That is a collection point for unburnt oil vapors and such. You are suppose to clean that out periodiacally as all kinds of nasty stuff will gather in there and it will get quite gooey. Also make sure to clean it out when ever it becomes submerged in water in case water and mud gets in there.
Nicholson 500x (Project)
Nicholson 185s
Nicholson 110 (That takes an atc70 tank)
Ascott 500 head/350x (Project)
Homebuilt Racer Chromoly Suspension 110 (Ragin Runt)
PK Racing Suspension 70
A+ Inc Suspension 70
AWS Aluminum 90 frame
Hi Performance ATC Suspension frame rd350
Hi Performance ATC Suspension frame 90