We used a #5 mower to cut hay for several years and it worked very good.
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We use it now to mow around trees and a few other areas and it works just fine.
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Most of the "universal" parts, such as pitmans, knives, sections, rivets, are available at Tractor Supply.
There are several folks on the YT forum that also have OEM new and used parts.
Be sure to get an operators manual for your mower.
The manual is a great help in setting up the mower to cut/mow at its best.
The key is to "register" the knife correctly.
We've got several #5 mowers that are "working" mowers and one just for "show".
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You can spend a lot of time and money on these old mowers, but if they serve your purpose then its time and money well spent.
Hope this helps and enjoy your <a href="http://youtu.be/rnH8n4KOYoU">John Deere #5 sickle mower</a>.