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Discussion Starter · #1 ·
Hello! New here and looking for repair advice.
Craftsman LT1000 (17.5hp B&S engine 31C707-0230-E1). I'm an idiot, did not research and pulled the flywheel with a harmonic balancer puller and side bolts were short so I removed the flywheel/crankshaft bolt and apparently pressed directly against the crankshaft end (with female threads). Seems I warped the end of the crankshaft such that I cannot get the bolt to start threading into the female threads in the end of the crankshaft. Everything looks absolutely fine, just can't get the bolt to start. Threads look perfect using a scope camera. Thinking maybe I just rolled the crankshaft end over a bit?
Looking for suggestions instead of tearing down and replacing crankshaft. Some ideas I've had:
1. Gently tap bolt in to try to push any "bulge" out until it can start to thread in
2. Drill out with a bit just smaller than the bolt to get rid of the bulge without going too far to mess up the threads
3. Drill out and tap to slightly larger and get new bolt. Would Grade 8 bolt be appropriate or non-hardened bolt? Doubt I could find a tapered/pointed bolt so would I just get a straight bolt 1/4" shorter?

Any suggestions on the above or other ideas much appreciated.
Thank you!
 

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Welcome to MTF. 馃嵒

Somehow your post got caught in one of the forum filters, just got back in and fixed 'er up... sorry for the delay.

Re. your crankshaft... have to see but maybe you could just clean up the existing threads with a tap?
 

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Welcome to MTF. 馃嵒

Somehow your post got caught in one of the forum filters, just got back in and fixed 'er up... sorry for the delay.

Re. your crankshaft... have to see but maybe you could just clean up the existing threads with a tap?
As above, use great care in starting clean up tap.
Walt Conner
 

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You can taper the original (or duplicate) bolt threads using a file, and slowly feed the bolt into the crankshaft. The taper may allow you to get past the damage from peening over the metal. Likely there is just a tiny bit of distortion preventing the threads from engaging when starting the bolt by hand. With a slight taper at the tip of the bolt, you may get past the mangled bit and connect with good threads. You can use a file to create a 'wedge' in the tip of the bolt. You would file parallel to the length of the bolt, and have a deeper cut at the tip that tapers as it comes up the threads. You may have seen 'self tapping' bolts with a cutout at the tip. They allow some metal to gather in the wedge area as the bolt is threaded into place. You don't want to cut threads, but want to get past the slightly damaged area(it could be just out of round) to an area of threads that are not damaged. You may have to clean up the outermost threads, which a tap or a 'wedged' bolt can both do. If you do taper a bolt, you should be able to feel the threads engage with 'good' threads down further into the bore, and once you feel the threads match, you can apply a bit more torque. Worst thing is you'd have to drill out the hole for the next larger size bolt, then tap and use that size, or drill for an insert(thread repair) i one can handle the torque needed to keep the tapers engaged.
tom.
 

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Discussion Starter · #5 ·
Thanks for the suggestions!
The bolt is already tapered and has no threads the first 1/4" but it won't grab. Thinking I rounded over the top mm or 2 or forced it out of round.
I'm afraid if I tap it, I may not start at the correct spot such that the new threads would not align to the existing good threads further down.
Thinking it's out of round, maybe I find a drill bit that just fits through then increase to 1/32 size larger and drill down a couple mm, try bolt & repeat? Essentially widenening, rounding & smoothing the upper damaged part to let the bolt get past and hopefully engage into the lower good threads.
Taking my time with research & advice so I hopefully don't make things worse.
 

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I would use a metal countersink bit,rotary file tapered bit, or maybe just a larger drill bit than the threaded hole is,to remove the mushroomed area on the end of the crankshaft (to create a funnel shaped taper,not too deep!)--until you reach good threads and the bolt will thread in again..there should be at least an inch of threads for that bolt,losing a few won't hurt anything..
 

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remember the tap end is threaded but a bit narrower and may go down and properly mesh with the good threads whereas the tip of your bolt has no threads.
 

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I like the ideas of carefully feeding in a tap. Maybe using a guide block to keep it straight.

Remember, the tap is meant to cut into a hole that's the size of the tap drill for this thread. As was said, a typical tap will start with a decent taper. You don't want a bottoming tap, they will have a much smaller taper.

Another option could be a small grinding bit on a Dremel. Maybe a small-diameter cylindrical stone bit. Go in and take just a little bit of metal off the inside surface of the first thread or two, then see if it will thread in.
 

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I'd try the tap.

