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Question for a machinist...

Discussion in 'Sound Off (MichiganForums.com)' started by jpollman, Jul 10, 2009.

  1. jpollman

    jpollman Premium Member

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    Hi all,

    I've been doing some work on my tractor and encountered a problem. I had to remove the mandrel/spindle assemblies from my mower deck. One came out fine but I broke off one of the three mounting bolts on the second one. It broke off so close to the top of the casting that I couldn't get a pair of vise grips on it. So I went to the grinder and ground it down nice and flat so that I could drill a hole down the center and use an easy out. Well I got the remainder of the bolt out but it WASN'T easy. ;)

    In the process though I ended up destroying the threads from the original hole. It's an aluminum casting. I have some Durafix aluminum repair rods that I used to repair an aluminum engine block casting last fall and they work great. I was able to fill the hole and get it ground down and cleaned up. I figured I could drill a new hole and tap it. The bolts are 5/16-18 bolts. If I pick up a 5/16-18 tap, would I drill a 5/16 hole? Or is that not correct? Maybe the tap would have directions on the size hole to drill but I just wondered if anyone here might know.

    Any input would be greatly appreciated.

    John
     
  2. WALLEYE MIKE

    WALLEYE MIKE Staff Member Mods

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    John, sometimes on the tap it will tell you the size of drill to use. It will be just a tad smaller than the tap. I'm thinking 1/64 less than the tap.
     

  3. Jasper302

    Jasper302

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  4. Michihunter

    Michihunter

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    I have to agree with this when it comes to this material. I've personally never had much luck tapping cast aluminum and the helicoil thread seems to get a better 'bite' on the aluminum itself.
     
    Last edited: Jul 10, 2009
  5. Ranger Ray

    Ranger Ray Smells like, Victory! Premium Member

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    A 5/16-18 tap uses a F drill (.2570). You could get by with a 1/4 drill also.
     
  6. Mitch

    Mitch

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    :yeahthat:

    RR is exactly right, an "F" drill is the proper tap drill. I would not advise going smaller (1/4) I would actually go a touch bigger with a "G" drill.

    Mitch
     
  7. bigcountrysg

    bigcountrysg

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    I would drill hole then use longer bolts. Put a longer bolt through the mower deck through the spindle then put flat washer with locking nut. Tighten down the bolt and locking nut down. Your done no worries about taps or drill bit sizes.

    BTW I just did this same repair on my cub cadet. Because the bolt holes for the spindle mounting bolts stripped out. I did all my spindle bolts like this.

    Oh the bolts used were grade 8 5/16-18x1.25 they cost me 40 cents each at the local hardware store. Then 20 cents each for the flat washers, then 96 cents each for the locking nuts. Total cost for 12 bolts, 12 washers, 12 locking nuts was around 19 dollars. But guess what I will never have a bolt break on the spindle again. Nor will I have to worry about the threads stripping out either.
     
    Last edited: Jul 10, 2009
  8. jpollman

    jpollman Premium Member

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    Cool. Thanks guys!

    I went back out there after I started this thread and then realized that a 5/16 drill would be too big. It looks like the "F" is the way to go.

    I never thought about a helicoil though. That may be the way to go. I'll have to check into it.

    Thanks again!

    John
     
  9. bigcountrysg

    bigcountrysg

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    I would not helicoil with cast aluminum. The metal is to soft and the helicoil may just come back out the next time you remove the bolt.
     
  10. tjstebb

    tjstebb

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    I would second the heli coil idea....with aluminum we use heli coils all the time and they work great plus you get everything you need in one kit.....except the drill bit but it tells you what one to use on the package....
    tjstebb
     
  11. Ranger Ray

    Ranger Ray Smells like, Victory! Premium Member

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    I doubt what he is going to use to make that hole, is going to give him a .250 or under hole. ;) Could very well end up with a 7mm hole. :lol:
     
    Last edited: Jul 10, 2009
  12. FERG 06

    FERG 06

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    John, those holes on the mandrels are not really, truely tapped.

    To explain it another way, the new ones aren't tapped. When I run a self tapping bolt into it and remove the bolt and look at the threads, they are barely cut into the metal.

    So it's not suprising that it stripped out. Happens all the time. I just drill out and put a bolt through it like Big said.

    Hope you didn't have to buy a mandrel, housing or shaft and bearing. You wouldn't believe the markup on those parts. If I sold you one at cost, you could have driven here and still paid less then retail.
     
  13. jpollman

    jpollman Premium Member

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    Yeah I just remembered that. A couple years ago I busted a mandrel and bought a new one. When it came it was smooth inside. The bolts pretty much cut the threads when you install them. There's acutally a hex recess in the under side of the casting. I think I may just drill it out and put a bolt through there and put a washer and lock nut on there. MUCH easier way to go.

    Thanks

    John
     
  14. FERG 06

    FERG 06

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    Yea I think that hex hole is their backup if they strip the hole at the factory. :lol: