Piston to valve clearance, like many other engine characteristics, is highly variable between individual engines. The consequence of not checking for yourself (or having your engine builder or mechanic check it) is that should your camshaft have more lift than clearance, you could easily wind up bending your valves, or worse, punching holes in your pistons with them -necessitating anything from a full engine teardown to a ground-up engine replacement.

Simple instructions for checking piston to valve clearance yourself can be found here -it is a very good idea to write down your clearance once you have measured it, so that if you decide to swap cams again, a little bit of math will tell you whether you have enough clearance to fit. Note that measurement will need to be performed again if you do any engine modification; swapping blocks or heads, machining your head or engine block deck surfaces, or changing to a different head gasket thickness will invalidate the original measurement. You will need to refer to your existing measurement to verify fit if you swap to a higher lift cam than that originally measured for, or want to change to a numerically higher rocker arm ratio.

Note the following equations for figuring out what effect altering rocker arm ratios has on valve lift -as an example, we'll look at a common rocker ratio swap. Small Block Chevy, switching from the factory 1.5-1.52 ratio to a 1.6 ratio. Assuming a .500" lift camshaft (based on a 1.5 ratio); we will first need to calculate camshaft lobe lift. The lobe lift equation is as follows:

Valve lift/Rocker Ratio = Lobe Lift

Plugging in the numbers for our current cam and rocker ratio gives us .500/1.5, which equals .333" of lobe lift.

Rearranging the equation a bit gives us the means of figuring our gross valve lift with the new rocker ratio:

Lobe Lift X Rocker Ratio = Valve Lift

Inserting the specs for our cam and prospective rockers gives us .333 X 1.6, which equals .534" -a substantial difference in lift. Note that a lot of guys like to say "switching to a 1.6 from a 1.5 gives .030" more lift!" -this is true in some cases, but as this number is a ratio, not a constant, it is unwise to assume that the lift increase will be any set number -do the math, record the numbers, and verify that you have clearance, and you'll never have to guess or worry about putting a valve through your brand new forged racing pistons!

Also, remember that if you are running a split pattern cam with a difference in lift between the intake and exhaust lobes, you'll want to do the equation twice (once for each lobe) in order to accurately calculate your gross valve lift.