[roadnsky]

Here's a post from Don back in 2010 about this very issue...


We receive a handful of reports each year in which an older Oberdorfer impeller has swelled to a size considerably larger than the dimensions of a new impeller. If an impeller is kept in service until it enlarges significantly beyond its original size (particularly in width), it will cause accelerated wear on the inside of the impeller chamber. In some few cases, assuming the impeller itself survives, wear within the impeller chamber will so closely match the aging/swelling impeller that the pump will continue to function longer than expected. Unfortunately, these pumps will usually not accept a new (properly sized) impeller.

New Oberdorfer impellers measure 2” in diameter, and 7/8” across the width. If your pump will not work with a new impeller with these nominal dimensions, you have several options:

1) Reinstall the old impeller and continue to use the pump (not really recommended except as an emergency measure to get to a safe port).
2) If you have an older Oberdorfer M3 series pump (usually with big raised letters on the back plate), you can try replacing its cam shoe with a new M7 shoe (product number - CSOB_08_69 in our online catalog at moyermarine.com). The M7 cam shoe is twice as thick as the M3 (1/8” versus 1/16”). The increased performance of the M7 shoe might allow the pump to continue working, at least as an emergency spare.
3) If wear within the impeller chamber shows up primarily as excessive depth, you might be able to work enough metal off the rear face of the housing (using sand paper on a flat surface) until seeing a small amount of “crush” on the new impeller when installing the back plate. In these cases, you will probably also have to reface the back plate using the sand paper on a flat surface to get rid of any grooves.
4) Replace the pump and begin changing the impeller after every second season or after 200 hours whichever comes first (after every third season or 300 hours for freshwater cooled engines)