I'v got my engines tore down to replace cams, can I do a compression test with the engine torn appart or does it have to be assembled and sealed?

When you say torn apart do you mean heads are off? If you heads are still one you can still do a leakdown test. Better of the 2 tests. A compression test needs the cam installed to do correctly.

heads are on, I just want to check and see if I need to replace rings or not while the engines are out.

As long as the heads are on, it is no problem to run a leakdown test.

check454mag, I am assuming that rocker arms have to be installed and properly adjusted??????

nope,,, actually it is easier with them off,,, dont have to worry about getting the cam in the right spot to insure fully closed valve

To do a leakdown test you need to have both valves closed. You screw the leakdown tester in the sparkplug hole. The tester has 2 gauges on it. You have a pressure regulator & adjust the pressure on it. I set mine at 100lbs. You then look at the second gauge. If it shows 85 then you have 15% leakage. The older the motor gets or if it has a problem the number will get lower. Liisten to the intake or exhaust. If you feel/hear air then the valve is not seating & that is where some of the leakage is going.

I agree with TurboJack. But, I will add making sure the piston is on T.D.C. for the cylinder you are checking.

checkmate- I was told the same but I thought it was to make sure the valves would stay shut. Hopefully someone that may know another reason will chime in.

Hello, Guys,
The rings seal best at TDC when in good condition. The seal at the bottom of the bore is not usually as good. Also, when the pressure is applied, unless you are at top center or bottom center, the piston will try to go to bottom and could spin unexpectedly.