MS241C worn crankshaft prevention and fix

[Shibby]

I started a couple threads previously that my MS241C saws chain brakes were killing the engine due to the crankshaft being worn down allowing the clutch drum to bind when the brake is engaged. I wanted to share my fix for it since I think it'll help other people down the road.

Alternative fixes for chain brake stalling engine?

MS241C/261C chain brake request

A summary (from second link above):

My theory is, if there was a tab or if a tab can be added, it will prevent the chain brake band from pushing out in that spot which is what is causing the drum to twist and bind. This would prevent the need for a new crankshaft (which would eventually have the same problem), and stop it from wearing further when the brake is engaged. I've done a rudiment test that suggests this is the case.

Well I finally got around to finishing my fix. A replacement for the triangular plastic insert that serves virtually no purpose other than to "complete the circle", so the band is held in place and won't twist the drum. This fixed the issue on my worn down 241s, and have good reason to believe it will help prevent wearing the crankshaft down in the first place since it keeps the drum and drum bearing from torquing on the crankshaft while the brake is engaged.

First, a look at the problem (first attachment). The band pushed past the small plastic cover piece. If the band is pushed back to be behind the tabs (second attachment), it will bounce back to the outer position after a couple cycles of the chain brake.





The fix is to replace the small triangular plastic cover piece with something that can retain the brake band and keep it aligned. This keeps the drum aligned, which ultimately prevents binding on the clutch. I made a couple 3D printed prototypes to test fit, functionality, etc. These worked really well, except the plastic always got hot and slowly deformed until it failed.



So I used the model to cnc a final version out of 6061-T6 aluminum. You can see the shelf/lip I added to keep the band from popping out, compared to the original that has nothing and is just for aesthetics.





I've run this for a couple hours bucking some logs and so far so good. My saw is back to being fully functional, and I don't have to replace a [seemingly impossible to find replacement] crankshaft!

 

[SimonHS]

Thanks for posting this. Very interesting.

I picked up two 261CMs with worn cranks. They were cheap. Would you be willing to sell me two of these wedges please?

 

[Shibby]

If the 261 has the same design and little triangle cover on the corner it should work. I'm not finding any pictures though so I can't verify it's the same as a 241. Would you be able to post a picture of your 261? I can certainly make a few more for anyone that wants them for their machines.

 

[SimonHS]

Thanks. I have photos that I took when I got the first one. Looks like the design is different?

 

[Shibby]

Definitely different design. Looks like the same problem though, the band is probably twisting the drum and torquing the bearing on the crankshaft. The 261 design looks more structurally sound though, going all the way around the drum instead of just one corner of it. There could probably be something to gain designing an alternate cover that has better support for the brake band. 3D printed ones might even work better on that because there is better support/mounting screws, but unfortunately I don't have any to experiment with...

If anyone wants to send me a 261 I'd be willing to play around with a solution for it

 

[Moparmyway]

I just dont really ever use the brake on any of my saws ............

 

[SimonHS]

Definitely different design.

Shame. Your bolt-on solution would have been ideal. I will have to use them as is or consider putting AM cranks in them.