Wednesday, December 16, 2020

Fixing Damaged Threads

  Working on chainsaws and other small 2 cycle engines means I deal with a lot of soft lightweight materials like aluminum and plastic. Because I deal with them every day I have a pretty good feel for when a screw is tight enough but not all my customers have that same feel. I get a lot of people that think "crank it till it strips then back off a quarter turn" is a real thing. They then bring it to me and ask if it can be saved. Pretty much all stripped threads can be saved but a lot of times the deal breaker is the amount of labor it takes to restore the threads. Is the bolt broke off in the threads? If so is the broken face flat so drilling is possible? Is the bolt exposed or recessed? Did the bolt vibrate out and leave a sloppy hole? All of these things require a different method of repair. 

 The following repair came across my bench yesterday. One bolt was holding the muffler on and the other was broke off just below the deck surface. A lot of times these bolts can be walked out by drilling a hole and using an "easy out" extractor. Some times a left handed drill bit is all thats needed. In this case it appears the bolt was cranked tight against the bottom of a blind hole and would need to be drilled.

 Don't adjust your picture. The saw was laying on its side on the bench. The upper muffler bolt hole is our concern.

 I taped the exhaust port to keep debris out of the engine. I center punched the bolt shank as best I could.

 The next step is to drill a nice straight hole through the bolt. I like to start with a small bit and work my way up.

 At this point I did try an easy out extractor just to see if the bolt would move but it was tighter than a camels butt in a sand storm. The answer would be to either drill it to size for a tap or drill it for a thread insert. Thread inserts have worked well for me.

spiral extractor

"Perma-coil" thread insert kit

Our new threads

 The kit requires drilling to a specific size depending on the insert being used. That hole is then tapped for the insert. Next the insert is put in a special holder that is held against the top of the hole while being turned in. Its all pretty easy.

 That blue piece is the holder and a driver runs through it turning our insert. Below is the finished results.

  "Why not just drill and tap for the next larger size bolt"? Sometimes that works but in this case a larger bolt would not have fit through the muffler.
These kits work pretty well and make a permanent repair for the original size bolt. The steel insert holds up better than the original aluminum threads and the only time I see come backs is on some sketchy spark plug thread repairs that probably shouldn't have been done in the first place but the owners felt any attempt at more run time was worth the effort.

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