Surface Finish

If a surface finish is too rough or coarse, the gasket cannot fully conform to the irregularities. Combustion gases and engine fluids cannot be properly sealed. In the past, the prevailing theory was if the surface finish was too smooth, friction between the surface and the gasket is reduced, allowing the gasket to move and be prone to blow out. Due to the improvement to composite materials in recent years, current thinking is that the smoother the finish that can be achieved, the better.  

Surface finish or perhaps more correctly, surface roughness values are normally expressed as roughness average or Ra, either in micrometers or microinches. Ra is the calculated value of the average roughness deviation from the mean roughness height. Another expression commonly used for surface finish is average peak to valley height or Rz, again either in micrometers or microinches. Rz is the average value of individual peak to valley heights in five continuous individual measurement sections.  Unfortunately Ra and Rz are not comparable and therefore the value of one cannot be converted to that of the other.  

There are two ways to check surface finish. One is to use a comparator gauge, which is a flat piece of metal upon which a number of specimens of standard surface finishes have been reproduced. By comparing the appearance and the feel of the machined surface to those of the specimens on the comparator, an approximate surface finish can by estimated. The other method is to actually measure the surface finish using a surface roughness tester, called a profilometer, which drags a diamond tipped stylus across the surface to measure and calculate the surface finish.