Lower Unit Shims

[NB Boater]

I have been reading and watching videos and I would like to confirm that I understand correctly.
It is my understanding that the gears and bearings are manufactured to very tight tolerances, but the cast casings are not. The 3 numbers stamped under the anode are different for each unit and are casing specific indicating what shims will be required.
Does this mean that, if the lower unit was shimmed correctly and then had to be disassembled to replace a bent prop shaft, the shims can stay in place without adjustment? When, if ever do you need to re-shim a lower unit?

[panasonic]

If you busted the side out of the case, bad corrosion, cracks..ect that made you have to get a new gear case and the internal parts were all still serviceable THEN you would possibly have to reshim.

I would reassemble with the old shims and check backlash and any specific clearances or dimensions that are called out in service manual and go from there. If that is what you are up to.

[boscoe99]

Although manufactured to tight tolerances, there can always be variances.

New gear case with existing gears, or new gears with an existing gear case, measurements should always be checked.

Yamaha service manuals have procedures and formulas for assisting in selecting shims but you always want to "trust but verify". By using feeler gauges, shimming gauges or a dial indicator to measure backlash. Just follow the applicable service manual.