View Full Version : Helicoil Problems-magnesium???

Yankee Dog
11-25-2007, 10:29 AM
OK, have a project chainsaw. Picked it up for a good price, has good compression, good condition. Just doing some "clean up" of the saw and discovered a problem.

It seems the bar bolt (the part where studs are threaded into the magnesium casting of the block and actually holds the chainsaw cutting bar) threaded holes are stripped and have been helicoiled. The chainsaw bar is 3 foot long (yes it's a biggie 87cc chainsaw).

The helicoil is holding in one of the stud holes and I'm gonna leave that alone. In the other stud hole (total of two), it seems the previous owner either used two helicoils in the same hole or the helicoil it self it failing. The stud (bar bolt) is loose and I don't want to monkey with it any more. I need a good solid mounting of this stud to make sure the cutting bar does not move (dangerous).

I plan on drilling out the hole, retapping the hole (magnesium) and inserting a solid steel or brass threaded stud. Then drilling and tapping the center of that that to receive the original stud. Of course when putting it all together I will use red loc-tite. I just want to make a good quality repair as this bar receives allot of stress during cutting of large diameter tree trunks.

I want to make a good solid, quality repair. So I have a few questions:

When tapping magnesium, is it better to tap with fine threads or course threads? I have 1 inch of depth to play with and the other side of the hole is open (in other words no bottoming tap needed).

Removal of the current helicoil: I tried to pic out the beginning portion or the helicoil in order to grab it with needle nose pliers and peel it out, but no luck so far. Can I just drill it out using progressivly larger drill bits?

If I am approaching this project the wrong way, then I am wide open to suggestions. Thanks for your time, Happy Holidays, YD.

EDIT: If it matters the original stud is a 8mm x 1.25 thread. The hole appears to be drilled and tapped for the helicoil so that leaves me with a tapped 10mm hole, so the plan was to use a 1/2" diameter threaded slug, with the original 8mm threads tapped in the center of the slug.

11-25-2007, 11:03 AM
Getting helicoils out can be a pain in the rear..

Drilling them out is not recomended..
Get a die grinder with a small bit and just "get under" the insert where it's closest to the top..
Grab it with some vice grips and uncoil it out of the hole..

The slug idea you have will work but after inserting the slug you might want to "dutchman" the slug to the main body..

Thats when you drill and tap half the slug with half the body 3 places around the slug and insert say 6-32 set screws to hold it from spinning and tearing out..


Sorry about the crappy drawing..:laugh:

Thats what I would do..


11-25-2007, 11:08 AM
By the way go with a course thread on the new slug..


Yankee Dog
11-25-2007, 12:25 PM
Zapster, thanks for the reply. Good suggestion about the dremel and helicoil.

Glad to see I am on track with the slug. The dutchman is an excellent idea as well. I am a little concerned about weakening things with installing the 3 dutchman holes.

I plan on using red high strength loc-tite, but I want it to never spin, out so I really like that idea. Can I get away with just one or two dutchman? Due to the position of the hole, a 3rd may be tough to do. One would be ideal.

"Drilling them out is not recomended.." To bad, I thought that would be the easy way to go. Thanks! YD

BTW, picture was right on the money, thanks.

Yankee Dog
11-25-2007, 02:02 PM
OK, helicoil is out. Worked like a charm. Time to hang a few Xmas lights so when the boss gets home she won't think I have just been messin around in the shop.

Will get to drilling and tapping a little later. Will a brass slug work OK? I already have some 1/2" x 13tpi threaded rod in brass. Besides, easier to drill and tap. YD

Rick Moran
11-25-2007, 03:51 PM
If you don't have enough meat for a complete Dutchman lock, red loctite will hold it fine. Also, you could use just one DM peg to stake it from moving, but 3 are stronger with equal pressure loading/support.

I would pre-make your "slug" insert on a lathe, drilling and tapping it to ensure it is concentric, and I would leave a small seating lip on it for more strength. You don't want to be messing with it once it's installed and chance drilling off-center. I think Zap would agree with that.

To install it, you use the stud w/ 2 nuts locked together to drive it in to the proper depth.

