Spindle shaft rethread

Discussion in 'Mechanic and Repair' started by saw n mow, Aug 8, 2011.

  1. saw n mow

    saw n mow LawnSite Senior Member
    Messages: 273

    I have a spindle shaft from my ZTR mower (Bad Boy MZ48) that has threads that are messed up part way up inside the shaft. I’m not sure how they got messed up…I’ve always tried to be careful when threading the blade bolt in when mounting the mower blades, and tightening the bolt to the mower manufacturers recommended 90 to 110 ft/lbs. I tighten the bolts as much as I can by hand, then give them a quick zap (tap on the trigger) with the electric impact gun, then torque with a torque wrench. The bolts are 5/8”-18 fine thread.

    Is my ‘quick zap’ with my impact gun before torqueing the bolts causing damage to the fine threads every time I mount the blades? Should I be tightening with a socket and ratchet (before torqueing) instead of the ‘quick zap’ with the impact gun?

    The reason for my questions are that I want to rethread the spindle shaft. I don’t know if I should go with the original 5/8”-18 fine thread or go with a 5/8”-11 coarse thread. I’m sure some of you have experience in this area, so what say you.

  2. Restrorob

    Restrorob LawnSite Fanatic
    Messages: 11,029


    For safety reasons (others) replace the shaft and bolt, Use Antisieze compound on the bolt threads every other removal to reduce thread galling. Run the bolts in by hand to spec.....
  3. saw n mow

    saw n mow LawnSite Senior Member
    Messages: 273

    All good advice...thank ya. Antisieze compound will be on my list for my next trip to town. I have replaced the spindle shaft and bolt. I just wanted to have the old one for a spare.

  4. samjdmosher

    samjdmosher LawnSite Member
    Messages: 10

    I ran into a similar circumstance on my Hustler 72" rough cut deck only someone else had been in there before me. Luckily they had taken it from 1/2"-20 to 5/8"-18 but they didn't have a bolt the right length so they just added washers and a spacer on the bottom and tightened the bejesus out of it. I ended up welding a bigger nut on it and using a 4' cheater on my breaker bar to get it out. The new correct length bolt went in with anti-seize after thoroughly cleaning the threads.

    In your situation, for a backup, I'd go to the next bigger size altogether, 11/16" fine thread (not sure of the threads per inch). That way you'd get a clean hole to re-thread rather than just wallowing around in what used to be a 5/8" one.
  5. saw n mow

    saw n mow LawnSite Senior Member
    Messages: 273

    Thanks for that. Something to think over for sure.

  6. Patriot Services

    Patriot Services LawnSite Fanatic
    Messages: 14,506

    Typically the rules for rethreading is drill out to next size over. Low stress applications preferably. A spindle is designed with a certain wall thickness for safety reasons. Overboring is not a good idea. Fine thread also holds its torque more consistenly (think lug nuts). Repated torqueing and loosening also cause stretch and can lead to failure. I change mine every couple months. Then again I tend to OCD on mechanical matters.
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  7. saw n mow

    saw n mow LawnSite Senior Member
    Messages: 273

    Thanks. Good sensible info.


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