Repair Internal Thread

R

RadiumRob

Well-Known Member
Joined
20 Sep 2011
Messages
148
Location
Portland Marina
Visit site
I've managed to damage the internal 8mm thread in my stainless steel rudder stock. I believe it is only the very start that has been cross-threaded.

I have an 8mm cutting tap (not thread chaser), but am anxious that I could do a lot more damage if it does not align perfectly with the deeper good thread.

An alternative would be to drill/grind out the top mm or so of damaged thread and let the bolt start slightly deeper in the hole - there's plenty of thread depth.

Any suggestions/recommendations gratefully received.
 
I can think of two alternatives. One is to make up a tool (by slicing up a screw) that you insert in parts and screw back out to restore the thread. Another is to drill on through your object with a pilot, then come back the other way a full size and run the tap through that way too.

But your plan is a lot easier and would work very well.
 
RadiumRob said:
I've managed to damage the internal 8mm thread in my stainless steel rudder stock. I believe it is only the very start that has been cross-threaded.

I have an 8mm cutting tap (not thread chaser), but am anxious that I could do a lot more damage if it does not align perfectly with the deeper good thread.

An alternative would be to drill/grind out the top mm or so of damaged thread and let the bolt start slightly deeper in the hole - there's plenty of thread depth.

Any suggestions/recommendations gratefully received.
Click to expand...

If the damaged thread is only a mm or so, I would risk the tap but use it very gently. After a 360 turn it should become easy enough turn with just fingers on the drive square. If it is not easy to turn after that first revolution I would wind it out and use your drilling idea.

Richard
 
I go along with the tap idea, but I think that I'd ensure that it was a taper tap, hoping that the truncated tap threads near the tip would engage with undamaged thread in the stock before any serious mis-match occurred.
 
Thanks for the replies.

Someone has suggested making up a cuboid block with a hole drilled perpendicular to the end surfaces through it, tapping this hole then using it as a guide to ensure the tap enters the hole at right angles to the top surface. Seems like a good approach.
 
Yes a good method of ensuring the tap is square to the work, which is easier said than done. The hole doesn't need to be tapped however, just a close fit on the tap OD. Best drilled in a good drill press or a lathe of course to ensure squareness.
 
penberth3 said:
On an internal thread?
Click to expand...

Yes. In a blind hole it is pretty much the only way you can engage with the undamaged lower threads to then sort out the bent upper threads.*
Running any sort of tap down it risks cutting away the existing good threads through misalignment of the thread start point.

On something as small as 8mm you may have to grind off three sides of the file to get it in the hole.
 
In my experience you will be able to guide a finishing tap through the top threads. The damage will not drag the tap to cut new threads because the remaining threads will still guid it. I use a back rotation first until I feel it jump, which is the tap disengaging, at the jump point, stop back ward rotating and lightly start to cut the threads as normal. Use lubrication. As you feel it bite cut about one 1/8 - 1/4 of circumference, then back out over the same interval, repeat for a full turn and then remove. Blow clean and inspect with a bright torch and magnifying glass, it is quite easy to see the threads aligning. Recovered a few threads this way on various motorbikes over the years. The main tip is to feel, it should not be much harder than a very stiff bolt. The mashes and dents in the damaged thread cut easily compared to cutting a new thread.
 
You must log in or register to reply here.

Share this page

Top