How To Do A Cylinder Leak Down Test
Pull the plugs. Get or make an adapter from a spark plug. Drop a 16" wooden dowel rod into #1 cylinder so it is touching the piston. 16" may be too long, I am not sure. Just be sure that there is enough sticking out when at BDC so that it is still visible. You can shorten it as needed. Put a wrench on the harmonic balancer bolt and rotate the engine while someone watches the dowel rod. When the dowel reaches its highest point, stop and remove it. Put the adapter in the plug hole and connect shop air to it and pressurize the cylinder. If the air keeps running, then you are on the exhaust stroke. Remove the hose and adapter and rotate the engine again. The piston (and dowel rod) will go down to BDC and then rise again to TDC. This time it will be on the compression stroke. Now reconnect the adapter and air hose and pressurize the cylinder to 125 - 150 psi. Let it set for :15 min. or so while you watch the surge tank (cap removed) for bubbles. If it is not on a Northstar, then watch for bubbles in the open radiator neck. Repeat this procedure on all 8 cylinders. If bubbles appear, the head gasket is breached at that cylinder. If no bubbles appear from any cylinder, the head gaskets are good.