How to fix crank threads

[n4mwd]

I rescued a new Ninja from a guy that wasn't too mechanically skilled. He has damaged the threads going into the crankshaft on the left/starter/alternator side. It looks a lot like he used some kind of wheel puller on the flywheel and crushed the first row of threads in the crankshaft.

As far as I can tell, this is a 10mm x 1.25mm thread pitch hole. I was hoping someone can confirm that thread pitch. Ron Ayers says its a M10 X 28mm Long bolt (92002-1688) but says nothing about the thread pitch.

So far I have not gotten an M10x1.25 tap to bite into it. I'm thinking about just drilling the first couple of damaged thread's out since it seems to have plenty.

And advice... I have attached a photo of the threads, but unfortunately I couldn't get a better photo.

[alex.s]

worse comes to worse, a used crank shaft is pretty cheap.

[n4mwd]

Not that cheap, but it looks like what I need to do.

I drilled out a few of the crushed threads and forced in a tap. It bit, but unfortunately, not in the right spot - so now its cross threaded.

I think my only options now are to either get a used crank or bore and thread it a size larger.

[Motofool]

The proper way to do it is with an interior cutting tool in a lathe.
That way, the existing thread can be repaired from inside out.

IMO, a tap could start biting at the wrong point and cause more damage, even if pre-drilled.

http://www.metrication.com/engineering/threads.htm

P.S.- Oops, three minutes too late

[n4mwd]

Now that you mention it, a lathe would be the proper way to go. It still might be possible if I take it to a machine shop.

I have also been looking up the part numbers for Ninja crankshafts. They are not all the same number. But I'm wondering exactly what is different. Not surprisingly, the newgens are different, but the pregens are different among themselves.

Kawi Part # . . . year
13031-1247 . . . 1988-2005
13031-0013 . . . 2006-2007
13031-0112 . . . 2008
13031-0120 . . . 2009-2010

Anybody know the difference?

[CynicalC]

The correct answer is Helicoil.

http://www.emhart.com/brands/heli-coil/

[alex.s]

Originally Posted by CynicalC

The correct answer is Helicoil.

http://www.emhart.com/brands/heli-coil/

i think the inside would still need to be cleaned up to stick a helicoil in. and if you are already putting it on the lathe, its not much work to setup the feed to rethread it. but thats a good point in that machining it, rethreading it, then inserting a helicoil would get it back to spec size.

[n4mwd]

My concern there would be if the helicoil would be able to take the high torque that goes with that nut - 50#. Other than having the bolt to OEM spec, is there anything that wrong with just a bigger bolt? A bigger bolt can usually carry more load - not that its needed or anything - but the point is that its not any weaker than the OEM bolt.

I guess the next step is to drop the engine, crack it in two and take the crank to the machine shop for an estimate. Sounds like fun.

Oh well, at least it gives me an excuse to look around inside for any other problems in there.

[alex.s]

that bolt holds the stator wheel on right? i think the only issue would be the hole size for the wheel. i think you would need to make that bigger also. i think helicoils are typically made with very strong metal... they were originally for aluminum holes i think that just werent strong enough. the helicoiled hole should be as strong as the original. they claim that it makes it stronger than most metals tapped holes.

don't forget the bolts inside the oil filter hole. took me awhile to figure out i hadn't taken those out when i was trying to split it.

[Motofool]

Originally Posted by n4mwd

..............Other than having the bolt to OEM spec, is there anything that wrong with just a bigger bolt? .........
I guess the next step is to drop the engine, crack it in two and take the crank to the machine shop for an estimate. Sounds like fun.
................

I would ask the machinist first.
They can test the hardness of the material and determine if they have the necessary tools.

Not trying to be negative, just pointing some considerations:

Fixing the entrance of the threaded hole is one thing, but going to a M12 (if there is enough material for it) or a M14 may prove difficult.
The reason is the hardness of the shaft.
The cone on which the generator fits is rectified after the crank undergoes heat treatment.
The thread and other lathe work is done before that heat treatment, while the steel is soft enough for the cutting tools.

Also consider that increasing the interior diameter of the shaft will make it weaker to torsional and bending stresses.

[alex.s]

thats actually a really good point.

[Racer x]

If it is just the first couple threads it can be repaired without much trouble. If you want to pull the engine. Leave it together. Machinest have thread tools that will chase the threads not a tap that cuts but a tool that goes in and streightens out the bent thread. You don't need to pull the crank. But if you do have to pull the crank leave the rods bolted to the crank. Otherwise you have to buy new rod bolts.

[n4mwd]

Thanks, I'll ask the machinist to see what his requirements are before I do anything.

The flywheel hole is about 20mm so clearance wouldn't be a problem, but stress might. But if the bolt is tight and locked in place, wouldn't the bolt itself become essentially part of the shaft in terms of mechanical stress?

Like you said, the crank is hardened. I'm not sure if it will be possible to cut new threads.
.

[CynicalC]

Originally Posted by alex.s

i think the inside would still need to be cleaned up to stick a helicoil in. and if you are already putting it on the lathe, its not much work to setup the feed to rethread it. but thats a good point in that machining it, rethreading it, then inserting a helicoil would get it back to spec size.

Honestly, a lathe isn't going to give you any advantage at all in this application. Some things are better off just being done by hand. Also, you can't just fix damage threads. Rethreading it the same size is just going to cut more metal off the already damaged threads, not push them back into place. You might be able to get a bolt to thread back in, that doesn't make it "fixed." It would need to be drilled out and threaded m12, drilled, tapped and helicoil installed, or some loctite thread repair solution.

[n4mwd]

I counted the threads. There are 15 total, 2 damaged, 13 still good.

