best bar length for 7900

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7900 is definitely better for milling than the 372, it has a lot more torque. Are you filing the normal 25° on the top plate, it will cut but for milling you want 10° on the top plate. I never could get great results with those Husky roller guides, and it's east to mount them wrong. I free hand file mostly, but for learning these files guides are simple, cheap and work well. These guides keep the file at the right height in the roots for normal cutting. https://www.stihlusa.com/products/accessories/chain-saws/fileguide/

I also prefer this depth gauge tool. https://amzn.to/2DbuU6l
 
Definitely some user error here on my part. Item 1 was the major problem. So three things to help anyone else that runs into similar issues:

1. If using 2x4 guide rails make sure they are straight . In my case they were not straight and me trying to keep the mill sliding flat on the guide rails was actually putting a bend force on the bar. As soon as I got everything straight, the saw chews through the wood at a reasonable rate. Slow and steady and about a tank of gas. 15-20 minutes for a 10' by 24" slab.

2. Get a tachometer and fix the idle on your 7900 if it's not idling per the manual spec. If it cuts out easily after revving, connect the tach and turn clockwise until it revs to 2500 rpm. Mine idles fine at 2350 and above in summer weather. Before that, mine was trying to idle at 2100.

3. If milling, sharpen the chain at 10 degree angle. I could not find a sharpener anywhere that would help maintain the right angle, so I made something simple out of scrap wood to keep the angle consistent. You can get yourself a protractor to measure the angle and cut a groove into some scrap wood and clamp to your bar to keep the angle right as you file. The file will gradually wear out the wood, but it should be good for a few sharpenings. In my case the angle change the my chain didn't affect the cut speed , but improved the smoothness of the cut a lot.

4. Make sure the saw is good and warm. With this saw, it should rev to 12300 when warm. When it's not warm it won't rev that high.
 
What is your saw model? Curious why it only goes to 12300rpm, I have a 7901, it is limited to 13,500 and I currently have it set to 13000
 

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