thanks to this forum and the great people here, I've learned a lot about these robust 240 and 300 engines. Some of the same basic questions come up over and over and it's nice to have a good resource to refer to.
Here, I am putting down some simple information regarding roller rockers, what they do, and fitment issues. These are oversized valves made for the 240 by SI Industries. They have stock length, stock 1/4" tips, and stock 1.7" spring height for the springs. The cam here is an Isky Mile-a-Mor, which has a 0.415" lift on the intake/exhaust. Lift on the stock cam is about 0.4" with a difference between the intake/exhaust.
Roller rockers can improve valve train functioning. Their main advantage to me, a simple daily driver guy that wants stepped up performance and economy compared to the OEM engine, is to reduce friction and increase longevity of the valve and valve guide. The only full aftermarket roller rocker that I know of still in production for the 240/300 is the Harland Sharp S4002, with a 1.6 ratio. That's the roller in the photos. Others have made BBChevy rollers work, and there are a number of ratios available. The geometry of the 300 head may be different than this early generation 240 head.
Friction is reduced by using needle bearings on the fulcrum point, and a roller tip on the valve stem. The roller tip allows the rocker to transfer force to the valve tip in a way that reduces side loading. The side loading of the valve stem in the valve guide increases friction and wears out the valve faster. Side loading occurs as the OEM rocker tip 'scrubs' across the valve tip.
Additional benefits, for the performance minded, are the ability to run high lift cam lobes and heavier spring loads, and maintain consistent functioning in the higher rpm range.
The geometry of the early 240 valve train is quite interesting. The lifter centers for a cylinder are approximately 2.07" apart. The pushrods maintain this 2.07" up and down motion via the use of built in valve guides. The following photo shows the general arrangement:
Note the tight clearance between the pushrod and pushrod guides in the cylinder head. The pushrod can move freely in the plane of the valve, but not laterally. On the OEM rocker or roller rocker setup, there is only one orientation that the rocker can assume.
The the valve train in the 240 head is less than ideal--the valve stems are 1.9" apart on the same cylinder. Since the pushrod only goes up and down (no lateral movement), there is a 'scrubbing' that takes place between the valve stem and the rocker tip to absorb the side movement. The after market roller reduces the scrubbing--much like how a tire has less 'scrubbing' when a steering wheel is turned at 5mph than when at a standstill.
Here's a photo of how the Harland Sharp 4002 roller rocker and OEM rocker sit on the stock valve height/spring setup. Note how the roller shows a definite off-center in the front and back sense. In subsequent photos, note how the roller also has an off-center in the side to side sense. Naturally, this will vary somewhat according to the cam and rocker ratio.
Note also how this photo show the approximately 7/16" height difference between the OEM and Harland Sharp Ford 240/300 roller. It's this adjusting nut on the top of the rocker that creates valve cover interference with the stock cover. The lock itself is inside the adjusting nut.
This photo shows the valve at rest. The roller should start off-center.
At full compression, the roller should pass through center and be fairly close to center. This is the point of peak spring tension and the roller should not be too far off center.
John in Fresno