rethreading crankshaft threads?

[thompson1600]

Hey,

I have a good 066 crankshaft, but the flywheel side threads have been stripped. I was wondering if anyone has rethreaded these? Don't know how hard the crankshaft is or whether it's able to be re-threaded.

Thanks for any info.

Tom

 

[Mange]

Good question.
This is a tuff one.

I tried to make a smaller diameter and thread, but it got to weak.
I tried to Silver weld a cylinder on, that i treaded, this actually worked, a while, but it to got weaker, and broke.
I can not think of anything that will last, and hold as good as new.

 

[Crofter]

It could be welded up and rethreaded but it is not easy or quick. Proper preheat and post heat needed to control hardness. Set up time and threading against a shoulder is time consuming. I would think it would cost a good part of the price of a new crank unless you find some retired tool and die maker. You wouldld likely lose too much diameter by rethreading to next smaller size.

 

[Al Smith]

If you are a real good lathe hand,a "bastard" thread could be turned along with matching nut.As Frank said,this is very time consuming,and because the crank assembly is one piece,the rod will have to be clamped during the turning process.Not a job for a novice.The best bet,in my opinion,is to get another parts saw off e-bay,with a good crank,of course.

 

[weimedog]

I had a special reverse thead 12mm x 1mm special built for my KTM motorcycle to repair a damaged crankshaft/flywheel side thread. It was partially stripped& part smashed from a fly wheel puller. There was enough left to save. The crank material wasn't as hard as I thought it would be. Worked well. I would shy away from the weld and re-thread...if your going to go that far, just get another crank!

 

[thompson1600]

OK, why would it matter if you went to a smaller size for the flywheel nut? It's not a bearing surface for any part, just needs to hold torque.

Mange, do you know why when you re-threaded it got weaker? Did you you just re-thread or did you untemper and retemper the steel?

The flywheel side of Stihl cranks should be regular thread, aren't they? It's the clutch side that is reverse, correct? So just rethreading to a regular thread of a smaller size should work?

Thanks

Tom

 

[Lewis Brander]

You can laugh it this if you want:

Years ago I had an old engine that the threads on the flywheel side ot the crank got completely stripped. I removed the crank, and put it up through the center hole in a drill press and clamped her vertical using a drill press vice with some brass sheet metal protecting the crank surfaces. Using a smaller drill, I drilled a pilot hole down through the center of the crank, using the centering hole in the end of the crank. I then ground off the threaded area on the crank end and then going back to the pilot hole, used it to drill a larger hole that could be tapped for a fine threaded bolt. I reinstalled the crank in the engine, put the flywheel and keyway back in place and using a fine threaded bolt, flat washer, lock washer and some lock tight reassembled the thing. It worked for me at the time. Don't remember haveing any problems with it loosening up. I know what a jury rig, but it worked. Don't know as I would even try it now. Especially with a high rpm engine unless I could somehow possibly pin the bolt in place like they do on some of the valve rocker studs when they're replaced. Things you do when you have no money. Take care. Lewis.

 

[Lawn Masters]

You know, I never thought of that. now we should find a donor saw and try it to see how it would perform with such a repair.

 

[thompson1600]

Lewis,

I have another crankshaft from an 044 that must have been beat on at one time by someone. When I went to torque the flywheel nut down, it broke clean off. I may just try your method on that crankshaft since the thread area is gone anyway.

But, still wondering about re-threading that 066 crankshaft.

Tom

 

[jimbo1490]

Excessive hardness will not be an issue when machining a crankshaft. The last Buick turbo project motor I built had a 4340 steel crank and rods. I had occasion to do a bit of grinding, filing and some stamping on these and found they were not very hard, though I never did a Rockwell or Brinnell test (why?) to quantify. It goes without saying that the crank steel is very tough and has very good UTS, it is just not super hard, except maybe for bearing journals, which are often specially treated to a high hardness to reduce wear.

Threaded parts of spinning shafts are generally not brought to a high hardness because of issues with britleness and the fact that the threads on these shafts are usually cut by a lathe. Bolts, for instance are always threaded by roll forming so that the threads do not create fatigue failure points the way cut threads do.

Jimbo

 

[jimbo1490]

Try this:

http://tinyurl.com/4llr6

Jimbo

 

[Lewis Brander]

Broken Crank threads:

If the threaded end is gone you'll need to make sure the hole you drill is perfectly centered in the crank. Haven't been near a lathe in years, but should be a way to put a centering hole in the end of the crank to use as a reference. Heck it the crank was out of the saw the whole proceedure could be done on a lathe, I think. As was stated the cranks usually are pretty workable or machineable, the bearing surface may be haardened, but the rest should be feasable. except for the clutch end of some cranks, which look to be also hardened, where the sprocket brgs ride or the threads where the clutch is screwed on. This is something that I tried years ago on an old engine that worked for me. If someone is going to try this, IT'S AT YOUR OWN RISK!!!!!!! Take care. Lewis.

 

[Lewis Brander]

To OSA:

Hello OSA. This worked for an old 4 cycle engine, but would be hesitant to try it on a high rpm saw. It would be like experimenting with a black powder charge in an old musket. I'd want to be several feet away from it in case shse blew. My father as a kid, completely filled a barrel on an old musket that hes grandfather had. At least he mounted (tied) the musket into 2 croched sticks that were shoved into the ground and using a long string tied to the trigger and standing several feet away, behind a barn door, touched her off. Needless to say, after the smoke cleared, only pieces of the old musket were found laying around. He never shot another gun after that untill he bought a 12 ga. Remington shotgun that had a poly choke on it for shooting ducks and phesents, years later, after he was married. Don't know what happened to that shotgun, mayby my mother still has it put away somewhere. Take cae. Lewis.

 

[Mange]

OK, why would it matter if you went to a smaller size for the flywheel nut? It's not a bearing surface for any part, just needs to hold torque.

Mange, do you know why when you re-threaded it got weaker? Did you you just re-thread or did you untemper and retemper the steel?

The flywheel side of Stihl cranks should be regular thread, aren't they? It's the clutch side that is reverse, correct? So just rethreading to a regular thread of a smaller size should work?

Thanks

Tom

I am not sure of all the tec words, so please be understanding.
I put the crank up in a lathe cut cown the old and then threaded, this is not a good idea, the diameter gets too small, and the crank I tested on was POS steel, very soft. It broke just inside the threads.
There is proberbly different hardenings between brands, Qualety, models.

 

[Lewis Brander]

Instead of a bolt:

I used a bolt in the repair I did to the old 4 cycle engine, but at the end of the crank a person could use a made up stud, out of a High grade type of bolt or one made say from some tool steel. It could then be installed, lock tight ues and also pinned in place (if you wanted). The stud could be of the same diameter as the original threaded end of the crank. I would drill the correct hole size, tap the hole for a particular stud size, then use a bottoming tap to finish up the hole, install the stud (torqued in and locktighted), possibly pin the stud and go from there. The only problem would be the keyway slot in the crank. It may tend to weaken and crack the crank in that area over a period of time. It would be interesting to try and put to the test in a safety setting. Crankshafts on lawnmowers are keyed and tapped in the area where the blade mounts and I haven't seen one fatique and crack in that area, from a blade hitting objects (only once). Just get bent. Just would be interesting to know if the bolt/stud Idea would be feasible and work. Any engineers out there willing to comment. Everyone take care. Lewis.

 

[Al Smith]

Odds are,the crank is some derivative of 4140 or 4340 steel .This is tough steel,they make wrenches,axles,crankshafts,etc out of this stuff.Cranks are not as a rule,hardened.The pin most likely is.It doesn't machine,that hard.