General rule of thumb is to take the break in shim out after you loose most of the metal pinch and the motor picks up in performance ( 1.5 - 2 gal). It is designed as a ¨break in¨shim to lower the compression and make it easier to start and run the motor when it is tight from new. Also takes a bit of strain off the con rod during this time. The motors are designed to run at the compression given with the original shims. For safety you should also replace the con rod after 2 gal due to the excessive strains placed on it during break in. They have been known to fail when the motor really starts to rev out later in it´s life. Cheap insurance, then you can give it death and not worry