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Replacing a lawn mower blade

I can't seem to get my new lawnmower blade on correct. The lawnmower is a lawnboy 20". I bought the correct blade but after installing, doesn't cut grass. What is the correct way? I did it as the picture on the box showed but still doesn't cut correctly. The machine is low enough to the grass. There should be more than what I find in the grass bag. What am I doing wrong ???

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Posted 2012-04-16T22:38:47+0000  by bobco908 bobco908
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Best Answer

Posted 2012-04-20T16:38:38+0000  by Travis_HD_ATL

Hello bobco908, 


Welcome to the community!  I do have a couple of questions.  What model Lawnboy do you have and what model blade did you purchase?  My first thought is that the blade might be upside down.  Do you have your manual?  If so, double check the installation instructions. 


Once you get back to us with the additional information, we can better assist you.

Posted 2012-04-17T12:17:16+0000  by Jen_HD_BOS

Hi bobco908,


I agree with Grow2girl about the blade being upside down. That is the most likely culprit since the blade can be installed that way and it won't be obvious until you attempt to cut the grass.

Here is a schematic of the LawnBoy blade configuration: Lawn Boy Blade schematic.gif


Notice the leading edge is the relatively straight part of the blade and the notched part is the "lift wing" to create air lift to move the clippings efficiently. Your mower may or may not have the extra support below the blade as shown here, but if if you do have it, be sure to re-install it.


As you observe the mower from the top, with the wheels on the ground, the blade rotates clock wise and the straight, sharp edge of the blade is the leading edge with the wing facing upward, toward the engine.


Here is a handy link for this procedure:




Posted 2012-04-17T12:59:14+0000  by Travis_HD_ATL
model lawnboy 10640 20 inch mulching rear bagger
Posted 2012-04-18T21:57:44+0000  by bobco908

Hello again bobco908,


Did you purchase blade model number 112-8841-03?  That is the blade for a 2009/2010 Lawnboy 10640.


I did have a chance to speak to a Lawnboy Tech and he suggested the following:

  1. The blade may be upside down.
  2. When mower is not running, check to see if the deck is clean underneath.
  3. When the bag is in place, check to see that the reflector isn't blocking grass from coming into the bag.

Good luck and let us know how you make out.



Posted 2012-04-19T15:35:44+0000  by Jen_HD_BOS

I'm looking at the attached demo and it seems to me that if the blade spins clockwise. As I look at the picture if that is true then the notched side of the blade should be the leading edge. If that's correct then my blade is on backward. I have the straight side of the blade as the leading edge.

Posted 2012-04-19T17:21:21+0000  by bobco908

Hi bobco,


You are looking at the demo correctly. The cutting edge is not notched. As I said in my previous responce, "Notice the leading edge is the relatively straight part of the blade and the notched part is the "lift wing" to create air lift to move the clippings efficiently."


You have it installed correctly if the smooth edge is leading as the blade rotates clockwise.


I have seen some of these mowers with the rear door closed onto the discharge shute and the bag hanging behind the door. It appears normal, but the door should be open to collect the clippings. If the black plastic door is closed, open it fully and then re-install the bag to allow the clippings to enter the bag.




Posted 2012-04-20T12:18:50+0000  by Travis_HD_ATL

Thanks for all your help. The bottom line is that my blade must be on backwards....I will reverse the blade and see what happens. It can't be any worse...Stay tuned !!!! If it doesn't work I'll get back to you.

Posted 2012-04-20T16:37:07+0000  by bobco908

After removing the blade I found that it can only go on one way. The blade doesn't fit any other way. I have to admit that I am the one who made the mistakes. I'm embarrassed to say that the blade was on correctly..The lawn wasn't long enough to cut. This past weekend my lawn needed cutting, it was much taller then last week. The blade did the job it was suppose to. It was operator error all along. Thanks for your help.

Posted 2012-04-24T20:17:41+0000  by bobco908
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