Mailbag: Checking Clearance Before Cam-Swapping a 307-Powered ’68 Nova

Q: I have a 1968 Chevy Nova with a stock 307 engine.

I have a brand new Summit cam and lifter kit that I’d like to install without any other modifications. The cam has a duration of 234 degrees intake and exhaust, with a .488″ of lift on both intake and exhaust. Since I plan to use the stock cylinder heads and valvetrain, do I risk the possibility of the valve striking the pistons?

Also, would you recommend that I change the rocker arms to a 1.5 or 1.6 ratio?

A: Your piston-to-valve clearance should not be a problem. However, you do have another problem—your stock valve springs will only handle about .465″ of lift if they’re in good condition, so you do run the risk of coil binding the springs. Switching to a higher-ratio rocker arm will, of course, increase the lift even more, creating a bigger problem.

We recommend upgrading to some Crane valve springs for a set of 16 heat-treated, corrosion-resistant springs capable of handling the increased lift.

We would also recommend some 1.5 ratio rocker arms too, as the stock rockers don’t have a slot large enough to accommodate all of the lift from that cam. A set of Summit steel roller-tip rockers will do the trick.

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  1. Jim Bellamy says:

    I agree!

  2. Is coil bind really the issue with the stock springs? Seems as if the geometry would bind stock springs, aftermarket springs would too. I would be as concerned or more with the chance of valve float with the weaker stock springs than coil bind, and definitely concerned about retainer to guide clearance with the higher lift. .488 lift is kinda pushing it without milling the stock guides isn’t it?

  3. Beth McLaurine says:

    Excellent advice

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