be sure to have the customer bring his core if you get a call for one of these engines.

GM says you shouldn’t bore the cylinders on the ’97-’98 blocks more than .004? and recommends not boring the ’99 and later ones more than .010?, because the iron liners are pretty thin to start with and they’re serrated on the outside, too, so there’s not very much material left to bore out. The only way to make a big bore motor out of one of these blocks is to machine the liner completely out of the block and install some new aftermarket liners, but that’s a major undertaking that requires special liners along with lots of time and experience, so we don’t recommend trying it.


1999 -’04 – 6.0L Cars and Trucks



All of the 6.0L blocks from ’99-’04 are made of cast iron and have the number 12551364 along with 6.0L cast on the block itself. Most of the ’05 blocks are cast iron, too, but the Chevy SSR, the Corvette and the GTO all came with an aluminum 6.0L block in ’05.

These blocks already have a 4.0? bore, so they’re the best alternative for the guy who wants a bigger motor for his car as long as he’s willing to put up with the additional weight (65 lbs.) of the iron block. The only other way to build a big bore motor with a stock block is to buy one of the new 6.0L aluminum blocks, but that’s bound to be real expensive.

No matter which 6.0L block you use, don’t plan on boring out it out more than .010?, because the cylinders are on 4.40? centers so the walls are pretty thin and the cylinders are so close together that head gasket sealing will become an issue, too.


There were only two different strokes used for the entire Gen III family, so the cranks should be pretty straightforward, but it’s not that easy because there are two different flange widths and one crank that has three or four different bobweights to deal with.


4.8L Cranks

There are two cranks for the 4.8L, one with a wide crank flange (1.250?) and one with a narrow flange (0.857?). Although we don’t know exactly what GM did, it appears that the ’99-’00 trucks with manual transmissions came with the 12553312 casting that had the wide flange and the ’99 – ’00 trucks with automatic transmissions came with the 12553482 crank that had the narrow flange. All of the 4.8L motors used the crank with the narrow flange starting in ’01.

5.3L Cranks

The 5.3L engines all came with automatic transmissions, so they all had the 12552216 casting with the narrow flange. This is the same casting that was used in both the 5.7L and the 6.0L, but it’s balanced specifically for this application, so it has its own unique GM part number. We don’t know what the bobweights are, but our sample piston weighed 406 grams.

5.7L Cranks

The 5.7L had the same stroke as the 5.3L, but the cars always with came the narrow flange, so it had the same 12552216 casting that was used for the 5.3L and 6.0L. The 5.7L cranks weighed about a pound less than the others because they had a 24.5 mm hole drilled right through the center of the second, third, fourth and fifth journals. We’re pretty sure that the balance was unique to this application, because the pistons weighed 434 grams which is more than the 5.3L and less than the 6.0L.


6.0L Cranks

The 6.0L shares the same 3.622? (92mm) stroke with the 5.3L and 5.7L, but there are two or three different cranks.



All of the ’99 – ’00 cranks had the 12552215 casting with the wide flange. This is the only application for a crank that has a 92mm stroke and a wide flange.

All of the ’01-’04 cranks used the 12552216 casting with the narrow flange, just like the 5.3L and 5.7L, but the 6.0L pistons weighed considerably more (470 grams) than the ones used in the 5.3L and 5.7L engines, so the bobweights had to be different. These may even be two different versions of this crank because the rods used in the LQ9 motors weighed 36 grams more than the ones in the LQ4 motors.

Exciter Rings

Be sure to replace the exciter ring anytime the crank is removed from the engine. GM sells it for about $15 under p/n 12559353. We install it with the tool that Larry Eriksson of Crankshaft Rebuilders designed, that is marketed by Goodson (p/n RRJ-350), because the engine will start, but it won’t run, if the total runout exceeds .020?, including crankshaft end play. Be sure to follow the installation instructions shown in the article and install the ring with the holes facing the rod journal because the engine won’t start if you install the exciter ring backwards.



This whole family of engines was designed with three different bore sizes and two different strokes, so GM only needed two rods, a long one and a short one, to cover all four different
displacements, but they ended up with three different rods anyway. These rods are all powdered metal forgings with cracked caps and they all had press-fit pins, except the ones used in the 6.0L LQ9 engines that came with full-floating pins. And, it appears that all the Gen III/Gen IV motors (a Gen III motor with “displacement on demand” is called a Gen IV motor) went to full-floating pins in ’05 according to some car magazines.



