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...I went with the CompCams Pushrod at 3/8"
There was plenty of Clearance with the Iron 4V Heads, But! I had to Grind/Polish the 'Bottom' of the 'U' Shaped Yoke, on the GuidePlates as the Rods Rubbed there. These were 3/8" GuidePlates, the 'Fork' was just a little Shallow, an Easy Fix!(I Think the Plates were Manley Or CompCams). The Rods were Standard Length, used with Roller Rockers. I Believe 5/16" pushrods ARE too Weak!! I also believe in 'Massive OverKill'!...
I know drag racers who swear by the Manley 3/8" push rods. I've always used custom ordered 5/16" push rods.

The outside diameter of a push rod is not the only consideration. The wall thickness of the tubing it is manufactured from is also important. A 5/16" OD push rod manufactured from tubing having 0.120" wall thickness will be very stiff. The hole in the middle of such a push rod is only 0.072" diameter ... its almost a solid push rod. It also serves as a nice way to meter the flow of oil to the valve gear. Finally, the material the tubing is manufactured from is also a consideration. A cromoly alloy that is light yet stiff is a good choice. The guys who manufacture custom push rods for race teams are normally well versed in the pros and cons of various materials, and can offer a recommendation.

I think my next set of push rods will be tapered, which is the hot tip today. Custom made tapered push rods are sold by at least two companies I'm aware of, Manton and Trend. Like beehive springs, tapered push rods cancel resonance by virtue of their shape. The rods will be 3/8" at one end, tapering to 5/16" at the other end. I'd like to still use 0.120" tubing if possible.

Choice quality stuff.

How much valve pressure can 5/16 rods stand?

Check with the specific manufacturer.

I'm using CompCams 7997-16 Hi-Tech™ 5/16" Diameter pushrods. More info here:

And those are being used with Lunati (74820-16) Pacaloy Beehive valve springs. My spring pressures follow:
Seat: 155 lbs.
Open: 377 lbs.
Rate: 370 lbs/in
That is completely dependent upon the wall thickness of the tubing, the material the tubing is made of, if the tubing has been heat treated, etc. In other words, knowing the tubing is 5/16" in diameter is not enough.

I agree with Garth, the manufacturer can provide guidelines, but it is also dependent upon the length of the pushrod, the maximum engine speed, and even the angle of the pushrod. The 351C with its canted valve heads is much harder on push rods than an in-line type cylinder head.

Great thread especially the post of Doug Bruan's issue (thanks Garth I would never have thought that would be an issue). I'm a bit surprised that the Scorpion rockers Doug had, would bind (depends on cam lift), but going from a 5/16 to a 3/8's rod diameter decreases the clearance by .0313 on each side of the pushrod. I was considering larger push rods, but now realize a high quality 5/16 rod will work just as well with fewer headaches with geometry and clearances.
I happen to like Smith Brothers or Manton custom pushrods, who also sell a cheap adjustable pushrod to find what length you really need; stock length normally is wrong. Custom-length pushrods also come in a bewildering variety of wall thicknesses that assist in stiffness. Garden-variety street engines usually get 0.065" or 0.080" wall 5/16" pushrods. But they also are available in 0.120" or even 0.160" walls.
There will be fitment issues in a host of places with oversized pushrods. For those concerned, weight of the pushrod and lifter has proven to be much less important to valve float levels than extra weight on the valve side of the rocker arm. I would suggest sticking to the cam grinder's recommendations for your spring strengths. They know their product better than we do.
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