How to measure LS accessory drive spacing

How we put this guide together

Of the 2,012 LS swap questions we pulled from forum threads, 51 touched the accessory drive, the belt, the balancer or the water pump. The ones that hurt most start the same way: a junkyard or swap-meet engine with no history, a balancer someone already changed, or a pile of brackets from three different donors. The accessory drive guide explains the three factory spacings and what to buy for each. This page is the measuring half: our own diagram of where to put the tape, the numbers to compare against, and what to do when your number lands between two.

Step 1: Know why the number matters

GM built LS engines with three accessory drive spacings. The only thing that differs is how far forward of the block the belt runs. The balancer, water pump, alternator, power steering pump, A/C compressor and idlers all have to sit on that same belt plane. Mix parts from two spacings and the belt runs crooked: it squeals, throws ribs and eventually comes off at rpm. So before you buy a single bracket, you need to know which spacing your balancer is.

If you know where the engine came from, you're done: look it up in the spacing picker. Measure when:

• the engine came without its accessories or its history,
• the balancer may have been swapped (common on swap engines and anything with an aftermarket damper),
• you're mixing a donor drive with a kit, or
• the belt already looks out of line.

Step 2: Take the two measurements

Measurement A: block face to belt centerline. This is the one that tells you where the belt runs.

1. Find the front face of the block: the machined surface the timing cover bolts to, not the face of the cover itself. Look just above or beside the cover, where the block's front edge is exposed.
2. Lay a straightedge across the crank pulley's belt grooves, standing up, and mark the middle of the grooved band. That's the belt centerline.
3. Measure straight forward, parallel to the crank, from the block face to that mark. A combination square or a depth gauge held against the block face is easier than a tape.

Measurement B: pulley front face back to the bolt seat. This one needs no belt and no block reference, so it's the best check on a bare balancer or one off the engine.

1. Lay a straightedge across the front face of the crank pulley.
2. Measure straight back through the center to the surface the crank bolt head seats on.

Write both down. They should point at the same spacing.

Step 3: Match your numbers

Spacing	Came on	A: block face to belt	B: pulley face to bolt seat
-1 Corvette	C5/C6 Corvette, CTS-V, G8, SS sedan, ZL1	2-1/8 in	1-1/2 in
-2 F-body / GTO	1998–2002 Camaro and Firebird, GTO	2-15/16 in	2-1/4 in
-3 Truck	Gen III and Gen IV trucks and SUVs, 2010–15 Camaro	3-11/16 in	3 in

Gen IV trucks use the same belt plane as Gen III trucks. What changes between them is the water pump (Gen III and Gen IV truck pumps are different parts, and only the Gen IV truck pump and the LS3/L99 Camaro pump clear a VVT front cover) and the thermostat housing, which differs because the pump bore and seal depth differ. Spacing is one question; the pump that fits your engine is another. More: accessory drive guide.

Step 4: Read a number that falls between two

The spacings sit about 3/4 in apart, so a careful measurement is never truly in the middle. If yours is:

• Within about 3/8 in of one value: that's your spacing.
• Close to halfway: something in the measurement is off. Check, in this order: 1. The reference point. Measuring from the timing cover instead of the block face, or to the edge of the groove band instead of its middle, moves A by a good fraction of an inch. 2. Measure B. Its steps are exactly 3/4 in (1-1/2, 2-1/4, 3 in), so it settles most arguments. 3. An aftermarket balancer. Performance dampers are sold in all three spacings and some have extra pulley rows. Measure the groove band that drives the accessories, not a supercharger or dry-sump row. 4. Supercharged engines. The LSA and LS9 add a supercharger drive ahead of the accessory belt. The LSA's accessory belt runs on Corvette spacing, and the truck plane sits one rib off the LSA's supercharger drive, so make sure you're on the accessory grooves.
• A and B disagree: the pulley and the balancer hub may not be the parts they look like. Trust B for the balancer, then check every other pulley against it (Step 5).

Step 5: Check the rest of the drive against the balancer

The balancer only tells you what it is. Before the belt goes on:

1. Lay a straightedge across the crank pulley grooves and the water pump pulley grooves. They must line up. If the pump is out, it's from a different spacing or wants a spacer.
2. Do the same from the crank to the alternator, power steering pump and A/C compressor. A bracket from a different spacing shows up here as an offset.
3. Idlers and tensioner too: a single misaligned idler is enough to walk a belt off.

Aftermarket drives

• Bracket kits for one spacing. Most swap bracket kits are sold as -1, -2 or -3 versions (ICT labels its kits that way) or need a spacing-specific install kit (Holley's high-mount brackets take one of three, for Corvette, F-body or truck). Order the one that matches your balancer.
• Complete drives with their own pump and balancer. Mid-mount drives such as Holley's come with their own pump and damper and pull the accessories in tighter than the factory drives. Measure them as a set and don't mix their pulleys with factory parts. Some complete kits are built only around Corvette spacing.
• Short on room? Corvette spacing is the most compact if the radiator is close; truck spacing puts the drive farthest forward and costs the most radiator and hood clearance. More: radiator, fans and steam line.

Common mistakes

• Measuring from the timing cover. Use the block face.
• Measuring to the edge of the belt. Use the middle of the groove band.
• Trusting the donor's paperwork. Engines get balancers changed. Measure B.
• Buying brackets before checking the balancer. Brackets are the expensive part to get wrong.
• Mixing a truck pump with a car balancer. The belt runs crooked no matter how tight it is.
• Forgetting VVT. On a VVT engine, the pump choice is decided by the front cover, not only by spacing.

Not sure what you're looking at? Pick where the engine came from in the spacing picker, or send us a correction if your drive measured something we haven't listed.