Reply to "Sanding block for cylinder heads"

I am using FelPro 8347PT1. From FelPro's installation guide: "Roughness Average (Ra) is the average micro-inch measurement of peak-to-valley roughness height of a flat surface. The lower the Ra number, the smoother the surface. Fel-Pro recommends a finish of 60 to 80 Ra for cast iron cylinder heads"

Using the chart below it show 80 grit puts the surface at 70 roughness, the ideal roughness per FelPro. Am I misinterpreting this?

US Grit   UK Grit   Ra    Ra 
ref       Ref.      µm    µ inch
          120       3     125
          180       2      85
80                  1.65   70
          240       1.50   50
          320       0.75   30
180                 0.62   25
240                 0.45   18
          500       0.40   15
320                 0.25   10

11/5/226:54 AM

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