Broke this weekend on my 70 351C. It just suddenly let go. Fortunately no further damage than the spring and a pushrod. Cause for concern because a spring broke on the other head a few months ago. I've never revved it over 4500.
If there's no more problems I'm taking it to Mesquite NV this weekend for Motor Mania.
If they are 2v heads, i have a really nice set for sale. All new guides,valve seats, valves, springs, pushrods, ported, decked, drilled and tapped for screw in studs and roller rockers. Let me know if your interested.
Since that is the second one you've broken, and you don't rev it past 4500, You better start checking things out. Quality of valve springs? Length of push rods? Lift of cam/ coil bind at full lift? Piston to valve clearence? Something seems very wrong, as you shouldn't be breaking valve springs at 4500??? I've ran several Clevelands from stock to 500 hp. Never have I seen a Cleveland break a valve spring. I'd start checking before you do some serious damage. JTS 71 Mach1
How old are those parts? I have seen springs break because of age/stress regardless of r.p.m. It appears those could be stock 2V springs (no inner spring). You might want to replace all of them with double springs. I would also opt for a new set of chrome moly pushrods (cheap insurance) GXR