Okay, you’ve already cut or ground your valve seats to the correct angles; you’ve ground the valve face in your valve refacer. Are you ready for final assembly of the cylinder head? Maybe, maybe not. The next step in many cases is to lap the valve to the valve seat for a final, perfect fit.
Lapping has been around for as long as engine building has been around. It’s a very simple, straightforward procedure.
All lapping tools work in the same manner. This means the tool is attached to the valve head, lapping compound is applied to the valve seat or the valve face and the tool is rotated left and right to lap the valve to the seat. It’s kind of like trying to start a campfire by spinning a stick fast enough to create enough heat to ignite your kindling. Of course, with valve lapping you’re not going to even try to start a fire, but the motion is the similar.
First, select the type of lapping tool you prefer. Goodson offers a variety of types and sizes including:
- Traditional Valve Lapping Sticks
- An Extra-Large Hand Valve Lapping Stick
- Vacuum Style Lapping Sticks
- Mechanical Valve Lapping Tool
- Powered Valve Lapping Tool
- Air Powered Valve Lapping Tool
Let’s take a look at each of them.
The traditional and extra-large lapping sticks are operated entirely by hand. They include a suction cup on one or both ends that you’ll use to attach it to the valve head. Sometimes a little spit will help to keep the suction cup attached. Of course, it’s always best to make sure all oils have been removed from the valve surface to ensure the suction cup mates to the valve. Suction cups are available in a variety of sizes to fit a range of valve heads.
Another type of valve lapping tool incorporates a pump to create vacuum to ensure the tool stays attached to the valve head. Don’t play with these unless you want to give yourself one heck of a hickie. They generate a lot of vacuum!
Next up is our Mechanical Valve Lapping Tool. Think fishing pole or egg beater. You crank the handle which creates the necessary oscillation.
If you’re looking for a powered lapping tool, you’re in luck. Goodson offers two of them. First is the Powered Lapping Tool that you drive with a power drill. The tool is designed to change the rotary action into an effective oscillating motion.
Last, but not least is the top shelf unit that most NHRA Top Fuel teams (and other race teams) use. The Air Powered Valve Lapping Tool. This heavy duty unit operates on 90 PSI of shop air and makes quick work of valve lapping.
The only thing left to talk about is Lapping Compound – a silicon-carbide, grease-based paste. Selecting the correct lapping compound is generally a matter of personal taste. Goodson offers several grits; from 120 to 1200 so you have lots to choose from. If you’re just getting started, you may want to experiment with different grits until you find what works best for you.
There are pros and cons to valve lapping and over time, you’ll find out which applications need to be lapped and which ones don’t. Practice makes perfect, but keep one thing in mind:
Lapping is a finishing step and should NOT be used to correct a bad seat, or worse yet, a valve that is imporoperly ground.
As always, if you have any questions about Valve Lapping or any other engine building topic, contact the Goodson Tech Services Department at 1-800-533-8010.