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During the steam engine era you wouldn't discuss steam engines very long with a steam engine machinist, especially a railroad machinist until he would mention, 'valve blow' and 'ring blow.' In plainer English it would mean leakage through the valve or the piston.

It is very easy to test a simple traction or stationary engine for leakage through the valve. Turn the crankshaft until the valve is at or near the center of its stroke. At this point the admission and exhaust ports will be closed. Close the steam-chest drain and open both cylinder drains. Open the throttle. If steam blows at either cylinder drain or at the exhaust nozzle, it will mean leakage through the valve. A good slide valve will often show no leak at all. Piston valves will nearly always show some leakage. Slight leaks will not amount to much but of course a big leak will mean poor performance and should be repaired.

It is a little more trouble to test for 'ring blow.' One old engineer who was considered an authority says: 'Turn the engine so that the crank is in the first quarter of either stroke, so that full pressure may be applied, block the cross-head so that the piston cannot move, disconnect the cylinder drain push-rod so that the drains can be opened independently of each other, open the drain on the end that should not be taking steam, then open the throttle. If steam shows, of course it must be blowing through the piston rings.'

There is one objection to this method. If the block is not carefully fitted to the cross-head it can slip and cause damage. In testing a traction engine an easier procedure can be followed. Block both drive wheels with a good piece of wood in front and back. If good pieces of wood are not at hand, dig a couple of holes about six or eight inches deep and back the engine into them. Do not dig deep enough to cause difficulty getting out, but enough so that considerable pressure can be applied. Engage the clutch at a point where the slack in the gears will be taken up with the crank position to give full pressure. Open the throttle enough to give considerable pressure, but not enough to pull over the blocks. In this way you can find out if steam is leaking through the rings. It is not often that you will find an engine that will not show a little leakage through the piston, but a big leak should be considered a problem and should not be tolerated.