I stripped the threads on a drain plug

[quazz]

I can offer no help and any advice might make matters worse but I have been there too and it is frustrating - an expensive tractor and a freaking .$25 bolt is keeping you from using it.

 

[Yooper]

Thanks again for all the replies and suggestions.
I have attached a couple pictures of the plug as requested. The threaded portion of the bolt beyond the washer is only about 3/8".
From what I'm hearing from you, finding out what the original threads were won't lead to the best solution because of the minimal good threads left in the hole.
Drilling the hole larger seems to be the best option. Since a helicoil requires drilling and tapping, it seems to me the best option is to drill and tap for a new bolt or fitting.
I like the idea of the NPT option, but right now the hole is larger than 7/16". It measures about .455 - .461" now where the threads are gone so I would have to go to a 3/8" NPT which requires a 37/64" hole (.578125"), not much larger than what I have.
Keeping chips out of the transmission will be a challenge, but I can flush it with hydraulic fluid before I seal it up. I might consider putting a piece of cloth in the hole prior to drilling. With a hole this large, I should be able to fish it out after the tapping.
Thanks again.

That little bit of oversize will still work with the 1/4 npt. Being that its tapered, the plug will just go further in before engaging. This is the ‘forgiving’ aspect of going with the npt.

 

[Charlie5320]

Thanks again for all the replies and suggestions.
I have attached a couple pictures of the plug as requested. The threaded portion of the bolt beyond the washer is only about 3/8".
From what I'm hearing from you, finding out what the original threads were won't lead to the best solution because of the minimal good threads left in the hole.
Drilling the hole larger seems to be the best option. Since a helicoil requires drilling and tapping, it seems to me the best option is to drill and tap for a new bolt or fitting.
I like the idea of the NPT option, but right now the hole is larger than 7/16". It measures about .455 - .461" now where the threads are gone so I would have to go to a 3/8" NPT which requires a 37/64" hole (.578125"), not much larger than what I have.
Keeping chips out of the transmission will be a challenge, but I can flush it with hydraulic fluid before I seal it up. I might consider putting a piece of cloth in the hole prior to drilling. With a hole this large, I should be able to fish it out after the tapping.
Thanks again.

If you use grease on the bit and tap you can capture almost all of the shavings.

 

[William1]

In my world, the repair is a Time-Sert. Drill, tap and install part (like a Heli-Coil, only multiple times stronger).
When drilling, coat the bit in grease. Remove frequently, clean off, re-apply the grease and drill some more. 99% if not 100% of the sparf will stick to the bit. Ditto when tapping.
In the future, a bolt with a oring does not ever need to be cranked down. The oring seals and holds the bolt from moving. Over tightened will completely crush the oring, and the bolt can be stuck, the oring leak. If you are worried about it loosening, safety wire it. A bolt with a oring is a lot like a oil filter. Tightened until the rubber meets then just a 1/4 or so more.

 

[AlanM]

That little bit of oversize will still work with the 1/4 npt. Being that its tapered, the plug will just go further in before engaging. This is the ‘forgiving’ aspect of going with the npt.

Your advice was excellent. It reduced the amount of chips produced plus I have the 1/4" NPT tap and plug.
A 7/16 " drill would not go through the remaining original threads so I did drill those to 7/16". Tapping the hole went well and I cleaned the tap about every 1/2 turn. I also used a shop vacuum to remove anything in the hole when the tap was out. I tapped the hole deep enough to install the plug up to just two threads not in the hole.
With the plug still out, I started filling the system and drained about a pint through the hole. After it quite dripping, I installed the plug with Permatex thread sealant on it. The system is now full and there are no leaks.

Regarding my original plan to use the remaining original threads with a longer flange bolt, I still do not know what the threads are. I called two Kubota dealers and neither knew the answer and neither had the flange bolt in stock. I ordered one just to find out what the threads are. One dealer told me he doubted that anyone there could identify the threads even after they have the bolt in hand. I'll have to borrow a metric thread gage or take it to a hardware store and see what size nut will fit it.

Thank you again for all your advice and excellent suggestions.

 

[Yooper]

Thanks for the follow up! You made my day!

 

[Lencho]

Hi Alan - nice photos and great follow through. Commendable!