It's too bad the opposite of this tool doesn't seem to exist (this is a split die, you open it, put it on good threads behind the damaged threads on a bolt or shaft, then turn towards the damage and it cleans it up without further damage).

Mike

41sm1u+mdhL._AC_SY400_.jpg
 

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Discussion Starter · #11 ·
Thank you all for your thoughts. I warped the tip of the CS such that it is either out of round or the edge rolled in so that the diameter of the opening is now smaller than the diameter of the threaded area. So my dilemma becomes, do I trust a tap to widen/round the opening as well as cut threads or do I need to round/widen the end another way. I like @Red's idea of grinding slightly with the Dremel, but drilling with a countersink bit would round the hole that I don't think I could do easily with a Dremel. I would take of the slightest bit at a time and test with a backup bolt and stop drilling/grinding once the bolt can get any type of grip, so that I know it fits through the opening.
@mike there are split restoring/reverse taps (back tap) available that get inserted completely, then expand to engage good threads and cuts/repairs threads as you back them out. I was thinking of using one for this, just not sure I can rely on it to widen/round the now-narrowed end.
I really appreciate all the feedback.
 

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The tool you just described is designed exactly for this problem. I have never seen one, but if you get one, please post an image.
In actuality, you could remove the threads totally from the outer surface down the bore some distance, leaving threadless bore diameter , and have the bolt threads engage sub-surface level. You would lose that number of threads, but there are more than enough left to handle the load. I personally don't like that idea, but it actually would / should work without problem. To imagine what I am describing, think of using a drill bit that is the outer diameter of the bolt threads exactly. Using it in the shaft would remove all threading, as far down as you went. Good(hopefully) threads would start at the end of the boring. Of course, getting the drill bit aligned perfectly is the problem. Theory<>reality, as I understand.
tom
 

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This is the sort of tool being described, I think.

Slip it into a spark plug hole with damaged outer threads (at the entrance to the hole), while the chaser is collapsed. Once inside, you tighten the nut, which expands the thread chaser back to normal size. Then you back it out of the hole.

It re-forms those threads, starting from deeper inside the hole, as it goes.

You'd just need to find one in the proper size. If it's expensive, I might explore other options. Grinding just a little off the inside diameter, and/or carefully using a tap.

 
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These are the bits I'd use for this dilema...or a dremel grinding stone with the same shape..
A larger drill than the hole is may work,but would probably
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want to jam up when it hits the first thread..Tap might work by itself if its not rolled over too badly--but I have had bad luck snapping taps off in blind holes so I avoid using them,only as a last resort..or after I used the countersink,if the bolt refused to start in the threads..
 

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Those would at least self-center, and wouldn't want to bite too hard. If a normal drill suddenly grabbed the threads, you might just make things worse.

I agree that breaking off a tap in it would be Bad. But use oil. And the fact that you're only trying to clean up a little bit of rolled steel means that hopefully the tap doesn't actually need to do a lot of cutting.
 

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Hey, I'm a genius!!! Unfortunately, every good idea I've ever had seems to have been done already...

Yes, try a back-tap.

Or, take it to a machine shop.

Mike
 
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Been there - done that.. Sounds like you didn't do too much damage.. Just mucked up the threads.. Yes, that fancy split die is all well and nice but most of us farmers don't have these in their toolbox.. What they do have usually is a thread chaser.. It's like a file but for threads.. HERE is one and I've got both english and metric..
 

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Good point. I've used one of those for helping external threads, like on a bolt.

But these are internal threads, like on a nut. Can you use a thread file on those?
 

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The crankshaft threads are external and the nut holds the flywheel on If he is having the trouble I'm thinking.. Still a valuable tool to have in your arsenal..

If it's internal threads either drill it out and use the next size larger bolt or use a heli-coil thread insert kit.. They are fairly cheap at most hardware stores
 

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... I removed the flywheel/crankshaft bolt and apparently pressed directly against the crankshaft end (with female threads). Seems I warped the end of the crankshaft such that I cannot get the bolt to start threading into the female threads in the end of the crankshaft.
The crankshaft threads are external and the nut holds the flywheel on If he is having the trouble I'm thinking.. Still a valuable tool to have in your arsenal.
Got it. Typically the crankshaft threads would be male, in my experience, with a female nut over them. But in this case, per the quote above, it's apparently reversed, and this crankshaft has female threads, and a bolt that goes into them.

But the thread restorer file is still a good thing to mention, as they can be a big help when needed.
 
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