Purchased this machine last year and am now the third owner. Came with two pony motors as I was told the one in the machine was not functioning and that the second pony was on a skid to replace it. Unfortunately I assumed wrong and have now broke both motors and am in the process of rebuilding one out of the two. Considering the electric conversion as well. My question is this: with the pony motor out and stripped down, it appears that the bottom of the crankcase has had the casting knocked out on both engines in order to allow oil to be shared? I have looked at several ponys that came out of D4 units and they do not have this other opening on the bottom of the block. I thought there should only be the one for the coolant and the oil in the pony was isolated as a stand alone. Anyone able to tell me if the coolant and the oil should have passages on a D6? 

The D6 Pony is different. The floor of the pony motor is actually the top of the flywheel housing and the oil drain is a passage through the flywheel housing.

All the original manuals I have indicate that the starting motor is splash lubricated and is a stand alone system, not connected to the D6 in any way.

So am I to understand that the hole is ONLY for and somehow connected to a drain plug?
I am contemplating brazing the hole shut and when an oil change is needed I can use suction to remove the old oil in the starting motor.

If you look at the front right side of the flywheel housing you should see a short pipe about three to four inches long with a cap on it. That is the drain for the pilot motor's engine oil. They do not share oil with the main. I don't think there is anything wrong with your pilot engine blocks.

I appreciate all the feedback. The starting motor has been the main issue since I acquired "Bessie". She was always a reliable machine for the previous owner and I was lucky enough to acquire her when my friend passed a few years back. We pull started her to get her loaded up and let her idle for the hour ride home. Purred like a big cat kitten. One of the pony blocks has a broken skirt on the bore and cannot be used unless I get it sleeved. It was the block I had wanted to use since the bore was untouched since new. The other block is already been bored .020 over and finding pistons and rings have become a challenge to say the least. I have two good pistons that can be used but they are standard bore. The oil issue with the pony is what caught my attention because it appears both ponys failed due to lack of lubrication. I should have NEVER tried to fire them without going through them. Some learn the hard way. She will come together eventually and I will get her out and about on the back forty to "play". If anyone knows of any sources for the pistons and rings I would be grateful for the lead.
Will keep the site posted on my progress. Thanks again

jp pistons make em what size you require. i have ordered a custom piston off them for a bsa before. came complete with new rings,gudgeon pin and retainers.