Rebuilding heads isnt too hard just need the right tools which include valve spring compressor, vacuum tester, straight edge, feeler gauge.

Aluminum heads should .003 or less when a straight edge is applied to the head surface. Check for cracks in your head by cleaning it throughly and inspecting closely.

You should buy new head bolts and measure the lower block with a straight edge to check for any warping.

Now would be a good time to do the timing chain since you have half of the motor apart.

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1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)