Here's a few tips.
#1 If the valve stem seals are newish and are the type that utilize a small garter spring around the top, squeezing onto the valve stem, then the valve won't drop unless pushed, and even then the seal is likely to catch the keeper groove.
#2 With the piston at TDC the valves cannot drop far enough so as that you cannot retrieve by hand or magnet. The valve is likely to drop only about 1/2 an inch or so before touching the top of the piston. Whoops, I now see Fast 68 plymouth's post...so , ditto.
#3 If using a compression tester hose to attach to the spark plug hole, be sure to remove the shrader valve in the end of the hose or you won't move air into the cylinder.
Best of luck.

I also use the Compression tester hose. Also, when you do it, you will probably get tempted to try to hold the piston at TDC. In my experience this wont work/can be a bad idea. I tried it one time, was using a little pancake compressor, car in gear (4 speed) with the e brake on. My e brake works GOOD too. Car kept lurching forward with that little compressor on one cylinder.

Also, if youve got 100 psi in the cylinder its really pretty hard to knock the valve down. Don't do anything crazy but it wont fall if you touch it (unless your valve job is terrible I suppose)

I have EZ heads with the 2" TTI headers and the EZ heads have the angled plugs also. I used my hose from my compression gauge as others and put air in the cyl. I also use this tool I bought from Mancini some years back. But I have no problem at all getting my comp gauge hose in the plug hole and putting air in every cyl with the TTI headers. You can see the black hose thats in cyl # 1 in this pic as its laying on the steering box. Ron

Placing the taped over screwdriver into a spark plug hole and rotating engine with a socket until it will not turn any more means that the piston has hit TDC. The only purpose for me with the screwdriver is a very simple way to establish TDC positioning,,,,my crank pulley or dampener is not degreernin quadrants and is tough to do with engine in car,,,else I would just turn 90 degrees(bringing each quadrant mark to timing marker) 4 times from number 1 TDC and deal with each pair of cylinders as described.

Cylinders at TDC are paired as thus,,,,meaning that two cylinders are always at TDC simultaneously. One at firing, one at overlap

Paired as follows:

1 & 6

8 & 5

4 & 7

3 & 2

Note:

I had said that easy to get screwdriver into driver side 1,3,5,7 which are paired each with passenger side where getting screwdriver is more problematic,,,,therefore need not even concern to insert screwdriver on that side.

Following firing order and one full engine rotation 1/4 turn at a time ang you have all 8 cylinders at TDC, hence only 4 screwdriver insertions.

Among other paranoia concerns with air technique for me is as previously noted that air can push a piston all the way down without your knowledge and any interruption to air supply could mean head removal time. Be certain to leave engine in gear with handbrake on in std. transmission car could prevent this if you remember at each cylinder to do so,,,,automatics I have no idea. Previously mentioned point well taken that oil seal might prevent valve falling into cylinder. Screwdriver method, can only fall an inch or so guaranteed.

On my engine numbers 2,4,6 cylinders as previously noted,,,,no way from above to get compression gauge hose or Lisle air supply hose into spark plug hole. Again, if anyone can, please let me know how you done it. I do not believe can be done but have often been surprised before.

I find the screwdriver method extremely safe and very fast,,,only have to deal with 4 cylinders, not 8 as required for air or rope. Your results may vary.

Someone had noted a rope getting knotted and not being able to remove. I had considered this,,,,particularly when I saw one cylinder swallow up 3 feet of 1/2 inch rope and it wanted to hangup as I removed it. It did come out,,,but thinking on the consequence if it did not gave me pause.

Others have noted with air or rope to have piston at near TDC. If you do this, you still have to deal with finding TDC(another issue) and then it getting it there. If you go thru that effort, why need either air or rope,,,and remembering cylinder pairing,,,,you need not deal with 8 cylinders, but only 4 as described.

Just for sake of conversation, don't worry about losing air pressure. Highly unlikely, and if you run any type of valve seal it shouldn't just fall out anyway. The seal will catch the valve at the lock groove.

A 2" valve has a surface area of just over 3". Multiply that by psi in the cylinder, 100psi on a 2' valve is 300 pounds of force holding it in place. Can't think of any stronger, more even, less likely to damage way to hold a valve.

By using a screw driver I thought they meant putting it under the valve to help hold it up

What I like about using full shop air is that if there is a compression leak somewhere, it will show up while you're changing out springs. Ie: vapors coming up through carb, bubbling, foaming in the radiator, hissing out the tailpipe. Might as well find any issues and correct them now, then at the track.
I have run 915 heads with straight and angled plugs, and Indy SR's all with Hooker 2 1/8" headers in my 70 'Cuda. It might be some work, but have always been able to replace plugs and do compression testing from above.

I am able to change spark plugs from above, number 6 being the toughest with number 4 not far behind. Getting a 8 plus inch long hose screwed in these cylinders impossible, if doable would take a lot of time.

The reason that I was removing valve springs was so that I could with a bench valve spring tester, be able to test each of my valve springs for seat pressure, wide open pressure, coil bind potential and ensuring that .060 safety margin was indeed there.

