Yesterday we covered why valve lash exists — the cold clearance that keeps valves from burning as the valvetrain expands. Today: how engineers actually set and maintain that gap. There are three dominant methods, each a compromise between precision, cost, and service interval.

1. Screw-and-Locknut (Adjustable Rockers)

The traditional pushrod solution. A threaded adjuster in the rocker arm rides on the pushrod tip; you back off the locknut, turn the adjuster with a screwdriver until a feeler gauge drags, then re-torque the nut. Cheap, infinitely adjustable, easy to service in the driveway. Small-block Chevys, Harley V-twins, and most aftermarket race heads use this. Downside: locknuts back off under vibration, and setting 16 valves takes patience.

2. Shim-Under-Bucket and Shim-Over-Bucket (DOHC)

The overhead-cam solution. The cam lobe presses directly on an inverted cup (the "bucket") over the valve stem. A hardened steel shim — typically 2.50–3.50 mm thick in 0.05 mm increments — sits either between the bucket and valve tip (under-bucket) or on top of the bucket (over-bucket). To adjust, you measure the current gap, calculate the required shim thickness, and swap shims from a kit. Honda B/K-series, Toyota 4A-GE, most motorcycle engines. Rule of thumb: shim change = current gap minus desired gap. If lash is 0.30 mm and spec is 0.20 mm, you need a shim 0.10 mm thicker. Over-bucket lets you swap shims without pulling the cam; under-bucket requires cam removal but keeps reciprocating mass lower for higher RPM.

3. Hydraulic Lash Adjusters (HLA)

Zero-maintenance option: a small oil-filled plunger inside the lifter or lash adjuster automatically takes up any clearance using engine oil pressure. Set-and-forget for 200,000 miles. Every modern GM LS engine, most OHC Fords, and virtually all luxury cars use HLAs. Trade-off: HLAs can "pump up" at high RPM (holding valves open slightly), limiting redline to roughly 6,500–7,000 RPM. That's why solid-lifter engines still dominate motorcycle and race applications.

Real-world example: The 1990s Honda B16A uses under-bucket shims with a spec of 0.15–0.19 mm intake, 0.17–0.21 mm exhaust. Skip the 60,000-mile adjustment and you'll often find intake valves at 0.05 mm — dangerously close to zero lash and eventual valve burning.