OK, post the 917. ???????? model number of the tractor and we can check to see if maybe you have "stacked" the parts back in the wrong sequence or something else. ***" I have changed my blades numerous times before"*** Changed as in "took them off to sharpen"? or as in "replaced with new blades"? If new, did you install the same blades this time that you used before?

If the star on the spindle seems to be in good shape, maybe the new blades are a touch thinner. I've seen that before. You can zip a touch off the star to reduce the thickness by a few thousandths. Sliding around would indicate that the star on the spindle is damaged. If the star is trashed, then a new spindle or shaft is needed if you can't clean if up enough to accept the blade. When installing the blade, I think you would do better holding the blade itself -either by hand or a block of wood.

It is a 917.273140 about 4-5 years old. Whenever I "change" the blades I am installing new ones. The last time I took the deck off and replaced the blades with the deck off. The manager of a local Sears store said that I need to install new blades while using a ramp, with the deck in tact and the mower blades engaged. He said if the mower blades are not engaged when you install them they will not install properly. I have never heard this before.

Well, something got to be different. I would first mic (using some type of measuring device that's +/- .001) the old blade thickness vs. the new blade thickness. Same I would now suspect that the bolt (you are using the old bolts right?)in not going far enough up into the spindle to sandwish the blades tight up against the spindle. Some new blades require spacers or washers especially if their is a difference in thickness?

These small diameter fine threaded bolts used on most craftmans will not take alot of humanly possible torque. you may of had dirty spindle thread holes or damaged threads which should of showed up when you removed them by one having to crank them all the way out or two by shinny roled threads on the bolt shanks?

You should of be able to break the mount bolts loose spin them out by hand, cleaned up the spindle theads and hole, star area, and bolt. install the new blades feeling them align with the star pattern on the spindle, hold the blade and install the bolt and run it down by hand till is seats against the blade, now all that is left is the one full turn or so when you torque the bolt to spec# (did I do good lurken).

Rarely knew a manager to be mechanic nor did they look like mechanic's, I suspect but will give some the benifit of the dought, that they get their information (about mechanically things) from hear say??

I don't think being engaged (with the belt) would make much difference when trying to hold the blade while torquing it to spec# with the deck on or off. IMO off would be much easier

This is from the owner's manual for that tractor:

1. Raise mower to highest position to allow
access to blades.
2. Remove blade bolt, lock washer and
flat washer securing blade.
3. Install new or resharpened blade
with trailing edge up towards deck as
shown.
IMPORTANT: To ensure proper assembly,
center hole in blade must align with star
on mandrel assembly.
4. Reassemble blade bolt, lock washer
and flat washer in exact order as
shown.
5. Tighten blade bolt securely (27-35 Ft.
Lbs. torque).
IMPORTANT: Blade bolt is grade 8 heat
treated.
>

Pay close attention to step #4, ensuring all components are assembled in perscribed order and then torqued to spec.

If you don't have a manual, take your 917.xxxx model number to sears parts webpage and download a copy.

I have to agree rcmoser on his suspicion of thinner blades.
It is possible that the mandrel has worn enough to let a standard blade not receive enough "crush" when the bolt is tightened down on the flat washer (which clamps against the blade). I also agree that if you want to take the deck off to do this blade R&R go ahead. Just don't let the pulleys get damaged from whatever surface you are working on. And I suggest a piece of 2 X 4 lumber (16" long or so) to block the tip of a blade against the deck edge wall instead of holding a mandrel with vise grips. Let's see if you can determine if the new blades are too thin, or if the mandrel has become worn. Do you typically replace blades due to heavy wear, or because you have maybe "hit something" and damaged a blade? Is this machine in commercial duty?

Here is a link that might be useful: 917.273140 OM

I think more harm than good is done to instruct someone to install these blades w/o pulling the deck. I see many of these stars on the mandrel shaft mushroomed from misalignment when being installed. You can't see that the stars align for 1 and have to do it by feel and 2 it is so much easier to clean below and above the deck by pulling 6 or 7 pins.

AMEN TP (are we allowed to say that?)

Also pay attention to Number 5. Use a torque wrench, then you KNOW it is tight.