On each cylinder I tested both intake and exhaust and composed a report. Yes, a gigantic misuse of time but I learned a lot in attempting to learn why engine had experienced 3 collapsed Scorpion hydraulic rollers. I did find that on 3 valves I had only some .050 margin before coil bind,,,,which I corrected with plus .060 keepers as recommended by Dwayne Porter. Furthermore replaced all lifters with Crower rollers.

Assessing each lifter on the bench and documentation of characteristics required some 30 minutes each including measurement of install height for each valve.

To have used air pressure to keep valves in place for 30 minutes and with its continual leakage about piston rings(compressor running continuously) was immediately obvious, hence the screwdriver idea. Yes, I know that perhaps a tight 'O' ring about each valve and turning off air would have solved this issue,,,,but the screwdriver idea came to mind and remembering learning that pistons were at TDC in pairs,,,,I paired these ideas and went with it. Adding the knowledge that I would not be able to air certain cylinders nor likely rope them easily the decision to use screwdriver idea was easy for me

As noted earlier, your results can vary.

Is it all done and back together?

There was never any follow up on your original thread.

If I ever have the springs off for any length of time, Cleco clamps or even cloths pins have worked. Was doing a buddies 762 BBC the other nite with one of those little pancake compressors, don't use one. Had an intake get loose, but as mentioned above , the seal caught it. Magnet was no use in that situation. Jim

I must have missed something... how much ring (or piston) damage did those cylinders have that your compressor had to run continuously??

When I installed my 451's inner springs after cam break-in using a compression tester hose, there was only a small amount of air audibly passing into the crankcase, none from the valves, and my 5 hp two-stage/60 gal compressor didn't bat an eye.

All done back together for some 6 weeks. No issues except passenger valve cover tends to loosen a bit at lower left and seep oil. Tighten it again and all is well to next time. Using Moroso gasket part number 93055 that consists of two vulcanized silicone components to a steel frame. Suspect that being so thick, silicone rubber may still be compressing, requiring further tightening. Will order the well reviewed Super Performance and give them a try. Aluminum valve covers,,,,Ede heads have a very short dam it appears to keep oil from overflowing.

Have a few hundred miles on engine since reassembly including 2 days at June Goodguys meet in Pleasanton.

Crower hydraulics replaced Scorpions,,,,no requirement to change pushrod length. Rocker arms turned down 1 full turn from initial spin the pushrod tension as per Johnson SEC(manufacturers of Crower lifter). Engine valve train appears less noisy than before where I had set 1/2 turn.

Had to use 3 or 4 plus .050 keepers to ensure .060 before coil bind.

I plan to tear down other 14 non-collapsed lifters to find if any other failure beyond the 3 previous failures.

All beehive valve spring pressures well within as new spec, at 145 to 160 pounds at valve seated and 360 to 400 pounds at full open at .549 lift intake and .544 exhaust.

Likely shot of blasphemy, I continue to run my very long time oil Mobil 1 synthetic. As hydraulic roller and not flat tappet and not a race car decided to stay synthetic because of low cost and very long interval's in time between oil changes,,,,up to 2 years but likely less than 2k miles. I subscribe to the view that synthetics are superior to Dino oils in this respect. However running 0/40 in deference to Porter recommendation). .

I wish to thank very much everyone of you on this forum and the multiple threads involved that offered suggestions and guidance to me on this journey. I went into it knowing very little of valve train science and have picked up one h*ll of an education, albeit still a rookie but far more comfortable than before.

Besides I ended up with a few very nice LSM valve tool for likely a quarter of the monies I would have been out if had paid a professional shop to deal with this issue.

Hopefully I can return the favors for many of you one day.

I'm curious as to what you'll find when the other lifters are dissected.

Glad to hear you've got it all back together.

Personally, not a huge fan of the Moroso v/c gaskets. Too "squishy" for me.

I've never tried the Superformance pieces, hopefully they'll work out.

What I've had the best success with, using Ede heads and MP cast v/c are the Fel-pro 1612, which are rubber coated fibre...... And they're fairly stiff, so the hardware seems "tight".
The down side of these is...... You can almost count on them getting damaged when you pull the covers, so I don't consider them "reusable".
On the plus side, they don't leak.

Compressor was not running continuously.

Air compressor 2 stage, 60 gallon, set at 110 pound pressure. Likely far more than required to hold a pair of valves in place.

Engine 512 440 Source, new pistons, rings etc. Excelllent condition, likely under 4K miles. No smoke, uses 0 oil.

Air leakage into oil pan not deemed excessive, but would likely be a lot of air wasted if were to keep a cylinder charged for 1/2 hour or up to possible 2 hours while evaluating springs, writing reports and other tasks. Yes as noted, an O ring or clothes pin as recommended would have worked to allow disconnecting or turning off compressor.

Just found that I screwdriver worked well for my needs,,,,and again no ability to get air into 3 of the cylinders.

I hope to get my good pal John Cross,,,jbc426 on forum,,,to show me how he might get air pressure or a compression gauge to cylinders 2, 4, 6. Until then I will believe not possible,,,perhaps from under car which was of no interest and I suspect would still be a pita to screw in the required hose.

Got all the springs swapped out and checked over. Car revs a lot crisper now. Thanks again!