Benz,

How long did you let the chain go (days, weeks, months?), when it was rattling?

When you drained the oil the last time, did any pieces or bits come out?

You really need to get that mechanical gauge in there. Why not give Earls or BAT (941 355-005, http://www.batinc.net/mocal), a call and see if they have an adaptor in stock.

Tinker