Over-bored ring gear end caps...

Over-boring ring gear / end caps, in an 18 Automatic or a D-180 to accommodate the larger diameter bolts may, seam like a worthwhile investment of one's time; if you already have to open up the transaxle for some other reason; however, it can prove to be a bit of a risk. I personally found out after I opened up a D-180 transaxle case.
The first image is two D-180 end caps the one on the right is broke. The second picture shows a later model end cap that was designed for the larger bolts compared to the earlier style. The third image focuses on the size differentiation of the holes. The forth is the result of over-boring. If an upgrade is required I suggest to replace the smaller bored ring gear and end caps with the later parts.

Good thing to know, thanks for the info Mark.