You certainly ask why should you replace your engine valves when rebuilding en engine? This paper will try to answer it for you.
A valve lapping is not always sufficient to solve leak issues operating during running. When lapping the valves at cold temperature, the sealing is obtained due to the conformation of the profile roughness between valve seat face and seat insert. Once all the parts at running temperature, components will dilate, deform, and take position in a different state as in cold temperature. Sealing is not guarantee anymore as roughness profiles created are not in front of each other. A valve seat and seat insert cutting (after checking the guides) allow to ensure a good sealing at cold temperature AND at operating temperature as the profiles angles, and roughness are extremely controlled. On the Porsche 924 models, the valves can’t be regrinded as the vlve face is reinforced with a specific addition element during the manufacturing process.
A leaking valve will considerably increase the component temperature as it can no longer evacuate heat through the seat. The alloy will quickly fatigue and the valve will burn (crack) or can sometimes break for the bi-metallic exhaust valves due to the repetitive seat collisions. This phenomenon can sometimes be observed with fuel system incorrectly calibrated (often due to a lean mixture). Here is why it’s primordial to be sure your injection system is in good condition first.
A reworked engine valve which has run for a long time in the engine can still have an important play in the guide, even if these ones are in good condition. The immediate sealing when touching the seat insert is not completely guarantee. In some cases, the valve train lift diagram can be affected especially at high revs. The valve will be excessively stressed because it suffers from excessive bending forces when dropping on the seat insert.
A worn or damaged valve stem when disassembling the engine can cause an important oil consumption as the sealing with valve stem will no longer be guaranteed.
