Oversized final drive mounting holes

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Oversized final drive mounting holes

Postby Mht » Thu Oct 29, 2020 8:36 pm

I’m replacing the final drives on my current cub project due to broken implement mounting holes. The left side final has several holes that have been tapped oversized to 7/16 bolts. I really hate to drill the mounting holes oversize in the replacement final. Should I repair the oversized holes with a helicoil to restore the holes to the original 3/8-16 thread or just stay with the 7/16 size and drill out the replacement final. I plan on working the tractor so I’m not really concerned with it being original but want it to hold up long term. The 7/16 bolts are a little bit of a pain as there is not enough clearance with the final drive housing to get a socket on them. Does anyone know off the top of their head if the proper drill size for a 3/8 helicoil will remove the threads from the 7/16 hole so I can get full threads for the helicoil to thread into.

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Re: Oversized final drive mounting holes

Postby Bob McCarty » Thu Oct 29, 2020 9:26 pm

A 3/8" helicoil takes an X drill bit (.3970). A 7/16" bit is .4375. I think you'd need to use a solid insert like an EZ-Lok or similar, the regular size takes a 1/2" drill size. The thin wall locking takes a 29/64' (.45) drill size which would remove minimal extra material. The non-locking version takes a 31/64" drill (.48). McMaster-Carr has different sizes in different materials, you might want to confirm what I've listed.
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Re: Oversized final drive mounting holes

Postby Gary Dotson » Fri Oct 30, 2020 7:51 am

Another option, for you, 7/16" bolts are available with a flanged head with an undersized hex. I see them with 7/16" hex, from McMaster, but other sources may have other sizes. There are also 12 point heads and socket head bolts. I would generally prefer to restore to the original size but when someone has already messed with them, it may not always be the best way.

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