Changing Valve Springs

[AMXguy]

I need to help a buddy change the valve springs in his 440 with the engine in the car, I have the air hose and compressor and I've done it before. I have all 4 spds so I always roll it up to TDC and leave it in gear with the brake set incase I'm off a little.  this is an auto, is there any reason not to just hook the air up and let it hit the bottom of the stroke? my theory has been at tdc you couldn't lose the valve if you lost the air, I'm not sure if you would anyway though.

 Is there a reason not to just let it hit the bottom and not worry about being tcd?

[dodge freak 2]

I do the old rope in the cylinder trick. Find a nice clean strong rope about a 1/4 wide and few feet long--making sure it has no snags at all. Turn the motor over until you see the piston going down a good amount, feed the rope in the spark plug hole until it won't easily take anymore and turn the crank back the other way to bring the piston up until it stops.

Now the rope is all cram up against the piston and valves holding the valves up to their seats. You can force loose sticking valve locks this way, even hitting the retainers with a hammer to break them free. The valves will stay forced up because the rope is holding them up.

You need someway to hold the valves up to their seats or how will you get the new springs lock on...if the valve drops a little ways, the spring will not compress enough to allow the keepers to get back on.    (yeah I guess new valve seals will hold them up too)

It works for me with no noisy air compressor running.

[Moparal]

I guess im old school. I always pull the heads.  Need to check the spring height open and closed. What if you need shims?

[Chryco Psycho]

I have used air & had the piston @ BDCwith no issues 

[Changin Gears]

I have used air & had the piston @ BDCwith no issues

I just did it this way yesterday.

[Aussie Challenger]

     If using air I always do it at BDC, the air will make sure the pistons is there and you can also check for valve leakage at the carby & tail pipe. ( I have seen a few bubbles of oil up the valve guide on one very worn head.)
  I have done it at TDC by removing spark plug and using a custom made hook that sat on the piston and held the valve up, this way pays to have a helper, crack the keepers seal on the collets before putting in the hook.