The Long and Short Rods

The 5.3L, 5.7L and the standard LQ4 6.0L used the 143 casting that was rounded off on one side of the shank and squared-off on the other side. Our sample rod weighed 609 grams.

The LQ9 version of the 6.0L that was used in the Escalade and Silverado SS came with the 123/145 casting that had a thicker beam, a stronger big end and a bushing in the small end for a full-floating pin. Our sample weighed 645 grams.


The original rod bolts in the 5.7L were more than adequate to start with and they were upgraded to take 450 hp in ’99, so they’re pretty reliable, but rebuilders should consider replacing all of them with ARP bolts when building a high performance motor.

Just for the record, the only parts from the Gen II engines that were carried over to the Gen III were the rod bearings; they’re the same for both families.


The 4.8L engine used a “long rod” that measured 6.275? center-to-center because it had a shorter stroke. Look for a 121BW casting number on the side of the cap.


The 5.3L, 5.7L and 6.0L all used the “short rod” that measured 6.096, center-to-center, but there were two different short rods used for the 6.0L engines.

Cam combinations for various Gen III applications


GM has used several different pistons in these engines because of the different displacements and applications. There are a few things you should know about them:



All of the Gen III pistons are cast, eutectic aluminum with 1.5/1.5/3.0mm ring grooves.

The 4.8L piston is a flat top, but the 5.3L is dished because they both use the same head, so the 5.3L with the longer stroke would have had too much compression with a flat top piston.


The LS1 pistons are made of a good, standard aluminum alloy that was more than adequate for a regular motor, but the pistons for the LS6 pistons were made of a premium alloy that had more copper and nickel in it so they were stronger and they didn’t expand quite as much when they were hot. The additional strength was important in a performance motor, but the lower expansion rate was more important because it allowed

GM to run tighter bore clearances which reduced noise and lowered oil consumption.

The 6.0L pistons were all flat tops, but the ones used for the 6.0L LQ9 that came in the Escalade in ’02 had a moly coating on the skirts and a thermal barrier on the top.

All of the Gen III pistons have had a moly coating on the skirts since 2003.


The Gen III cams are all steel billets that are gun-drilled and designed for hydraulic roller tappets. The Gen III cams are considerably larger in diameter and a lot stronger so the whole valve train is more stable as a result.


There have been several cams used for these engines with a variety of specifications, but rebuilders may be able to consolidate some applications. Staying within a three degree window for lobe separation seems to have worked okay with the earlier OBD II systems from our experience, so these consolidations should work okay, but it’s best to sneak up on them carefully, especially for the later cars and trucks, because the computers are becoming more sophisticated and less forgiving! Just remember, it’s always better to put the cam back in the same motor it came out of if there’s any doubt. With that in mind, the combinations that look like they should work okay are spelled out in the chart.



Rebuilders should note that the 0967 fits the 4.8L/5.3 and the 6.0L in certain years and the 1721 was used in both cars and trucks at times. They should also be aware that the cam used in the ’02-’04 LS6 had a smaller base circle so the valves were 0.6 mm longer to compensate for the difference. That means this cam can’t be interchanged with any other cam unless all the matching components are used, too.


GM has used several different heads on these engines, depending on both the year and the application. They’re made of cast iron or aluminum and they come with a variety of chambers and ports, so they’re not all interchangeable even though you can bolt them on any of the blocks. These heads are unique because they have “cathedral intake ports” that are tall and narrow instead of the rectangular design we’re all used to seeing. GM spent a lot of time designing the heads with “replicated ports,” making sure that all the intake ports and exhaust ports were identical to each other so the airflow was exactly the same for every cylinder. The tall ports allowed them to target the injectors for the best possible spray pattern on the back of the intake valve, too. These heads are the key to the success of the Gen III motors and the horsepower they make.


Although all these heads look very similar, there are significant differences:

4.8L/5.3L Trucks

Both of these engines share a common aluminum head with 71cc chambers.. The 12559862 casting was used from ’99-’04, but the 12561706 showed up on some engines in ’04, too. The 862 casting came with and without the recess on the edge of the deck surface, so rebuilders need to make sure they use the right head gasket when installing one of these heads.

