Stihl 661 Chain And Sharpener File 2 in 1
Standard Stihl 661 chainsaw - (if you bought the standard model without changing the bar or sprocket):
The chain you need for a Stihl 661 with a 25 inch bar is: 33RS 84 (84 is the number of links).
The Stihl part number at the moment (February 2020) is: 3623 00X 0084 (where the X would appear to be the Country Code: 2 is Canada and 5 is USA) so for USA the part number is 36230050084.
The pitch of this chain is: 3/8 inch or 9.32 mm.
The track (gauge) of this chain is: .050 inch or 1.3 mm.
The Stihl 661 chainsaw comes standard (at time of writing) with a 25 " bar with a Stihl part number of (for me anyway) of : 30030008830. Look for the bar part number on the bar close to the motor.
To see more variations on the chains and bars (Stihl Part Numbers and configurations) or to verify the above, you can try checking here:
As at February 2020, the typical price for this chain would be about $25 USD (currently 2 listings on Ebay, and Amazon for this price).
Note - you can often get a better deal at the Stealership if you are buying 2 or 3 (or more) chains. At one place I called, if you bought 2 they gave you the 3rd free. Another place had 1 chain for $34 USD, but 2 for $48 (24$ each).
And what file is to be used to sharpen our Stihl 33RS 84 chain? The best Stihl file would appear to be the 2 in one file with the part number: 5605 750 4305. These are $35 USD at the stealership last I checked. Note also that this file works with specific models of saws only. If you have multiple saws, carefully check that this file is compatible with the others in your collection.
The Stihl chain, bar, sharpener equation is a mess - very difficult to get an answer to a simple question of what part, part number, etc. Hopefully this article helped you quickly figure it out the answer for the Stihl 661.