Idler gear shimming You need to check the clearance before you mount the engine. Here's how: Insert the idler gear with the thrust washers it came with into the gearbox. Bolt the end casing in place with the gasket. Tighten to the spec torque. Measure the idler gear float with a feeler gauge. It should be ~0.003". If it's more than that, and it always is, insert the appropriate OVERSIZE thrust washer. These are available from Mini Mania. I forget what the over-sizes are but they are stated in most parts manuals. Thanks to Johnny B I have a complete selection in stock for all my rebuild jobs :-) BTW, don't try to shim the thrust with brass shims. Hardened steel is the proper material for the application. Brass will disintegrate from the load, leaving you with not only too much idler gear movement, but also debris in the gearbox