Thread repair epoxy sounds interesting and may be adequate for fixing the bad threads. However I still can't get the bolt in to the original threads.

I called the machine shop and he said I had to removed the crank and just bring that in. I kind of expected that. He said that he didn't have a lathe capable of cutting internal threads, but that he thought he could probably do the job with the hand tools that he has.

He said there are three option depending on how things go. Basically, its what we have already discussed here. 1. Chase originals. 2. Drill and tap M12. 3. Drill and tap for helicoil - With the #1 option being the cheapest.

So now I need to get busy dropping the engine.

[Racer x]

If it was my bike I would use a demml tool and remove the damaged thread enough to start the bolt. 13 good threads and two that have the peak cut off will hold.

[Motofool]

Originally Posted by n4mwd

.........But if the bolt is tight and locked in place, wouldn't the bolt itself become essentially part of the shaft in terms of mechanical stress?........

Not 100% sure, but I believe that it doesn't work that way.
Friction in the threads is the only force joining bolt and shaft; hence, they don't work as a solid piece.

The bolt imposes a traction stress on the shaft, because its only function is to create an axial force.

That force, combined with the wedge effect of the conical surfaces, develops enough friction between both parts as to avoid rotation due to inertia.

I have seen keys sheared when the bolt has not been torqued-in enough.

This is the tool that a machinist may have:

http://www.toolsource.com/spark-plug...rsbghs632busr3

This is how it works:
http://toolmonger.com/2007/05/22/re-...hreads-safely/

Google local "engine rebuilders"

I would leave the dimensions of the shaft as is, considering a heavy mass spinning at 12,000 turns a minute (200 per second) on the extreme of a shaft, and will try to repair that blind hole thread the best I can.
That way, the worst can happen is the bolt can't pull hard enough, shearing the key.

Best!

[Motofool]

Originally Posted by n4mwd

............He said there are three option depending on how things go. Basically, its what we have already discussed here. 1. Chase originals. 2. Drill and tap M12. 3. Drill and tap for helicoil - With the #1 option being the cheapest.
..............

Sorry, I didn't see your previous post; I was looking for that tool shown in post #17 above.
That tool is made for repairing spark plugs and any blind hole in M14, M12 and M10, being M10 the less common.

I still don't see the need for opening the engine.

I would try other shops and opinions.

[n4mwd]

Thanks for the tip on the back tappers. Unfortunately, I couldn't find anyone that sells one for 1.25 thread pitch (M10x1.25). I think they would be ideal in this situation if I could get one to fit.

I've decided to take the engine (intact) to the machine shop first and see if I can get him to work on it without me having to split it. I've got no clue how many gaskets, seals and bearings I'll need to replace if I do that. I think he is concerned about weight, but from what I've heard, its only about 90# and is much lighter than the cast iron engine blocks they're used to dealing with.

[Motofool]

Good choice!
Drain all the fluids carefully.

Any shop equipped with a vertical lathe could easily fix it.

One year ago, I had a very happy experience with these guys that repaired some parts of the engine of my car:

Engine Rebuilders of Palm Beach
1722 Donna Rd, West Palm Beach, FL 33409
(561) 478-0800

I don't know if they have the specific tools for this case or a lathe, but they are experienced in these type of repairs.

Best!

[n4mwd]

I've been talking to Lake Park Machine Shop. They did some work for me a few years back. I'll call your guy next if my guy doesn't work out.

[alex.s]

it feels lighter than 90lbs empty... i would guestimate closer to 60... i picked it up without issue and im a weakling

[CynicalC]

Originally Posted by Motofool

Good choice!
Drain all the fluids carefully.

Any shop equipped with a vertical lathe could easily fix it.

One year ago, I had a very happy experience with these guys that repaired some parts of the engine of my car:

Engine Rebuilders of Palm Beach
1722 Donna Rd, West Palm Beach, FL 33409
(561) 478-0800

I don't know if they have the specific tools for this case or a lathe, but they are experienced in these type of repairs.

Best!

Again, you don't need any machine to fix a couple damaged threads. Especially if it's only 2 threads. >_< seriously.

[n4mwd]

Got engine out. Its draining in a bucket right now. I'll take it over tomorrow.

I would do it myself but I don't believe that my skill level is adequate to complete the job successfully. I have already proven that I can make it worse.

I wish they made one of those back tappers in my size. That would make things A LOT easier.

[n4mwd]

Sort of. I took it to my machinist and he used several hand tools to get it the old threads chased. He said I didn't cross thread it with my tap, but rather, my tap was too soft to cut the hardened steel. He had a better tap.

Anyhow, $20 later, I have a clean bolt hole. The only thing is that it seems a bit wobbly. He said that most of the material that came out was thread compound and not metal.

I'm hoping that some loctite will tighten the threads back up when I put it back together.

Anyhow, the machinist guy was awesome. He literally dropped everything to come and work on my engine.

[Motofool]

Kudos to the machinist

[CynicalC]

Originally Posted by n4mwd

Sort of. I took it to my machinist and he used several hand tools to get it the old threads chased. He said I didn't cross thread it with my tap, but rather, my tap was too soft to cut the hardened steel. He had a better tap.

Anyhow, $20 later, I have a clean bolt hole. The only thing is that it seems a bit wobbly. He said that most of the material that came out was thread compound and not metal.

I'm hoping that some loctite will tighten the threads back up when I put it back together.

Anyhow, the machinist guy was awesome. He literally dropped everything to come and work on my engine.

Production work is monotonous. G-jobs make it more interesting. I don't buy the bit about your tap being too soft. A bolt was hard enough to damage the threads in the first place and a tap is no softer than a bolt. They're hardened and brittle and would break before flexing for the base metal.