Cog be slipping
 

ok so my friends cog on his masi keeps slipping...we've tightened it extremely tight every time...his lock ring is fine...we really dont know what it is...he weights 240 lbs he says its too much stress for the cog because he weighs alot...is this true? the tread is not stripped on the hub or on the cog or lock ring...any ideas or suggestions to fix this? thankss

Use this and it won't slip any more- rotafix http://204.73.203.34/fisso/eng/schpignone.htm
I would still use the lock ring as well.

you grease the threads on the cog and lockring first?

rotafix or a good chainwhip/lockring should be fine (seriously, i'm in love with the shimano track tool). hunker down on it with all your (or his) weight

I'd double check that there is an interference between the lockring and the cog. Tighten the cog, then the lockring, then try to unscrew the cog.

Also, double check the hub threads. Is it slipping while backpedalling/accellerating a half turn or so? Or is it slipping-slipping as in if you pedal hard, it slips forward every time?

i've had a problem with this, which seems simple but it didn't initially occur to me...

my cog was slipping a little, when i pedal hard forward, it would slip, then "lock" against the hub. when i would slow down/skid, it would slip backwards, and "lock" against the lockring. obviously i was like "well **** i need to tighten it down" so when i got home, i tried to tighten down the cog and lockring, but everything seemed snug. next day, same problem. then it occurred to me what was going on. i was coming to a stop before getting off my bike (duh) and in doing so, locking my cog against my lockring. neither wanted to budge under the force of my tools. so i got on the bike, took a good hard pedal forward (anticipating a slip). cog slips forward. got off the bike, then tightened the lockring. haven't had a problem since then.

don't underestimate the power of your legs over your arms!