Which way do the mower blades mount, on a Craftsman DT4000, 19HP Mower 42" deck?

Sounds like the blade mounting bracket mount in engine shaft no longer has wodreff key.

Terry, Deck full of debris around belt tensioner not allowing it to release, tension arm seized/spring broke, wrong/small belt installed.
Check the attached links,instruction and guides, Good luck
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CRAFTSMAN 42 MOWER REPAIR
My mower deck will not disengage
Blade will not disengage Craftsman 917 270662
https://answers.yahoo.com/question/index?qid=20090407155731AACRvsq
Sears Craftsman 917 272481 LTX1000 Riding Mower Deck Belt Blade...

I HAVE A CRAFTSMAN 18 H.P. 3 BLADE 46" MOWER DECK.
1. THE PLASTIC PULLEY COVERS ON BOTH SIDES OF MOWER...WILL TRAP STICKS AND DEBRIS WHICH VERY RAPIDLY WEARS THOSE BELTS..I REMOVED MINE
IF IT IS RUBBING:
1. EITHER THE BELT HAS STRETCHED-REPLACE
2.THE SPRING MOUNT (WELDED TO DECK) GOT BENT...STRAIGHTEN

just replaced some pulleys on my craftsman 42 inch cut and now the blades are ingages all the time, but it will start

Are your blades upside down? Mower deck level?Proper air pressure all 4 tires?Correct blades for the diameter of cut? Deck bent? Deck hangers bent?

Here's a link I found Doug on your belts. https://www.shopyourway.com/questions/1126482

Download the product manual from the Sears website. Murray manufactured these tractors so you can download the manual from the Murray website also. Procedure for replacing the mower deck belt is quite simple. No need to remove the deck. Spring location also on diagrams. Mower deck parts diagram breaks down the whole assembly. Easier way to remove pulleys is using an impact wrench, otherwise hold the blade while removig the nut that holds the pulley in place. Use a heavy glove to hold the blade for safety!

You have to post the model number from under seat. Not all Craftsman decks are the same. There are 38", 42", 46" w/3 blades, 46" w/ 2 blades, 52", etc.. Better yet, Get the model number under the seat (starts with 917) and go to... http://www.searspartsdirect.com/partsdirect/index.action Then enter the model #, hit search, then scroll down to Browse Subcomponent List and click on Mower Deck. There's your diagram.

maybe the safty switch under seat might be starting to go out if that switch doesn't work the blades wont come on at all for safety

The first thing to check is your blades. If one is bent, it will never cut even. It happens a lot hitting roots, antbeds, etc. Mulching blades will bend easier than a straight blade. It's best to take them off and lay one on top of the other to see if they're bent.