Broken screw thread.

EricV

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You are creating a flat bottom taper tap. It will get the job done if you are careful, and that's ok. But it's not a bottom tap. Different designs.
 

EricV

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Could probably go to studs instead of bolts. That way you won't have to work that damaged hole any more than needed at a later date. Rancho Cucamonga used to have a top notch dealership.
Which is what most people do when installing a skid plate that mounts to the engine bolts. Really helps avoid issues later.
 

Xclimation

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Which is what most people do when installing a skid plate that mounts to the engine bolts. Really helps avoid issues later.
Thank You for answer my question before about retaping! Sometimes I wish I were a machinist! Soooo many things I come across that need a machinist and when I take my problem/project to a machinist...y'all make it look easy!

I didn't know it was (THOSE) bolts on the engine stay!!! Before the Rumbux, I had the altrider skid plate and crash bars. I mounted a stud, myself. Biggest reason I ended up mounted a stud is because EVERY TIME I would go to reinstall those engine stay bolts...they had a hard time catching the thread!!!! My next machinist question....why are some threads more difficult to catch or get the thread started? (Not sure of the proper terminology.) Sometimes I have luck with threads like this using a thread chaser or I even retap using a tap the same size...then usually the hard threading problem goes away. I didn't want to try this on these particular threads (engine stay). When I tap...I'm slow and meticulous...using thread oil, going in straight and backing out every 5 threads or so...
 

Clawdog60

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Just grab a tap and cut the end off and carefully file the ends. Making it a bottom tap. Re-tap the hole.
I have done this with success on similar cycle problems ruins the tap but.... A bottom tap is preferable if obtainable. You’ll have to go up to next size bolt with drill and bottom tap. Time sert. Tig closed the hole and drill and tap. A next larger self tapping bolt if you could find one. An aerospace epoxy and retap maybe.
 

EricV

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My next machinist question....why are some threads more difficult to catch or get the thread started? (Not sure of the proper terminology.)
A large part of this is alignment and the shape of the end of the bolt. The engine mount bolts can be a real bear because you are putting the bolt thru that steel plate and often skid plate too, and trying to get it into the engine. Even though it may not seem like much, when you remove one of those bolts the plate sags a little. If you remove both, it's absolutely going to sag some. Which presents a real hazard to getting them aligned correctly for easy threading when you put them back in. Steel bolts, aluminum engine block. All too easy to damage things. These were never intended to be removed on a regular basis.

Bolts that are intended to be removed and replaced for maintenance or other reasons often have a bit of taper in the tip to aid in starting the threads. Bolts that are intended to be factory installed and either never removed or only for major, rare occasions often have a more square tip profile to allow maximum thread contact. You will notice this next time you work on the bike and consider the area you are working on.

Another reason is that threads are classed in three categories. External threads are A, internal are B. Classes are 1 - Loose and spin on easily, (general purpose hardware or Ikea furniture ;) ), 2 - Best fit to allow excellent engagement and security, but still go together easily. (this is 90% of what we use) and 3 - close tolerance. (this is a friction fit where it requires some minor effort to thread things together. Used on critical components, often where safety is an issue.) More info - LINK

One manufacturing trick that you've likely seen is to use 2A fasteners in a safety area, but also use dowel pins to maintain alignment. This way the part is assembled precisely, and the fasteners are only holding it in place, not dictating the alignment.

Sometimes I have luck with threads like this using a thread chaser or I even retap using a tap the same size...then usually the hard threading problem goes away. I didn't want to try this on these particular threads (engine stay). When I tap...I'm slow and meticulous...using thread oil, going in straight and backing out every 5 threads or so...
If the threads are not stripped out, just lightly damaged you can often chase the threads. They make thread chasing taps that are designed not to cut metal, but to smooth it back into correct shape. A poor mans thread chaser can be made by taking a bolt the correct size and pitch and cutting or filing a notch in the end that goes from the tip approximately up 5 threads at a slight angle from the center line of the bolt. The notch is to give some room for debris as you thread this into the damaged hole. If damage is not too severe the bolt will clean up the threads and allow for smooth engagement again. Like the tap or chase tap, you go in a little, back out and clean, repeat until past the damage or deep enough for the intended fastener.

This is an example of a professional thread chasing tap. You can see it looks more like a standard tap for cutting threads, with much more relief. (note to those in the trades - This is not a turbo tap, but does look very similar.) The design, like a bottom tap, is created to pull debris out of the hole, not push it forward. (A standard taper tap pushes debris forward because it is intended to be used on a thru hole.)
chaser tap.jpg

This is an easier type to use, intended for garage or DIY users rather than manufacturing. Note the tip shape and relief groove. You can mimic this making your own from a bolt. These are much harder steel then an average bolt and push/roll the damaged threads back into the correct shape. Sometimes the better quality of this style will be lightly heat treated.
Thread chasing tap 2.jpg

When you run a same size tap back into a damaged hole you are going to cut some material away, cleaning up the threads in the process to allow smooth fastener engagement again. Most of the time this is minor enough to not cause an issue. However, it aways weakens the thread because there will be less secure engagement after cutting some material out of the threads.

It's hard to tell from the OP's pictures how much damage there is. It appeared to me that the threads were completely stripped out, thus my suggestion for a timesert or helicoil insert. If the threads have not been cross threaded and are only mildly damaged, it may be possible to chase the threads back into good enough condition to still be used, but as Don mentioned, using Studs instead would be a Good Idea™.
 
