An 18" or 20" bar would be ideal on a Stihl 041
http://store.chainsawr.com/
A chainsaw can be measured by Engine size or Bar length. Both are important to the weight and cutting applications
20 in. Bar
you have to see what size the bar is.
The longest recommended was 31 inches. http://store.chainsawr.com/
A STIHL 045 av is 75cc and the max recommended bar size for it is 32". However... If you have a 045 av Super then it would be 87cc and the max recommended bar length would be 36". So it depends if you have a SUPER or not. It is easy to tell as it will say SUPER on the top tag right below 045 av.
First remove the clutch cover to expose the clutch, bar studs and area of the tensioner. Next you need to remove the bar studs. To do this tighten two nuts agains each other on the bar stud, then put a wrench on the lower nut and use it to loosen the whole stud. Repeat for the 2nd bar stud. Sometimes if you are lucky you can use vice grips on the shoulder of the stud to unscrew them, be careful not to damage the threads. After removing the bar studs, the inner bar plate and tensioner assembly lift right out. http://store.chainsawr.com/
any quality bar oil will work fine.
I have an 034AV Super and it came stock with a 20" bar. An 034 isn't quite as powerful (56.5cc vs 61.5cc), but a 20" bar should still work great on it.
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A gun sight on a chainsaw is a mark across the top and side of the chainsaw case. This mark is made perpendicular to the saw bar and on some saws is molded on the case. This can be characterized as the letter "T". The top of the T is your mark with the vertical line of the T being the bar of the saw.