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Old 05-01-2013, 01:24 PM
MouseFink MouseFink is offline
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The base circle of the camshaft determines the distance from the rocker arm jamming shoulder so the pre load can be set and locked by the rocker arm nut tapered seat. If the distance is changed by using a camshaft with a smaller base circle, the 20 lb. of torque needed to lock the rocker nut to the bottleneck rocker stud cannot be met. You can still lock the pre load setting by using SB Chevrolet 3/8"-24 lock nuts a couple of times before the treads are worn out.

You can measure the cam lobe base circle with a pair of cheap vernier calipers ($10 at Harbor Freight) and it can be done with the camshaft in the engine on Pontiacs. If you call Lunati, they probably don't know what a cam lobe base circle is and if they did, you should not trust the informational they provide.

The 3/8"-24 bottleneck stud gives the stock screw-in or pressed in rocker studs a weak point and a place to break. They have a 7/16" shank but that is not where they break. The stock studs break at the tapered rocker nut seat between the 3/8"-24 threads and the 7/16" shank.

It is not the .450" valve lift that snaps a stud but the increased valve spring pressure caused by the extra lift can break a stock bottleneck rocker stud. Stock Pontiac valve springs coil bind at .526" valve lift which makes .470" the maximum valve lift with stock valve springs. An extra 125 lb. open pressure with stock valve springs is enough to break a bottleneck rocker arm stud. Some cam grinders prefer not to press the limit of .470" valve lift but use .450" valve lift as the limit with a 1.330" base circle for an added margin of safety. That was done in order to sell more camshafts to happy customers that are using stock bottleneck rocker studs. You can break stock bottleneck studs just by switching to 1.6:1 rocker arms.


The reason a 1.330" base circle and .313" lobe lift is the limit is because 1.643" is at the design limit for the camshaft to fit through 1.8687" diameter cam bearings. That leaves .113" clearance between the cam lobes and cam bearings. Lunati holds that limit to .0150" for their cams before they reduce the diameter of the cam lobe base circles.

Last edited by MouseFink; 05-01-2013 at 01:35 PM.
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