How Tp Repair Damaged Crankshaft Theards

• #one

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 information technology's able to be re-threaded.

Thanks for whatever info.

Tom

Mange

Addicted to ArboristSite

• #ii

Good question.
This is a tuff one.

I tried to brand a smaller bore and thread, but information technology got to weak.
I tried to Silverish weld a cylinder on, that i treaded, this actually worked, a while, just information technology to got weaker, and broke.
I can not retrieve of annihilation that will last, and concur as good as new.

• #three

It could be welded up and rethreaded but it is not easy or quick. Proper preheat and mail service heat needed to command hardness. Set upwardly time and threading against a shoulder is time consuming. I would think it would price a skillful role of the price of a new creepo unless y'all detect some retired tool and die maker. You wouldld likely lose likewise much diameter by rethreading to adjacent smaller size.

• #4

If you are a existent good lathe mitt,a "bastard" thread could be turned along with matching nut.As Frank said,this is very time consuming,and because the crank associates is i piece,the rod will have to be clamped during the turning process.Not a task for a novice.The all-time bet,in my opinion,is to get some other parts saw off e-bay,with a good crank,of class.

• #five

I had a special opposite thead 12mm ten 1mm special built for my KTM motorcycle to repair a damaged crankshaft/flywheel side thread. It was partially stripped& part smashed from a fly bike puller. There was enough left to save. The crank material wasn't as hard as I thought information technology 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!

• #half dozen

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

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

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

Thank you

Tom

• #vii

Yous tin can laugh it this if you want:

Years ago I had an erstwhile engine that the threads on the flywheel side ot the creepo got completely stripped. I removed the crank, and put it up through the centre 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 airplane pilot hole down through the center of the creepo, using the centering hole in the cease of the creepo. I then ground off the threaded surface area on the crank terminate and then going dorsum to the pilot hole, used it to drill a larger hole that could exist 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 commodities, flat washer, lock washer and some lock tight reassembled the thing. It worked for me at the time. Don't think haveing any problems with it loosening up. I know what a jury rig, just it worked. Don't know as I would even endeavour it now. Specially with a high rpm engine unless I could somehow perhaps pin the bolt in place like they do on some of the valve rocker studs when they're replaced. Things you exercise when you have no coin. Take care. Lewis.

• #eight

You know, I never thought of that. now we should detect a donor saw and effort information technology to run across how it would perform with such a repair.

• #9

Lewis,

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

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

Tom

• #ten

Excessive hardness will not be an issue when machining a crankshaft. The concluding Buick turbo project motor I built had a 4340 steel crank and rods. I had occasion to exercise a fleck of grinding, filing and some stamping on these and plant 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, information technology is just not super hard, except perhaps for bearing journals, which are often specially treated to a high hardness to reduce article of clothing.

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 ringlet forming then that the threads practise not create fatigue failure points the fashion cut threads do.

Jimbo

• #12

Broken Creepo threads:

If the threaded cease is gone yous'll need to make sure the hole you drill is perfectly centered in the crank. Haven't been most a lathe in years, but should be a way to put a centering hole in the terminate of the crank to apply as a reference. Heck it the crank was out of the saw the whole proceedure could exist washed on a lathe, I think. Equally was stated the cranks usually are pretty workable or machineable, the bearing surface may be haardened, merely the rest should be feasable. except for the clutch end of some cranks, which expect 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.

• #13

To OSA:

Hello OSA. This worked for an old 4 cycle engine, but would be hesitant to attempt information technology on a loftier rpm saw. It would be like experimenting with a black powder accuse in an old musket. I'd desire to be several feet away from it in case shse blew. My begetter as a child, completely filled a barrel on an old musket that hes granddad had. At to the lowest degree he mounted (tied) the musket into ii croched sticks that were shoved into the ground and using a long string tied to the trigger and standing several feet away, backside a barn door, touched her off. Needless to say, afterward the smoke cleared, only pieces of the onetime musket were found laying effectually. He never shot another gun after that untill he bought a 12 ga. Remington shotgun that had a poly choke on information technology for shooting ducks and phesents, years later, after he was married. Don't know what happened to that shotgun, mayby my female parent nonetheless has it put abroad somewhere. Take cae. Lewis.

Last edited: Apr iv, 2005

Mange

Addicted to ArboristSite

• #14

thompson1600 said:

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

Mange, practise 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 exist regular thread, aren't they? It'southward the clutch side that is reverse, correct? So just rethreading to a regular thread of a smaller size should work?

Cheers

Tom

I am not certain of all the tec words, so please be understanding.
I put the creepo upward in a lathe cutting cown the former and then threaded, this is not a good thought, the diameter gets besides pocket-sized, and the crank I tested on was POS steel, very soft. It broke but inside the threads.
There is proberbly different hardenings betwixt brands, Qualety, models.

• #15

Instead of a commodities:

I used a bolt in the repair I did to the old 4 cycle engine, only at the cease of the crank a person could use a made up stud, out of a High grade blazon of bolt or ane made say from some tool steel. Information technology could then be installed, lock tight ues and likewise pinned in place (if yous wanted). The stud could be of the aforementioned diameter as the original threaded end of the crank. I would drill the correct pigsty size, tap the hole for a particular stud size, then use a bottoming tap to cease up the hole, install the stud (torqued in and locktighted), maybe pin the stud and go from there. The only trouble would exist the keyway slot in the crank. Information technology may tend to weaken and fissure the creepo in that area over a catamenia of fourth dimension. It would exist interesting to try and put to the test in a safety setting. Crankshafts on lawnmowers are keyed and tapped in the surface area where the blade mounts and I haven't seen ane fatique and scissure in that expanse, from a blade hit objects (just once). Just go bent. Just would be interesting to know if the commodities/stud Thought would be feasible and piece of work. Any engineers out there willing to comment. Everyone take care. Lewis.

• #16

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 non as a rule,hardened.The pivot near probable is.It doesn't machine,that hard.

How Tp Repair Damaged Crankshaft Theards,

Source: https://www.arboristsite.com/threads/rethreading-crankshaft-threads.21155/

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