DO NOT TAP the valves with the piston parked at TDC!
Ok, at the top of the compression stroke you can, but be careful that you do not tap a valve deep enough to smash into a piston.....
To do a LD test, the piston has to be at the top of the bore, and on the compression stroke so that both valves are closed.
If you have a solid-lifter cam, there has to be at least some lash evident.
When you inject the air, it likes to blow the piston down, so; the piston has to be almost exactly at TDC. Furthermore, the LD test is usually done at a low enough pressure (30psi) that you can control that pressure surge.
I inject 80psi because it gives me a reasonably accurate result; but
I installed a valve on my tester that allows me to bring it in slowly, so the dang piston stays at the top. And my tester has only one gauge on it so that I only have one error to deal with.
So, then I deadhead my gauge, and adjust the regulator so it reads 80psi. Then without moving anything, I open my valve to exhaust the gauge, then slowly bring the pressure back up. If the piston moves, I back it up and start over. If the piston stays, then the gauge will read less than 80psi, because some of it always leaks somewhere.
So then, if the gauge reads say 76 psi, then the pressure loss is;
(80 less 76)/80= 5%LD
Had you tested with the typical 30psi;
then 5% would be (1 less 5%) x30=28.5psi. But most of us would not be able to read the .5 psi on the stinking little gauge supplied. Maybe not even a full psi. So we might round it to something like 28 or 29 psi and call it done. But 28psi is ;
(30 less 28)/30=6.7% LD, and
29 is; (30 less 29)/30= 3.3% LD
Both numbers of which are way out to lunch, compared to the correct number of 5%.
So the bottom line is to always test with as much pressure as you dare,
BUT
Always make sure there is NO BAR on the balancer to slam thru the rad, or smash your hand...
I am comfortable with 80psi, which, on a 4" piston, is 1005 pounds of downward force;
that's a lot of force.
If you ever can't unscrew your balancer-bolt, now you know the secret.......
30psi is 377pounds of force, a relatively tame-looking number, in comparison. Don't let it fool you; you cannot hang on to the bar very long after it starts moving; so I highly recommend that you don't even try..
To avoid all the rig-maroll,
you can unscrew the rocker shafts, and do the LD with the pistons anywhere; the pressure will push them to the bottom, and then the LD number will stabilize. With the pistons at the bottom, you can rap the valves to your hearts content.
