Just got a 377 with a rim sprocket so I see what it is, but why is it better?
In normal use, the chain will wear grooves in the sprocket so it has to be replaced. This happens long before the clutch is worn out, but with a spur sprocket, it is part of the clutch and the assembly has to be replaced. With a rim sprocket, you can wear out 3 or more rims before the clutch needs replacing. You can also get different amount of teeth on a rim that will fit a standard (is there any such thing?) clutch, so you can change out the rim for different performance.
When the sprocket is worn out, do you want to pay $5 or $15? If you are a total cheapskate like me, that is an easy question. I had found a place that made a 2 piece set-up for my Super XL. That one is next. Just have to find the site again.
Changing out a rim sprocket is also a lot easier and a lot less work than a complete clutch drum.
I changed my Stihl out to a rim sprocket just cause Bailey's had one and it saved a trip to the dealer. Didn't even think or know about the other advantages. Cool. I guess I'll get a spare sprocket the next time I order from them.
An example ,about the rim sprockets .I can take my 048,run it with an 8 tooth,on a 20" bar,for general usage.In the matter of several minutes,I can change it to a 7 tooth,and a 32" bar,for large cuts.Besides all that,the rims are cheap,about $3 ,on sale.As far as the drum portion wearing out,I've never seen it happen.I have several 125 Macs,that were used in logging service,in Oregon.I don't see much wear on these drums,in spite of the hard usage .
Rim sprockets, aside form the aforementioned advantage of easy and cheap gear changes, also support the chain better lessening wear to it`s straps. Wear to the straps carries over to all other cutting components and shortens their lives.
Russ
OK ... so we can tell when the inside of a rim sprocket is worn (the little warning gooves on the outside of the rim get worn down, there is side wear & tooth wear inside the sprocket, etc.)
but (a) how do you tell if the "inside" bore of a rim sprocket is worn, e.g. the rim slides too loosely onto the splined-hub of the clutch drum, so that the sprocket can "wobble* or rock side-to-side, e.g. the inside spline of the rim no longer fits the outside hub of the drum, so the sprocket "cocks" to one side or the other, and, when the chain is under tension such as in a cut, the tension pulls the spinning sprocket to a skewed position, which then twists the chain as it comes around the sprocket and heads for the bar groove, which then has to twist the drivers back upright again, wearing out the driver tips and bar groove from the inside?
that's probably the same question as, how do you know when the spline hub on the clutch drum is worn down?
and (b) how do you know when the clutch bearing surface of a clutch drum is worn?
It's occuring to me a new bar also deserves a new clutch hub, to keep the rim sprockets sitting up straight, and the chain sitting up straight. No sense to wearing out a new bar with a cocked sprocket on a worn drum.
A properly tensioned will keep that rim sprocket lined up with the splines worn to the point that the sprocket spins on them. That tells you when it is worn to far, when the motor makes the right noises but the chain don't move. :D :D