Saturday, July 20, 2013

Repairing Threads (On The Cheap)

Getting you out of a hole.
At some point of working with bikes or cars, you'll be faced with a thread that's been damaged through over tightening, cross-threading, or simply full of corrosion/paint. A good set of taps and dies, or a thread chaser set, is invaluable for these times, but here's a neat trick that can get you out of that hole when you have neither.

But first, here's a few of the sets I use regularly. Well worth the investment, even it only saves you once.

Snap On Thread Chaser Set
M10x1.25 Chaser, Nut andTap
Beta Tap Set
Now the cheap method.
1) Take a bolt of the correct thread. 
In this case an M8x1.25 - this refers to the diameter of the bolt, not the size of the head (M8 = 8mm), and the pitch (distance between each raised point in millimetres).

2) Cover the thread in rag, and place the bolt in the vice. 

3) Take a square, triangular or flat file and start to cut a slot into the end of the bolt.

4) Once it starts to look like this, turn it 120ยบ and cut another slot. 

5) Repeat one last time so you have three slots around the bolt. These are going to carry the swarf away from the problem thread, and prevent further damage. 

I use plenty of WD40 when clearing threads in aluminium, and take my time. If it tightens up, work it back and fore, and spray with lube until it frees. 
If you can't work out the correct angle the bolt should be screwed in (due to cross-threading damage), see if you can screw some other bolts in surrounding holes. This will give you a guide to follow when you're trying to get it back on track. Similarly, a spark plug will normally follow the same angle as the others in a multi-cylinder engine. 

Take your time to work out angles before screwing the chaser, or tap, into the hole or you'll cut a good thread at a crap angle!

The budget thread chaser. 
This can also be put to good use with old spark plugs if somebody's managed to cross-thread them or the holes are full of old carbon etc. Just slot an old plug in the same way. One thing to remember, your budget chaser isn't as hard as the correct tool, so don't expect it to keep clearing in the same way. This is more about getting you out of a jam on a Saturday afternoon when those vital stores have closed.