Can someone give me the lowdown on valve clearance vs rocker ratio

If i buy a set of pistons that quote a compression distance of 1.250" and i get a cam for the sake of argument with a .500/.500 lift - I can use a 1.75 ratio rocker (which makes .875 on the lift on the cam)??


Thanks

 

IMO if you are changing the cam, get your lift with the cam. Most SBF cams are spaced for 1.6 rockers, and have had their advertised lift adjusted taking this into account.

In other words if you buy a cam advertised at .600 valve lift, the lobes will lift the lifters only .375... with 1.7rockers the lift will be .6375. 1.75 rockers = .656 lift

Unless you are building one hell of a hoss,, .600 valve lift is plenty.

As far as checking piston to valve clearance goes, I have used the clay method. Put some little clay "stools" on the piston while lowered in the bore, put the gasket and head on, turn the engine, remove the head, dissect the clay, measure at the thinnest point and wallah!!

I am sure there are more sophisticated ways of doing this, but it works for me...

 

Might have to be the clay method - i've done a search and i cant come up with a formula for it at all.

Thanks

 

There is no formula to calculate it that I know of. It's based on the size and shape of the valve releifs that are cut into the piston, the lift and the valve timing of the cam. It's primarily a valve timing thing though, the valve needs to be opened at the right time to avoid hitting the piston.