Big journal crank has 3 radial bolts. Two secure the flywheel to the crank. The third bolt, pointing towards the big end journals also secures the flywheel, but in addition locates and secures a removable tube which acts as an additional trap. Looking through the hole, you think its nice and clear, but remove that third bolt and extract the tube. Some are reluctant to come out and require considerable violence, guile, threats etc to get out. More sludge will then be revealed.
Established wisdom is to replace that bolt with new and seal with Loctite. Measure the journals, wear limit before a regrind is 2 thou.
Swarfy.