Maybe to oil the outer wall and keep it cool?

Frank/Rebop

Bristol, In ( by Elkhart) 

I took two old, original and presumably untouched gerotor pumps apart and both had the slotted outer rotor.
Both Melling repair kits I bought two weeks ago (and rejected because of gouges) had no slot.
I found a rebuilt pump on Ebay that I took apart to verify clearances, and it had a slotted outer rotor, so that's the one I used.
It seems like it would function a small reservoir of oil that rotates around the circumference of the body to keep it lubed (?).
Probably make little difference.  My thought was that it's cheaper to manufacture them without the slot, so that's how they do it, though it 
raises the question:  Are they still really manufacturing these kits?

That outer slot in the gerotor set baffles me too.  The next time I talk to Verne Schumann, I’ll ask him about that slot and what his thoughts are on it.  As he’s in the oil pump manufacturing business, he’ll come closer to knowing the right answer than most.

Lorena, Texas (South of Waco)

So, What about that slot?


"FREE SAMPLE"

Steve.  I feel your pain with the noise generated from some of those spur gear oil pumps.  The one on my ’55 Customline has a nice clicking noise at idle and if not knowing what it was, I’d be concerned.  As it is, the engine has over a quarter of a million miles on it now and I’ve learned to live with it.  But when the time comes, I will change it over to a gerotor style of oil pump.

Lorena, Texas (South of Waco)

The pump is back together now, and the pan has no leak at all. I used a Best gasket and a sealer called "Dirko" which I used on the rebuild of a VW motor I just finished. It's a very good product, and the tube even comes with a small plastic tool that fits on the end that allows one to roll the tube up very tightly from the end. I coated the bottom of the block, and the pan rail with a thin coat. I put thread sealer on the main retainer studs too. 
The oil pressure with my old gear pump using 15w-40 Rotella was 70 psi cold and 55-60 psi hot at 3,000 rpm, and was a noisy 20-25 psi hot at 700 rpm. With 10w-40 Bradd Penn oil the rotor pump puts out 60 psi cold and 50 psi hot at 3,000 rpm and about 15-20 psi hot at 700 rpm. The pump is quiet.

FWIW there is a permatex product called "the right stuff" that works better than ANY product I have ever used on pan gaskets.

A difference exists between the original outer rotor and the new Melling in that the new one lacks the cut-out in the outside surface.
Is the intent for this slot to allow an oil supply to be available between the inside surface of the pump body and the rotor?

Ted-
Exactly my thought.  I checked the contents in the store (Autozone !), and decided to hang on to this kit until I see the second one I ordered.
Hopefully the second one will contain all parts in new condition.  I guess I could pick the best bits for the two sets, and then return the bad set as "defective".
As you imply though, there is a high likelihood that the kit containing the damaged parts will simply be put right back into circulation.

Steve.  Not sure where you got your Melling K-56 kit from but you’re running into a problem that’s becoming more frequent as time goes on.  And that problem is the old parts being turned back into the stores in their new boxes by unscrupulous buyers who are simply turning the old parts back in for refund or credit as new parts not used.  The contents within those boxes are obviously not being checked by the stores when being received back which has those used parts going back into stock and hence, being resold as new parts.  It has become necessary to simply check many of the items as soon as they are received to insure that the new parts are indeed new.

Lorena, Texas (South of Waco)