Guide updates and tuning notes by email → Follow us
Plan a swap

Home › Q&A › Identifying engines

What actually changed between Gen III and Gen IV LS blocks?

The main Gen III → Gen IV changes that matter for swaps:

Last updated

  • 58x crank reluctor (gray sensor) and a 4x cam sensor moved to the front cover (Gen III: 24x, black sensor, rear cam sensor).
  • Drive-by-wire throttle on all Gen IV, and EV6/USCAR injectors.
  • Options on many Gen IV engines: Active Fuel Management (e.g., LH6, LY5, LC9, LMG, L94, L99) and variable valve timing (e.g., LY6, L92, L20, L96).
  • New engine families: 6.2 L (L92, LS3, L99, LSA, LS9) and rectangle-port heads on the 6.2s and on the LY6/L96 6.0s.

Mounts, bellhousing pattern and most external dimensions carry over.

What we recommend

The main changes: Gen III is generally 24x and Gen IV is 58x, knock sensors moved from the valley to the outside of the block, and the cam sensor moved from the rear of the block to the front timing cover. Head bolt lengths changed, and Gen IV blocks got more strengthening ribs plus better oiling and bay breathing. The Gen IV water jacket was raised about 0.750 inch with true siamesed bores, which is useful for dry sleeves, and Gen IV engines can have AFM/DoD and VVT provisions. Heads bolt across generations, but big-valve L92/LS7 heads can hit on small-bore blocks.

Check this first:

  • Confirm head bolt length for the block generation
  • Check valve-to-bore clearance before putting L92/LS7 heads on a small-bore block

Based on what we've seen work across our own builds, our friends' builds and the wider LS community, reviewed by our lead shop tuner (over 20 years of LS and LT tuning experience). Every combination is different, so use at your own risk.

Go deeper

Check this first

  1. Confirm head bolt length for the block generation
  2. Check valve-to-bore clearance before putting L92/LS7 heads on a small-bore block

Try this, then this

  1. ID the block by cam sensor location and knock sensor position — visible without teardown. Worked if: generation confirmed

How you know it's fixed

  • Block generation identified; correct bolts/gaskets ordered

Still not fixed?

  • Heads bolt across generations but big-valve L92/LS7 heads can hit small bores - check clearance

Key numbers

WhatValue
Reluctor changeGen III 24x, Gen IV 58x
Sensor movesknock sensors valley to block sides; cam sensor rear to front cover

Time and cost: Visual inspection

Something wrong on this page? Tell us