The was an article in "THE STAR" and it showed a picture of an aftermarket MB waterpump vs. a factory MB waterpump.
The factory pump had curved fins/fingers in the impellar to provide increased flow. The aftermarket did NOT.
Also, a rebuilt factory MB waterpump was only $89; came with a 12-month warranty; AND since purchased from the dealer, LABOR would be covered to replace the pump should it fail, and anything else that a failed pump could damage.
In short, sometimes it makes sense to go with factory MB parts such as: tranmissions, engines and computers. Almost anytime the LABOR and collateral damage could be very expensive.
1988 360TE AMG