Definition - What does Sub Shim mean?
Sub shims are thin pieces of metal installed in drill strings fitted with a bent sub to ensure the correct alignment of jet nozzles. They are installed on the threads connecting the bit to the bent sub and align the jetting assembly on the bit in the desired direction. Correct alignment guarantees optimum control in directional capabilities while drilling.
Trenchlesspedia explains Sub Shim
Bent sub assemblies are used when drilling to either correct the direction due to a deflection, or intentionally to produce a bore that is not lateral. Bent subs create lift in the drilling. Sub shims aid in this endeavor by providing proper alignment of the jet nozzles on the drill bit and in some cases blanking out one or more nozzles on the drill bit. If this is done, the jetting assembly on the "high side" of the bit will be left open to assist in the desired directional change. If the bit has only a single, centered nozzle then shims are not needed.