The lapping process will last as long as the valves do it is basically a grind job without the machining process you use the compound as the process. You can test the springs with a spring tester. A valve spring tester is nothing but a scale on a device to compress the springs to tell you how much force is required to compress them. you test for installed height pressure then for rate by compressing the spring a specific amount. Springs do not generally wear out but if you are putting new valves etc in a head I would get new springs they are not expensive around 75 to 100 bucks for a set. What valve spring pressure and rate you use is determined by cam duration and lift and what type of rpm the motor is going to see. F
From the Crane Cam web site
"A vehicle owner wants to use a .520" valve lift camshaft in an application and is considering different valve springs.
Spring A has an installed pressure of 125# at 1.750" installed height and has a rate of 280#/in.
Spring B has an installed pressure of 115# at 1.750" installed height with a rate of 410#/in.
At .520" lift, Spring A has an open pressure of 271# (this is 125# of seat pressure plus [.520" x 280#/in] = 146# from spring compression). At .520" lift, Spring B has an open pressure of 328# (this is 115# of seat pressure plus [.520" x 410#/in] = 213# from spring compression). Both of these springs would work on a street performance application requiring good performance and reliability.
However, Spring A with a lower open pressure of 271# could probably be used on a cylinder head with pressed in rocker studs; while Spring B would definitely require screw in studs for adequate reliability. Spring B would probably provide better performance above 6000 RPM (especially with relatively heavy valves) because of its higher open pressure of 328#. Spring A would probably idle a little smoother with higher vacuum, especially if a high pressure oil pump or thicker oil is used. This is a result of Spring A's higher seat pressure of 125#."
It is rather complicated but not really once you get the idea, with a marine motor we do not see the high rpm's so we are looking for a smoother idle etc.. The rate of compression of the spring is what controls it from letting the rocker bounce at the top of the cam lift which is what ruins pushrods. You do not want bounce so high rpm's and high lift/duration cams need heavy springs. In my situation the springs are actually compressible with your hand on the bench not much but some they are in the 80 to 90 lbs installed range with a rate of around 200 lbs per inch. so that is relatively light.
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