Cleaning up a thread in a blind hole

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Avocet

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Not actually on a boat, but I'd like to seek opinions from the engineeringly knowledgeable on here!

This is an M12 x 1.5 tapped, blind hole - about 30 or 40mm deep, in (what I am pretty certain is) mild steel. Someone has either tried to put an M12 x 1.75 in there, or has cross-threaded a bolt at some point. The top 10-15mm of thread is a bit of a mess. I'm worried that if I bung an M12 x 1.5 taper tap down there, it MIGHT follow the wrong course and trash the rest of the threads on its way down. Part of me is thinking I should maybe just drill out the first bit of damaged thread and forget about it. (Hopefully, the remaining 20-odd mm of thread will be enough to hold the job together, if I get a longer bolt). If at all possible though, I'd like to save as much of the thread as I can. Any suggestions please?
 
The remaining 10mm or more of thread should be stronger than the bolt AIUI.
Recoil kit is about £35, you know you want one....

Personally I might take a chance with a tap and then see where I could borrow the recoil kit....
 
lw395 said:
Recoil kit is about £35, you know you want one....
Personally I might take a chance with a tap and then see where I could borrow the recoil kit....
Click to expand...

I'd go with a Recoil, but then I'm lucky in having some aboard. (You'd be welcome to borrow, Avocet, but I suspect distance might be an issue).
If even 20mm of the old thread is in good condition, it should be plenty strong enough.
 
macd said:
I'd go with a Recoil, but then I'm lucky in having some aboard. (You'd be welcome to borrow, Avocet, but I suspect distance might be an issue).
If even 20mm of the old thread is in good condition, it should be plenty strong enough.
Click to expand...

Do consider that the thickness of standard nut is 0.8 times the diameter of the bolt so 10 mm would be OK but due to the potential damage thus loss of thread area I would want longer so 20 mm would be more than OK
 
I recently repaired a damaged thread in aluminium plate. The top of the thread was mashed and crossed. I used a tapered tap. It took a few attempts and it was obvious that the taper tap was cross threading, but by feel and manipulating the tap around the axis it caught deeper at the good threads and subsequently recut the top threads. It was all done with the hand tap. I used a method where I screwed in slightly and felt the hand tap pulling off centre, then stopped and reversed until I felt it easily adopt the vertical position, then forward again until it gripped. I varied the hand pressure down as well and eventually I felt that it was engaging correctly and turning with less force than the crossed thread position. Worked well, the bottom tap would not have worked in my case as the top was mashed bad and it would just have started cutting a new helical profile that probably would not match the original.
 
If you can get it to a drill press, just drill down for 10mm with a 12mm drill to get rid of the badly damaged threads. After that you will still have enough good thread and be able to screw the correct bolt in. If there is still some damaged thread the plain 12mm hole will hold a plug tap upright to enable you to clean the remaining damaged threads.
As an aside to the above bolt failure is likely to happen in two ways, either the threads strip or the core of the bolt fails in tension. With metric coarse threads the core will fail before the threads if threads are engaged for a depth greaer that the bolt outside diameter, assuming both bolt and threaded hole are in steel. For finer threads the core is greater in diameter, and a slightly greater length of thread engagement is required. What this means is that if the 12mm fine thread bolt is engaged 10 full turns (i.e 15mm) it will be at full strength. If the tapped hole is in a weaker metal, cast iron for example, then greater thread engagement is needed as the threads in the hole will fail before those on the steel bolt.
 
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Thanks all. These are car dampers. In fact, the stripped thread is in an aftermarket "performance" insert. You cut the top off your old strut, empty the oil out, and shove this "insert" into the remaining tube. You then need to drill a hole in the bottom of your old strut and put this M12 screw through it and into the bottom of the insert, to stop it lifting out of the tube. Looking at them, it's obvious that whoever drilled the hole in the bottom of the strut, didn't get it central, so part of it overhangs the tapped hole in the insert. I've opened them out with a die grinder, and now I just need to clean the threads up.

Because it's a damper, there's not much chance of getting it set up in a pillar drill. If I could get the insert out, I'm sure it would go in a lathe, but it doesn't look like they're EVER coming out now! They'll be at least 20 years old and I imagine, rusted to the inside of the old strut. (Which makes my M12 screw through the bottom somewhat redundant, but I'd feel better if it was there)! That also means the helicoil isn't an option - I don't know how much "meat" there is around the hole. Strength of the bolt won't be an issue, it's a 10.9 cap head. I'm sure I could lift the car with it!
 
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