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That is the question.... In the middle of a head gasket job. Installing new springs and seals also. Just wondering if I should lap the valves while I'm at it. The motor has been relatively stock its whole life with 135k miles.
How much will lapping the valves affect pushrod length? Is it negligible?
I wouldn't lap in the valves to bandaid a sealing problem.
I usually just lap the valves to check the concentricity and contact area of the seat angle. It won't remove any noticeable amount of material if done correctly, and that does not include using a drill. You should use the handle with the suction cups to hand lap the valves.
I agree with above except im impatient and use a drill. Takes about 10 secs per stem and gets the faces nice and pretty. Generally if I'm installing a used head with no issues I will do this just to check the sealing surfaces and to slightly renew them. With compound, you get a very good look at how the two parts are mating, and therefore how well they will seal.
I'd consider soaking the valves in some lacquer thinner and use a bronze bristle brush. Most of the crap will come off and you may not even need to lap. Try to soak the seats as well.
Without a hot tank, lacquer thinner is the next best thing. I was really surprised how much crap came off.
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Valves: To lap or not to lap? 
