Valve Seats
After the new guides are inserted, the valve seats need to be ground (or cut). Valve seats have three angles - the primary angle refers to the area of the seat that makes contact with the valve. This angle is usually cut one degree different from the valve itself - this assures an area of contact that is very narrow and seals very well on initial startup after the valve job is complete, and prevents burning of the valve that would result if contact was poor. Over time, with repeated pounding of the valve into the seat during normal use, the primary angle on the seat and the the valve will wear to be the same. The other two angles that need to be cut are done at a later stage, and will be explained in context.
The valve seat (arrowed) is seen here. It has carbon deposits and evidence of corrosion from exhaust gasses.
Concentricity of the valve guide, the valve and the valve seat is absolutely critical to proper fit and sealing.
All grinding or cutting needs to be done with a central pilot that ensures this concentricity. Any movement in the pilot cannot be accepted, and therefore a pilot of exactly the right size is needed. Pilots are tapered very slightly to ensure a tight fit . There are adjustable pilots that expand to fit but these are less stable and discouraged. Coat very lightly with oil to ensure removal.
The grinding stone (A) is chosen for correct diameter and coarseness - I used a very fine grit. Before use, the stone needs to be dressed - this entails cutting the surface of the stone with a diamond (B) slowly moving across the stone while it spins. This is done to both create a clean grinding surface, and also to customize the angle at which you want the seats cut. I dressed the stone to 44 degrees.
The stone and holder is then slid down over the pilot to rest on the valve seat.
A power driver spins the stone at high speed. No downward pressure is applied - it’s weight is plenty.
As the goal is to remove the very least material possible, I simply pressed the trigger and immediately released it allowing the stone to spin to a stop - remove and evaluate - repeat as necessary.
Apply blue machinist dye to a valve and then install the valve in place. Slight rotation of the valve and then removal will transfer dye to the valve seat at all points of contact.
Here, full circumference contact is seen (A) and well centered on the seat. If any gaps in the dye were present, further grinding would be needed.
Also note the other valve in place (B) and a rag (C) stuffed in - both of these are there to prevent ground seat metal and stone dust from getting into the cylinder or manifolds.
A freshly ground valve seat has a uniform appearance of clean metal all around. However, this cannot be assumed to be enough - a contact test with a valve is needed to properly assess
The seat is now completed in the primary angle. The valves need to be assessed and adjusted next.
At this point, with a good seat and a good guide, it is essential to check that there is no excessive runout in the newly ground seat. This is the best way to assess true concentricity of the guide and the seat, as well as ensure that the seat has been ground such that the valve will make even contact all the way around.
The runout gauge fits into the guide like a pilot, and then the measuring needle (arrowed) is positioned to touch the seat. As the gauge is rotated, any runout can be measured on the dial.