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What they did was fine, but the wear in an oil pump happens at the top and bottom of the 'pocket' the rotors sit in (not usually across the diameter of the rotors). When new, what we call the head clearance (the gap allowed between the rotors and one end of the pocket) is kept at about 0.0015 inch (min) to 0.002 inch (max) for best efficiency - less and the pump will sieze through lack of lubrication, more and the oil squeezes past the rotors, and pump efficiency drops, as it becomes a 'stirrer' not a pump. At the age our tractors are, there is likely to be 0.005 inch or more of head clearance, so the pump isn't doing as much good as it could do. When new, the Standard engine's oil pump should have shifted 5.5 UK gallons per minute at 2250 rpm - that's a lot of oil. As you have already taken the pump out, check this clearance, and if necessary ask the shop that rebuilt it to machine the pump body and or the cover plate to get the head clearance back to 0.0015 to 0.002 inch. Then re-check your oil pressure. I know that a few thou wear sounds like nothing to worry about, but it makes a huge difference to pump performance (and could save you splitting the tractor to do the main bearings). How do I know this? Because where I work now is where the oil pumps for the TE and 35 series Standard engines were designed and manufactured. Just changing the internal parts will not necessarily improve things - I still have to do my pump properly as described above, but then I still have 10 psi on the gauge after a day working hard.
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