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EricV

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Gents what do you think about places like that one ? It is close to me. Looks like they could do the job right.

http://brokentap.com
They should have plenty of experience. Call them and talk to them about the issue. They may not want to deal with it on the bike and it's not cost effective to bring the individual part to them. If they don't want to deal with the entire bike, ask if they could recommend someone, or if one of their guys might be interested in a side job after he gets off work.

For best results you will want the mounting plate and skid plate off and out of the way so they can get direct access to the damaged hole.
 

Clawdog60

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A large part of this is alignment and the shape of the end of the bolt. The engine mount bolts can be a real bear because you are putting the bolt thru that steel plate and often skid plate too, and trying to get it into the engine. Even though it may not seem like much, when you remove one of those bolts the plate sags a little. If you remove both, it's absolutely going to sag some. Which presents a real hazard to getting them aligned correctly for easy threading when you put them back in. Steel bolts, aluminum engine block. All too easy to damage things. These were never intended to be removed on a regular basis.

Bolts that are intended to be removed and replaced for maintenance or other reasons often have a bit of taper in the tip to aid in starting the threads. Bolts that are intended to be factory installed and either never removed or only for major, rare occasions often have a more square tip profile to allow maximum thread contact. You will notice this next time you work on the bike and consider the area you are working on.

Another reason is that threads are classed in three categories. External threads are A, internal are B. Classes are 1 - Loose and spin on easily, (general purpose hardware or Ikea furniture ;) ), 2 - Best fit to allow excellent engagement and security, but still go together easily. (this is 90% of what we use) and 3 - close tolerance. (this is a friction fit where it requires some minor effort to thread things together. Used on critical components, often where safety is an issue.) More info - LINK

One manufacturing trick that you've likely seen is to use 2A fasteners in a safety area, but also use dowel pins to maintain alignment. This way the part is assembled precisely, and the fasteners are only holding it in place, not dictating the alignment.



If the threads are not stripped out, just lightly damaged you can often chase the threads. They make thread chasing taps that are designed not to cut metal, but to smooth it back into correct shape. A poor mans thread chaser can be made by taking a bolt the correct size and pitch and cutting or filing a notch in the end that goes from the tip approximately up 5 threads at a slight angle from the center line of the bolt. The notch is to give some room for debris as you thread this into the damaged hole. If damage is not too severe the bolt will clean up the threads and allow for smooth engagement again. Like the tap or chase tap, you go in a little, back out and clean, repeat until past the damage or deep enough for the intended fastener.

This is an example of a professional thread chasing tap. You can see it looks more like a standard tap for cutting threads, with much more relief. (note to those in the trades - This is not a turbo tap, but does look very similar.) The design, like a bottom tap, is created to pull debris out of the hole, not push it forward. (A standard taper tap pushes debris forward because it is intended to be used on a thru hole.)
View attachment 63538

This is an easier type to use, intended for garage or DIY users rather than manufacturing. Note the tip shape and relief groove. You can mimic this making your own from a bolt. These are much harder steel then an average bolt and push/roll the damaged threads back into the correct shape. Sometimes the better quality of this style will be lightly heat treated.
View attachment 63540

When you run a same size tap back into a damaged hole you are going to cut some material away, cleaning up the threads in the process to allow smooth fastener engagement again. Most of the time this is minor enough to not cause an issue. However, it aways weakens the thread because there will be less secure engagement after cutting some material out of the threads.

It's hard to tell from the OP's pictures how much damage there is. It appeared to me that the threads were completely stripped out, thus my suggestion for a timesert or helicoil insert. If the threads have not been cross threaded and are only mildly damaged, it may be possible to chase the threads back into good enough condition to still be used, but as Don mentioned, using Studs instead would be a Good Idea™.
WOW
 

AlsoRan

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Gents what do you think about places like that one ? It is close to me. Looks like they could do the job right.

http://brokentap.com
That could be a good place to get some direction. They likely will not want to deal with the bike but if they have a process in mind, ask if there is anything you can do (remove components etc.) in order to prepare the work area better so they will work on it. If not see if have any suggestions for an alternate plan. One thing is that if this repair is completed on the bike it will be done by hand, and any reputable shop will not like doing that without some extra tooling. Since it's already a mess with the material type and ruined threads etc., any attempt to drill or tap by hand is not for the faint of heart and will create a high probability of making the situation worse.

If they do not already have specific tooling they may likely recommend to have a jig / guide bushing made in order to create a more stable process. The jig will mount to another "good" threaded hole opposite or near the damaged one. It will look like a ring or an arm and it will have a guide bushing for the specific drill size (suggested by timesert or whoever) in order to get the hole drilled straight. Ideally they will have another tool or make a separate jig for tap alignment (especially since it's a bottoming tap) in order to get the first few new cut threads started. It may seem like a lot of hassle but it's not really that big of a deal to a skilled machine shop.
 

Kruzzin5

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What a great topic! And good to have all the experts on here to offer their advice! Wish I was smart enough to post my problem several days ago…
 

DanST1200

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Hello, can i revive this topic? Unfortunately, the same problem happened to me. When assembling the drop frames I used the original short screws and destroyed one thread. Thanks to the use of a short screw, I destroyed approx. 12 mm of thread. There is still about 10-12mm of thread left. So I used longer screws and attached the frames. The undamaged screw was able to be tightened well and firmly, for a moment of approx. 60 Nm. Of course, the damaged thread is a bit soft when tightening, so I tightened it with feel. Tightened to approx. 28 Nm, then it would probably start spinning slowly and I didn't dare to tighten any more. Do you think I can leave it like this? In the manual, the torque is up to 75 Nm. He would certainly not remove the damaged thread. I'll check to see if the screw loosens when I start riding. My point is, can a less tight bolt do something? After all, it holds the engine to the frame. Thanks a lot for the information.
 
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