What is a Valve Spring Coil Bind and how does it relate to spring travel and valve lift?
When the valve spring is compressed until its coils touch one another and can travel no further, it is said to be in coil bind. The catalog shows the approximate coil bind height for the various Crane Cams valve springs. To measure this you must install the retainer in the valve spring, then compress the spring until it coil binds. Now measure from the bottom side of the retainer to the bottom of the spring. This measurement is the coil bind height. This can be done on the cylinder head with a spring compression tool (part number 99417-1), in a bench vise, or in a professional valve spring tester.
Using the above figure, subtract the coil bind height "B" from the valve spring installed height "A". The difference "C" is the maximum spring travel. The spring travel must always be at least .060" greater than the full lift of the valve. This safety margin of .060" (or more) is necessary to avoid the dangers of coil bind and over-stressing the spring.
If coil bind occurs, the resulting mechanical interference will severely damage the camshaft and valve train components.
How do you increase spring travel?
The valve spring must have sufficient travel (plus .060" safety margin) to accommodate the amount of valve lift created by the camshaft and/or an increase in rocker arm ratio. To increase spring travel you can either raise the installed height (but this will lessen the spring tension), or change to a spring with additional travel. If there is not a standard diameter spring available with enough travel, the cylinder heads will have to be machined and a larger outside diameter (O.D.) spring installed.
Crane Cams offers some special valve springs in standard diameters which eliminates having to machine the cylinder heads. For example, a small block Chevrolet engine can use spring kit part number 11309-1 to handle .550" to .600" valve lift. The 85-00 302 Ford hydraulic roller engines can use spring kit part number 44308-1 to handle .550" lift.
Besides coil bind, what other types of mechanical interference should you look for?
When you increase the valve lift with a bigger cam or increased rocker arm ratio, you must be sure there is no interference between any of the moving parts. Some of the components that must be inspected for clearance are:
1) The distance from the bottom of the valve spring retainer and the top of the valve stem guide, or the top of the valve stem seal, must be equal to the net valve lift of the valve, plus at least .060" more for clearance.
2) When using rocker arms mounted on a stud, the length of the slot in the rocker arm body must be inspected to be sure it is long enough to avoid binding on the stud. The ends of the slot must be at least .060" away from the stud when the rocker is at full valve lift and when the valve is closed. Be especially careful when using stock Chevy stamped steel rockers and any high performance stock or aftermarket cam. These rockers will typically not provide enough clearance at full-lift, and will bind on the rocker stud.
Crane Cams offers long slot and extra long slot steel rocker arms to relieve this interference problem. Aluminum roller rocker arms may be required to provide sufficient travel on larger lift camshafts or when using longer ratio rockers.
3) The underside of the rocker arm body cannot touch the valve spring retainer. You will need at least .040" clearance to the retainer throughout the full movement of the rocker arm. If necessary, a different shape retainer or rocker arm design will be required. In some cases, installing a lash cap on the tip of the valve stem can provide the clearance required.
4) Valve to piston clearance must be checked to be sure there is sufficient clearance. The intake valve must have at least .100" clearance to the piston and at least .120" clearance on the exhaust valve.
What is the critical point of crank rotation for checking valve to piston clearance?
The critical point for both valves is the "Overlap Period" as the exhaust cycle is ending and the intake cycle is beginning. You must start checking the clearance before and continue after T.D.C. on both the intake and exhaust valves to be sure you have the correct readings through the overlap period.
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You can also use a drill with a grinding stone.
