It is absolutely essential that the cylinder is on the compression stroke when performing engine timing or compression tests. Failure to get this step right will cause maintenance nightmares for you! 

That's where this handy little tool comes in. It helps you detect the compression stroke in a completely foolproof way. Just remove one of the spark plug from the #1 cylinder, insert this tool, then rotate the prop and listen for the whistle. That is your indication that the piston is on the upstroke of its compression cycle and ready for maintenance. Comes complete with adapters for both aircraft engines (18mm) and automotive engines (14mm).