A "go/no go" gauge will show...
A "go/no go" gauge will show if your valve guides are worn or loose.
* Check concentricity and perform a pressure test. The valve seat must be concentric-as round as possible-for the valve to seat properly and obtain the necessary seal in the combustion chamber. A concentricity gauge is a must. Once the angles are cut in the head, use the gauge to determine where you need to grind the seats to true them up. Go beyond what the guy down at the local garage or machine shop might be doing with heads off street car engines.
Once any concentricity issues are addressed, check to see that the valve is seating at the desired point. Lap the seat with a fine compound to see that the valve is seating right on its outside edge. Vacuum testing the head to make sure the valves are seating properly is critical. If you have a pressure leak, then you've got a problem-recheck the concentricity, the angles, etc.
* Preventive maintenance is crucial. Valve adjustments and spring checks should be part of your valve maintenance routine. If the valves are too tight, the engine won't run efficiently. A loose valve will pound and actually start to bounce, thus destroying the seat.
"Naturally, once the valve job is done and the engine has run, it's always critical to routinely each week adjust the valve lash and stay on top of that," says Poe. "Oftentimes that will tell you if you have something going wrong. If the valve adjustment starts closing up, then you feel like the valve is either sinking in the seat or a valve stem could be stretching or you've got a problem somewhere. It's critical to stay on top of the valve adjustment every week."
Lapping the valve with a fine...
Lapping the valve with a fine compound will tell you if the valve is seating properly and verify location.
Keep fresh valvesprings in the engine. Gauge the springs when they're new so you'll have a baseline figure to compare to when you check pressure after each run. "That way you can tell if one starts to fall off," Dorton says. "Usually, they won't all go at one time. One will go, then another one. It's critical to check every one because, generally, they won't all fall flat at once, just one here and there."
A valve job should be done in the range of 1,000 laps for short-track racers, although some engine builders may go as high as 2,000 laps. The key is to not take anything for granted when doing a rebuild. Even if the heads run like you think they should, or the engine runs like you think it should, don't take the valve job for granted. And, preferably, make sure the same person is doing the work each time. If you're taking the heads to an outside shop, make sure they're checking the concentricity and truing the seats as needed. Remember, a valve job for a racing engine should go above and beyond a valve job for a street car engine.
Valve maintenance is crucial...
Valve maintenance is crucial to peak engine performance.
* Stick to the basics. Avoid the temptation to get radical when machining heads. A 30-degree top angle, 45-degree seat angle and a 60-degree bottom cut are standard. "A lot of guys are playing with steeper seat angles, which tend to work good for a little while, but they tend to be detrimental to the seat," says Poe. "It hammers the seat out pretty badly when you start increasing the seat angle just to gain a little additional flow."
"You can experiment with different valve configurations, but in general we find an exhaust valve with a radius on the chamber side and a flat intake works the best," Dorton says. "Check with the valve manufacturer on recommended minimum margins or valve thickness. It is not recommended to thin valves to even valve heights in the cylinder head. The seats in the head should be machined to the same height plus or minus 0.001-inch."