Keith Black flat top piston set is .040 oversize with an 8.9:1 compression ratio for 1988-up XL 1200cc Evolution models. The Keith Black 390 hyper eutectic alloy is cast in a permanent steel mold, making it possible to build a lightweight piston that produces more power, uses less fuel, and lasts longer. Pistons are .040 oversize for a 3.498 bore and have a 9:1 compression ratio. The low heat transfer of the alloy keeps the skirts cool, so piston expansion is minimal. Combining a low heat transfer piston alloy and high compression ring placement increases power and economy. The elevated ring placement alone reduces detonation and increases the maximum ring temperature. **Note: Run the engine at moderate speeds for at least 500 miles avoiding excessive running in lower gears to properly break in new pistons and rings.
- 1988 / UP XL