Thorp Air Command - T18.net

That is pretty common among mechanical tachs. It is also why good prop balancing systems use their own calibrated tach and why it isn’t recommended to set up new or overhauled engines using the aircraft instruments. As long as it is consistent it is pretty easy to live with. One aircraft I don’t own but fly quite a bit is pretty accurate on cool days but off by as much as 200 rpm on hot days. I use manifold and fuel pressures to set rpm on that one and verify with a “true tach”, if I remember to grab it.

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Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

The type certificate for the O-320 lists max continuous power at 2700. (Depending on the suffix, some models are as low as 2450.) Dynamometer curves in the owner's manual go to 2800. I didn't notice a specific "red line" number in either document.

You won’t. RPM “limits” are set in the aircraft TC but you would be hard pressed to find one that exceeds the “max continuous” listed in the engine documents.

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Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson