Pressure at the tappet increases in a linear fashion and is directly related to the increase in valve lift that’s accomplished by increasing the rocker ratio.

Frank has pretty much nailed how to calculate the actual load on the lifter based on the spring pressure. As a short cut and while not absolutely accurate, the valve spring over the nose pressures are typically used as a guide of what’s permissible for tappet loading. As a general rule, the valve spring pressure at 330-350 lbs is the limit for flat tappet camshafts. That would equate to 528-560 lbs actual pressure at the tappet using 1.6:1 rockers. But you’ll not hear the 560 lb value being used, only the 350 lb value. This is just a matter of semantics and the average person just needs to be aware of what the numbers actually represent so those numbers can be kept in their proper perspective.

Valve springs have a rate per inch value that can be used to calculate the amount of valve spring pressure at a given valve opening. It’s this value that can also be used to calculate ‘over the nose’ or lobe/tappet pressures. If the rate per inch isn’t known, it can be calculated from any two known compressed points on the spring. The linearity of the valve spring pressure rate makes this possible.

For a performance Y, the Isky p/n 165-A beehive valve springs are a popular choice so I’ll use it in this example. This spring is rated at 130lbs. @ 1.800” and 320 lbs. at 1.200. This is a 190 lb increase in pressure with a .600” lift. Doing the math for a 1.000” valve lift knowing these two values, the rate per inch would be 316.67 lbs. Going to the catalogue and looking up this spring shows its rate per inch listed at 310 lbs. While slightly different, it’s close enoughto be comfortable with using the catalogue value.

Knowing that particular rate per inch allows other valve spring compression values to be calculated. If the net valve lift is 0.531”, then the ‘over the nose’ pressure will be 130+ (.531 X 310) or 295 lbs. This is of course assuming the closed pressure is being set at the recommended 130 lb level. Likewise, if the net valve lift is 0.650”, then the ‘over the nose’ pressure will be 130+ (.650 X 310) or 332 lbs. If using my calculated value of 316.67, then the ‘over the nose’ pressure for the 0.650” lift cam will be 336 lbs. We’ll not quibble over 4 lbs. When in doubt, the valve spring pressures should always be checked with a valve spring tester at both the intended closed installed height and at the maximum valve opening.

The service manual shows the wooden clothes pins that John is talking about being used to hold the lifters in place while the camshaft is being serviced in the vehicle. Here’s the excerpt from the 1957 Service Manual.

And here’s the link to the thread showing a picture of the lifter tool used to install the tappets by way of the cam tunnel. Clothes pins still have to be used as the lifters are placed in their respective holes.

John, my wife could supply the Canadian market if your wife wants to go International. On this subject, not that I plan to remove a cam this way, I have wondered if you couldn't fabricate a tool with rare earth magnets to hold the lifter up during the cam change. I can visualize the cloths pins slipping off at the worst possible time. However, the problem with magnets, although I see that Ford advocates the use of magnets in the shop manual, is that they attract metal debris. Since the magnet will induce some magnetism into the lifter any metal debris will be difficult to extract and will undoubtedly find its way into the lifter bore. I suppose one could lay a sheet of saran wrap over the lifters before getting the magnet anywhere near them.