Regarding early sbc heads I believe the valve springs are 1 and 1/4" in diameter, what reason is there to go to a larger diameter? 1 and 1/2" is what I measure on a pair of very nice, fresh 3917291 heads I picked up last year.
I use 1.50's on every thing. .100" longer valves of any diameter valve too. I like to set the close pressure with at least a .015" shim under to protect the cast iron from the bottom of the spring. Good Luck...
Curious what the cast dates are on those, are they 2.02's? The "291" heads were the last of the true camel humps, also first year for the temp sender. There were some early in the production year that did not have the sender. I have a set on my 327 that were NOS, the GM box was labeled "initial 1968 Corvette stock".
You can get the Comp cam beehive spring 26981-16 and still use the 1.250 pocket. Use with flat tappet or roller cam profiles up to .525 lift.
The answer to your Question is more seat Pressure. Say 120lbs vs. 180lbs, so when you Rev your Lifters are not jumping off Cam Lobes.
30tudor - As others have said, it's a "pressure thing". If you have an early, low lift cam and don't do any racing around, you're fine with the smaller springs. But if you have a higher lift, and or higher duration cam than the 60's "standard performance" cams, opening the pockets and installing a larger spring is a good option. The 1-1/4" "beehive" springs are ok, but I'd still run a larger diameter spring for more stability, even running a 1-1/2" dia. beehive spring..! Mike
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