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I had to replace a head gasket and needed to do a better job of preparing surfaces that contact the gaskets.  I could have taken the cylinder head to a shop where they could put it on their machine that essentially sands the surface but I came across a way to do the same in my garage.  The block is a thick glass shelf (8" x 24") with two sheets of 80 grit sand paper attached with spray adhesive.  I laid the block on the garage floor with a piece of old yoga mat in between to keep the glass from sliding and then moved the cylinder head back and forth on the block.  It only took about one minute of sanding and the surface looked perfect.  The sanding block can also be used on the deck of the cylinder bank but you have to make sure no debris gets into the bores and lifter valley.



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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

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