[7] A valve stem height gauge is a simple tool that makes accurate measurements a snap. You must make sure you are using the same retainers and locks that you will be using in the final build in order for your measurements to be accurate.

[8] Like on the chamber side, Troutman records the height of each valve, this time from the lock to the spring seat. Now, because he knows how deep the valve is sitting in the chamber, he can determine if any variances are because of the valve seat height or because of the height of the spring seat and can cut the seats accordingly.

[9] The point of all this is to be able to minimize or eliminate the use of any shims underneath the spring seats (right). Shims are a band-aid that serve only to help achieve the correct valvespring installed height. The problem is that shims create a couple of compromises that can harm the valvetrain. First, stacking shims weakens the foundation that the valvesprings sit on. It allows more spring “wobble” which hurts the spring’s ability to maintain proper valve motion at high rpm levels. Second, shims serve as a heat insulator. One of the biggest enemies to spring life is heat, which is generated every time the spring is compressed. Besides oil splashing the spring, the only other way to remove heat from the spring is through the head to be carried away by the coolant flowing through the water jacket. Shims help hold that heat against the spring. And finally, cutting shims can often mean using a shorter valve stem, which cuts overall weight and also helps stabilize the valvetrain.

[10] Notice that we haven’t talked about valvesprings yet. That’s because Troutman says the valvesprings shouldn’t be chosen ahead of time and the valvetrain made to fit them. Instead, all works together. Many spring manufacturers publish the specs of all their valvesprings in their catalogs, as Comp Cams has done here. Now that he knows the valve stem height, Troutman can choose the spring that has the correct rate to create the correct seat pressure at the spring’s installed height along with the optimum open pressure. Troutman says Comp offers enough springs that he almost always can find the right spring to meet the needs of the engine and almost never has to resort to shims to make the installed height fits the spring’s requirements.

[11] You should also check and make sure that the spring seat you are using also works as a locator. If it is right, the spring will be a snug fit over the locator on the spring seat. The picture shows how the fit is snug enough to hold the edge of the spring up on its own.

[12] Entire articles can be written on seat angles alone, and we won’t touch on them much here. But before beginning final assembly, Troutman laps the valves. The first step is to apply a small amount of lapping compound to the valves around the seat area.

[13] Troutman spins the valve in its seat. The slight abrasiveness of the lapping compound helps provide a better seal by smoothing away any tool marks left by the cutter while it also shows the area where the valve meets the seat.

[14] After the valves have been lapped, you can see exactly where the valve contacts the seat in the combustion chamber. Notice it is approximately in the center of the seat face. Some engine builders try to gain a little extra flow by moving the seat area to the extreme edge of the valve. But this means that when it comes time for the rebuild, recutting the seat will drop the valve into the port, which will hurt performance. So to help maintain peak performance over the course of a season—or even longer—you must plan ahead.

[15] Besides sealing the combustion chamber, the area where the valve contacts the seat is also the primary way heat moves from the valve into the head where it can be swept away by the coolant flowing through the jackets. The seat area should be at least 0.040-inch wide for the intakes and 0.060 for the exhausts(they will grow slightly as the engine is run). But be careful that the mating area isn't too wide. Besides hurting flow, a seat area that's too wide will spread the loading provided by the spring over a larger area. Troutman says he's seen occasions where a seat that's too wide will allow a piece of carbon or trash to damage the seat, while a narrower seating area will create enough pressure between the valve and the seat to break up any carbon before any damage occurs.

[16] Finally, a note about valve guide clearance. With an 11/32-inch diameter valve stem, a good rule of thumb for the valve guide clearance is to be between 0.0015- and 0.002-inch. If you are running cast-iron guides, go for the larger end of the range at 0.002 to avoid galling or sticking. With bronze valve guide inserts you can go for the tighter side. Generally, you don’t want any more than 0.002 clearance because that will allow the valve extra movement in the guide which will beat up the seat.

[17] To avoid valve stems galling in the guides from the extra heat, some valve manufacturers will actually make the exhaust valve stems a few tenths of a thousandth undersized. Because of this your machinist should always set up his measurement tools and measure all the intake guides, then recalibrate and go through the exhausts separately.

Comp Cams
3406 Democrat Road
TN  38118
KT Engine Development
384 Industrial CT
NC  28025