Low-buck DIY: Valve Lapping

We recently refreshed a set of 30,000-mile heads for our budget LT-1 rebuild. While we did splurge on new springs to match our cam, the valves cleaned up nicely and we decided to re-use them. Inspection revealed a little pitting on the sealing surface, and while they might have been just fine, we knew they could be a little better.


The brown ring is the contact area and was pitted and worn.

Hand-lapping valves was part of regular vehicle maintenance in the early 1900s, but the time-honored ceremony has fallen out of fashion with the advent of modern technology. Today’s engine builders have advanced machinery that perfectly cuts valve seats and valves better than any hand ever could. However, there is a time and a place for all things, and many top engine builders still give them a once-over just to be sure.

In cases like our engine, it’s a step you shouldn’t skip. Any time you change the seats, re-grind the valves or put new valves into an existing seat, it’s a good idea to lap them in and make sure they are seated.

Since valve lapping paste is a highly abrasive pumice, there is a right way, and a wrong way, to do it. When done properly, the pumice breaks down between the surfaces as they wear in together. This is why the sound goes from gritty to smooth. The valve is then picked up and turned a quarter of a turn to introduce new pumice to the surface, and the motion is repeated.

The proper motion is a quick back-and-forth “fire-starting” hand motion that evenly works in the pumice. Cheating by spinning them in a drill carves a groove in the seat and should be avoided. If you MUST use a power tool, pulse gently and be sure to lift and turn several times. The object is a uniform seating surface, not to machine them down. Slow and steady wins the race.

The newly-matched area will be a smooth gray ring on both the valve and seat, and allow the valve to make a solid cupping sound when dropped into place. Be sure to thoroughly clean any traces of the compound and debris when finished. Failure to do so will leave grit behind that will cause serious damage. Follow along and see how it’s done.

Tools you’ll need: