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John, I'm not sure about the M pumps - I'm working on an H and they are all the same for that model. What I would do is check the clearance on each. Pay close attention to the following: 1 - gear backlash should probably be around 0.006 or so. You can still get NOS gears if needed. 2 - Side to side play in the shaft. This should be minimal. If you have play check the clearance between the the casting and gears; if it's out of spec look for a new pump. 3 - Check for end play in the shaft. Should be 0.005 or less. If its out, you can knock out the pin in the drive gear at the top, pull the gear with a puller and get some shims to place under the gear from someplace like McMaster and Carr to make up the play. If you had some play you will have some galling on the bottom end plate that will need to be taken out. Either grind it out yourself using grindind compound and a flat surface ot take it to a machine shop. Ive heard some people argue that thes pump is submersed in oil and so don't wear out, but that's not true. I think that many people believe this because those pumps were really over-engineered and will give good oil pressure even if they are worn. So I guess that the take-home message is, that the pump dosen't need to be perfect to do a respectable job.
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