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Sanding block for cylinder heads

stevebuchananstevebuchananMember

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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stevebuchananstevebuchananMember

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
lf-tp2511lf-tp2511Member

Steve

there are two critical things when it comes to refreshing a cylinder head surface ….. how clean they are, and how uniformly flat they are.

Those heads certainly appear to be clean, but have you done any checks for flatness?

A machine shop test would be best, but a good straight edge, and some back lighting could give a good seat-of-the-pants evaluation.

Is it possible to do some sort of a test like is done on body work, using two colors of primer to find high and low spots? Metal bluing solution and crocus cloth?

🤷‍♂️🤷‍♂️

Larry

stevebuchananstevebuchananMember

Larry,

These heads were at a shop 1,000 miles ago for a complete valve job including head resurfacing. The engine was assembled at that time and has not been overheated (just over rev'd a bit). It looks like this head was still flat when I removed it this week. There was the typical amount of carbon and gasket material on the head. If it was warped, that would have shown up while sanding on the flat glass block. There would have been pockets of dirty areas surrounded by clean surfaces.

I also used a straight edge to ensure flatness of this cylinder head.  To your point It is better to know now if there is a flatness problem than after the engine is assembled and being tested.

Thanks, Steve

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