5.7L Cars

There have been three different heads used on the 5.7L motors.



’97-’98 LS1 – The original LS1 heads were 10215339 or 12558806 castings that had 66.7cc chambers and nine bolts around the perimeter to hold the rocker covers on. These early heads are unique because they had a recessed area right below the No. 3 or No. 6 cylinder, so rebuilders must use the early style, composition head gaskets with them. If the later MLS gaskets are installed with the heads that have the recess, the engine will leak coolant to the outside from the recessed area in the deck surface.

’99-’04 LS1 – The LS1 switched to center-bolt rocker covers in ’99, so these engines came with a new head. This 12559853 casting was used up through ’01, but the 12564241 was also found on some engines beginning in ’01. We’re told that these heads didn’t have the recessed area on the outer edge, so the MLS gasket should work okay, but the parts book calls for the composition gasket up through ’01 so that’s probably what you should use. These heads have 66.7cc chambers, too.


’01-’04 LS6 – The LS6 performance engines had the 12564243 castings that flowed more air due to better intake ports and the “Dee” shaped exhaust ports. The chamber was a little smaller (61.15cc) so the engine had a slightly higher compression ratio, too.

6.0L Cars and Trucks

There have been two heads used on the regular 6.0L engine along with a third one that was used only on the engines that were designed to run on natural gas.



The cast iron head on the ’99-’01 engines was a 12561873 casting.

All of the ’01- ’04 6.0L engines had an aluminum head with a 12562317 or 12572035 casting number. These heads were used on all the 6.0L motors including the standard LQ4 and the LQ9 performance motor. They’re easy to recognize because they have the same “Dee” shaped exhaust ports that were found on the LS6 heads.

All of the 6.0L heads have 71cc chambers.

GM offered a 6.0L engine for natural gas applications so there’s a 12562317 casting with special valves and seats, too. This head can be identified by the unique pattern of bumps found on the rocker rail (see the story, including photo and chart, in last month’s “Core Corner” Engine Builder, March 2005).



The exhaust valves are all the same size (1.55?), but the intakes came in two different sizes. The 4.8L and 5.3L engines used 1.890? intake valves, while the 6.0L, LS1 and LS6 5.7L engines used 2.00? intake valves.

The LS6 used lightweight valves with hollow stems starting in ’02. The hollow intakes saved some weight, but the hollow exhausts were filled with sodium so they ran cooler. Both of these valves are 0.6 mm longer than the others because the cam for the ’02 LS6 had a smaller base circle.

Valve Springs

All of these heads had beehive valve springs, but there are three different part numbers:

GM Part Number 12553696


’99-’04 4.8L LR4

’99-’04 5.3L LM7

’97-’04 5.7L LS1

’99-’04 6.0L LQ4

’02-’04 6.0L LQ9

GM Part Number 12565117


’01 5.7L LS6

GM Part Number 12565313


’02-’04 5.7l LS6

Crankshaft Fitment Chart

That covers all the major parts along with the castings, but there are a few more things you need to know about these engines:




Be sure to use the correct head gaskets for all of these engines because they will leak to the outside if you don’t. If the head has a recess, you must use a composition gasket, but if it doesn’t, you can use the MLS gasket.

The 5.7L head gaskets were made of multilayered steel (MLS) starting in ’02. They’re imprinted, too, but the imprint is same on both sides so they can be used on either side of the engine. However, these MLS gaskets can’t be used on the early motors because the recess in the outer edge of the heads isn’t sealed by the imprint on the MLS gaskets.

The 5.3L engine that’s available in the ’05 Grand Prix GXP will be used in the Impala SS and the Monte Carlo SS in ’06. It’s designed specifically for a FWD application so it has a shorter crankshaft and a revised accessory drive system that includes an offset housing for the water pump. It has an aluminum block, heads, oil pan and front cover, so it’s pretty light. In fact, its supposed to weigh about the same as the supercharged Buick 3800 and it makes a lot more horsepower, so it will probably become the new performance motor for the FWD cars.


Be sure to install the plastic plug in the oil passage in the back of the block on the driver’s side. It divides the oil passage so the oil flows through the filter and it plugs the back of the main oil galley. It’s a p/n 12573460 and it sells around $5.

