Water pump package number 556283 available from WDDGM, now contains a pump with a sheet metal impellor. This pump is also being used on production engines.
Dealer Technical Bulletin 81-T-40 defined criteria for the cast iron impellor. It must have square vanes and .045 inch clearance. It is no longer necessary to be concerned about this if a new pump with the sheet metal impellor is used.
General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.