I wouldn't recommend red for the final installation of the stud if you may want to get it out someday. Instead, use blue loctite as it holds fine and can be removed easily. My experience w/ red is that it's great for final installation of studs, but they generally need heat to get it them release if you need to change them. You wouldn't want to bring the slug out with it.

Also, you always use coarse threads for softer metals, alum, mag, and any other alloys. With steel, you have your choice depending on what kind of strength you need.

My favorite insets for alum and alloys are Time-Serts. Extremely thin hardened steel threaded inserts with a seating lip and they have a slight expansion thread on the bottom. Red loctite and they never come out unless you try w/ heat. Used a a lot of these for Top Fuel Hemis to hold the blower manifolds down and headers on.

Heli-Coils suck and have a tendancy to come out. Keen -Serts have four steel stakes driven down and cut the aluminum up even more. :nono:

11-25-2007, 04:42 PM
I would pre-make your "slug" insert on a lathe, drilling and tapping it to ensure it is concentric, and I would leave a small seating lip on it for more strength. You don't want to be messing with it once it's installed and chance drilling off-center. I think Zap would agree with that.

Agreed! 100%..

Keenserts are a good choice also..

If doing the "dutchman" thing 2 would be good opposite of each other..
You'll be OK either way..


Yankee Dog
11-25-2007, 07:35 PM
OK, slug is made using some 1/2" BRASS threaded rod. I drilled and tapped the center of the slug for 8mm x 1.25. That turned out good. Strength wise, is the brass similiar to the magnesium?

I went with brass as I have it on hand. At first I tried it with a steel bolt but it ate up drill bits.

Tomorrow I plan on drilling and tapping the hole in the chainsaw, but now I am having second thoughts on the size of the hole required for the 1/2 inch slug. I an kind of close to an edge (about 1/8" after drilling). So a little nervous about that.

My neighbor has a mini lathe, and while, I don't know what I'm doing with it, I think I'm gonna get a 7/16 inch bolt, and try to turn down one end to the 8mm size, then run a die to cut 8 x 1.25 threads.

So, I will try to make a stud that is 7/16 x14 tpi on one end, and 8mm x 1.25 on the other. At least that is what I am thinking at this point. Thanks for all the help so far, YD

11-25-2007, 08:02 PM
Sounds good so far..
1.25 is close to 3/8-16..
Why not tap it that size and use a stud with a nut instead of a bolt?

Just a thought..


Yankee Dog
11-25-2007, 10:40 PM
Remember, it was tapped for a helicoil prior to me owning it, so the holes are pretty darn close to 10mm (helicoil is removed) now. It is a stud currently. It is a stud with about 1 inch fully threaded length that goes into crankcase (8mm), and on the other end it is about 2 inches long and partially threaded (8mm). Has a shoulder that is about 12mm wide in the middle of the stud. Needs to be a stud to change bars.

The portion that threads into the crankcase was originally 8 x 1.25 and then drilled out and tapped for a helicoil. So as far as I can tell, probably a 10mm tap for the helicoil insert. Don't think a 3/8th will work. So now I will make it 7/16. The other end needs to remain 8mm as that is the size of the holes drilled from the factory on the chainsaw bar. That is a replaceable item, so pretty much have to stick with that.

I turned down a 7/16 bolt this eve. Need to go get a 8 x 1.25 die to thread the end that the bar and cover slips onto. Will drill and tap 7/16 threads in the crankcase side in the morning and hope for the best. Thanks, YD

Yankee Dog
11-26-2007, 06:53 PM
Well, it's a done deal. Borrowed my neighbors mini lathe and turned a new stud. 10mm x 1.5 on the bad threads end, and 8 x 1.25 on the end that receives the saw bar and cover.

I tapped the hole that had the helicoil with a 10mm x 1.5 tap. While the threads in there were not perfect, the stud is in place with red loctite and it took a descent amount of torque.

So at this point, I will let it all set up and just hope for the best. If it does not work out, I can always go up to 7/16. I cut one of those with the lathe for back up. After that, the saw will become a boat anchor.

Thanks for all the help, you guys are great. YD

08-14-2009, 11:15 PM
Will get to drilling and tapping a little later. Will a brass slug work OK? I already have some 1/2" x 13tpi threaded rod in brass http://www.xhcljx.com/eng

08-15-2009, 02:11 AM
Dead thread! Dead thread!