I've recently installed a Shimano ES-51 BB on an old early 90s Specialed Crossroads Cromoly frame. The shell is 68mm. My concern is that the right side (non-drive) plastic adapter, fully tightened, is not flush with the shell rim. Does anyone know how this should fit? I can fit a fingernail between the shell and the adapter flange. There doesn't seem to be any play in the crankset, but of course, I haven't ridden it yet.
I've inspected a couple other bikes with what appear to be Shimano BBs, and the plastic adapter on those all seemed to be seated much deeper, almost flush with the shell.
There was light rust on the threads after I removed the old BB, but I brushed it and cleaned it up. However, at the joint between the chainstay and shell, there appeared to be a small area of raised corrosion that I couldn't remove. It's possible the adapter is stopping at this point.
I really haven't been able to find much about proper fit (even on the Shimano website, which gives rather brief instructions for installation.) Am I concerned about nothing? Is it likely to be tight enough as it is?