How much do they cost and are they more expensive than OEM gaskets?
Q) Costs for valve cover gaskets range from under $4 each for TCM 360s (M20Ks) to $8 to $10+/- each for Lycomings. Since they are guaranteed to last the life of the engine and not counting the cost of loss of oil wasted by leaks by other gasket materials, REAL gaskets are far cheaper and more economical in the long run than the stock cork or neoprene that commonly have to be replaced several times just to get to TBO.
