Pilot Bearing hole?

Just typing out loud here............

With the converter back maybe you could snake a small diameter spring, such as one used to rod out a coffee maker, into the bore to determine any real depth.

If your bent on seeing, and surely it will be dark in there, maybe a dental mirror and a mini flex light will do what you need.

http://www.ebay.com/itm/Steelman-10...-Flexible-Mini-Inspection-Light-/281341271477