Since a lot of you guys work on your equipment here, and since the mower engines are so simple, I figure most you would appreciate this tool. A store bought leak down tester will run $100+, but you can build your own for much cheaper. First off, you need to get a pressure regulator. This one from Sears (part number 282-16025) is the same one that Snap-On uses on their leakdown tester. It even comes with the gauge on it already. Attach a male Quick Connect fitting to the inlet side of the regulator and a female quick connect fitting on the outlet side of the regulator. Now all you need to do is make a small hose that adapts from a male quick connect to spark plug threads. A easy way to do this is to butcher up a old (or cheap) compression tester to use its line. Or you could have any store that makes hydraulic lines make you one. If you do use a compression tester hose, you need to remove the check valve out of the fitting. To use it, attach your air compressor to the inlet of the regulator and adjust the regulator until the gauge reads 100 PSI, this is your 0% leakage indicator. Make sure the cylinder is at TDC of the compression stroke so all the valves are closed and attach the leakdown tester to the cylinder head. All engines leak somewhat, so even on a healthy engine, the gauge will drop down to 90 PSI (10% leakage) or so. If it drops a large amount, then you have a leak (from chipped valves or a bad headgasket etc). If it leaks significantly, you can feel the air coming out of the intake or exhaust manifold, indicating which set of valves is damaged. If the headgasket is leaking, you can sometimes feel it between the head and the block. Anyways, have fun building a tool for $20 that is pretty much the same as the several hundred dollar Snap-On equivalent.