The roller lifter guides are available from GM under p/n 12551162 for less than $5 apiece, but there are four in each engine, so it adds up if they’re missing or damaged. They’re a little bit unusual, because they will hold the lifters up out of the way so the cam can be replaced without removing the intake manifold.

The hydraulic roller lifters are unique to these engines, too. They have more plunger travel because of the bolt down, “net lash” design of the valve train and they meter less oil topside because the roller rockers don’t need as much oil as the old, ball style rockers did. GM offers them under p/n 17122490, but they’re over $30 apiece, so you may want to consider some alternatives.

The pushrods are longer than the ones for the Gen I/II Motors and they have thicker walls.

The Gen III heads can be identified by casting numbers and by the “bumps” found on the outside of the rocker rail. Last month’s “Core Corner” explain how to break the code.


These engines have had some oil pressure problems. Some people are blueprinting the oil pump to get more pressure and flow, but there are two other areas that may be contributing to the problem: 1)

Rebuilders should be very particular with the main bearing clearances with an aluminum block because it will increase when the engine gets hot, and 2) they should be sure to check the cam bearing clearance.

We’ve seen three engines that had .005? to .006? bearing clearance on the front and back cam journals and we even ended up custom-making the cam bearings for one engine to get it within factory specs.

GM changed the length of some of the head bolts in 2004. The ’99-’03 engines had 16 long ones, (6.50? overall length) and four short ones, (4.40? overall length) along with 10 small 8.0 mm bolts. In ’04, GM started using all short bolts (4.40?) to avoid the stripped threads that were caused when people put the short ones in the deep holes and tried to torque them down.

There have been at least six oil pans used on the Gen III motors. They’re all made out of aluminum and they’re all a structural part of the engine:

The Gen IV small block motors that were introduced in 2005 were redesigned to accommodate GM’s new “displacement-on-demand” (DOD) technology that allows cylinder deactivation during light loads. There were several changes made on these Gen IV engines:


All of the 5.3L engines got the LS1-style heads with the “Dee”-shaped exhaust ports in 2004. The dished pistons were replaced by flat-tops, too.

The new 6.0L aluminum block has the extra mounting boss for the A/C compressor so it should be a bolt-in replacement for the earlier iron blocks, but be sure to check it out before you sell one.

* The Corvette has a shallow two-piece pan with two wings that stick out from the sides. It’s usually called the “batwing” pan.

* The pan for the Camaro and Firebird had a shallow rear sump (5.25?) that was squared up on the bottom edges. It’s a 1255889 casting.

* The pickups, full sized vans and big sport utilities had a deep, rear sump pan that sloped from the front of the pan down to the sump that was 8.5? deep and measured 8? x 9? on the bottom. It’s a 12560392 or 12573704 casting.

* The pan for the smaller sport-utilities, including the Envoy and Trailblazer has a hole in it because the front axle goes through it.

* The GTO has a special pan with a front sump.

* The FWD cars will probably have their own unique oil pan for these applications.

* A “lifter oil manifold assembly” (LOMA) was installed in the valley under the intake manifold so the knock sensors were moved to the left and right sides of the block and the cam position sensor was moved to the front of the cam.


* The cam sprocket was redesigned so it provided the signal for the cam sensor.

* A new, high-volume oil pump replaced the original design.

* Additional oil galleys in the block were added to provide high pressure oil to the LOMA.


So, that’s the “Gen III” story. It’s a good motor that’s already been used in lots of cars and trucks. GM has built about 1.4 million of these engines a year since ’99, so there’s a bunch of them out there and they should start wearing out pretty soon. With that in mind, you may want to keep this information handy so you’ll know what to look for when they show up on your dock.

The information for this article came from many sources, including the original SAE paper published by GM, a book on the LS1 that was written by Will Handzel and several magazine articles plus a lot of personal experience, but the scope of what we did wouldn’t have been possible without the help we got from Roy Berndt at PERA and all his contacts in the industry. Thanks to Roy and everyone else who helped.


Doug Anderson is Manager of Technical Services for
Grooms Engines, located in Nashville, TN. He has authored numerous
technical articles on engine rebuilding for Engine Builder magazine for
more than 20 years. Anderson has also made many technical presentations
on engine building at AERA and PERA conventions and seminars.

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