Just a follow up post to this thread. Since getting this grinder up and running, I've had a negging surface finish issue that I haven't been able to track down, until now. It's always had at least a hint of a "herringbone" type surface finish, and it would really jump out after hitting it with a flat stone. Drove me nuts.
Pulled the spindle, removed the motor( making note of the orientation of the Lovejoy coupler as its all balanced from the factory ) and found the rubber spider in the Lovejoy was mostly gone and the consistency of modeling clay! Found a NOS replacement on Ebay, and put it in this evening and did a test grind on a small block of 4140. Comparing it to a previous grind on a similar block with the same wheel looks promising. The spindle is now virtually silent, and taking a cut of a couple tenths up a half thou, I can't even feel it, and the sound level of the grind is way less than before. With the bad coupler, I could noticeably feel when the wheel was in the cut, and it was pretty loud. Basically with all that slop in the coupler, the wheel was being driven by an impact driver of sorts. There was just enough of the rubber left so there was never any metal on metal contact of the two halves of the Lovejoy. Pretty relieved to finally track down this last piece of the puzzle on this grinder.
Here's what I found when I separated the motor from the spindle. This has to be the original coupler/spider, as the relief grooves in the spindle only had a single, very crisp set of witness marks from the five set screws that secure it. This area of the spindle is dry, and there was no evidence of any chemicals or lubricant in there, that could have attacked the rubber spider( or urethane, or whatever it was made of ). I guess just old age. It was clay like, and smeared and flung all over the cavity its in.
Here's the NOS replacement spider I fond on Ebay. Same manufacture/part no as the original. Fit right up.
Before/after test blocks of 4140. Same 5SG46-J wheel, and both given a fresh stoning before the photo. In addition to the prominent lines on the before block on the left, it has ripple/chatter marks running perpendicular, that aren't showing up well in the photo. The appearance of the after block were unchanged after stoning. Pretty relieved to finally track this down.