Thread Chaser or Tap ?

[R_W_B]

Ok I've read all the data I've found about why you should use Thread Chasers to clean threads instead of just a tap or die. They say the chaser has duller cutting edges (less angle) and is slightly less diameter than the tap and terms like less aggressive than the tap.

Here is what gets me. If I use a tap (instead of chaser) to clean my threads, it says that it will cut my threads too much causing the bolt to be slightly wobbling around in the hole. If this is the case, then why doesn't the first OEM tap cut the same exact depth and cause the same exact scenario. I don't get this ?? I mean isn't it the same thing as the tap I would be using ?

Appreciate any insight on this .

 

[ChrisAU]

I used a tap to clean the threads on my intake, and ran the bolts through a die. Those bolts are still very snug. If the tap and die are the right size, then how could it possibly be wrong?

 

[Rewind]

I'm thinking the same thing Chris.

 

[bluex]

When you first tap a hole you drill it undersize. The tap finishes out the hole to the correct size sorta. You want a certian thread depth to hold the torque of the fastener. When you retap a hole with a normal tap it will again remove some material from the threads. That makes the thread depth shallower/weaker and able to hold less torque before failure.

I work more with metric fasters so I can give this example easier. An M8 threaded hole requires a 6.7mm drill bit. The tap finishes the hole to the correct size and thread depth which is 66% of the pitch of the fastener if I remember right. The reason for dilling the hole undersize is to leave enough material to get the correct thread depth.

Does that make sense?

edit: Read your post somemore and no a thread chaser and a thread tap are not the same. Its hard to explain, but a thread chaser is not a sharp if you will as a tap. Thats why it dosent remove any material. Its easy to see the difference if you have both in your hands. I can see the difference in pictures but I know already what it is. A tap has sharp edges to create the threads. A chaser is more rounded. If you have a local tool supply they should carry both and you could see the difference in aggressiveness between the tap and a chaser.

 

[ChrisAU]

What I did with my manifold holes was turn it by hand; if it required a wrench to clean the already tapped holes with a tap, then damage is probably being done. You may snag up, but pull the tap out and clean it, its just from the gunk building up.

 

[R_W_B]

When you first tap a hole you drill it undersize. The tap finishes out the hole to the correct size sorta. You want a certian thread depth to hold the torque of the fastener. When you retap a hole with a normal tap it will again remove some material from the threads. That makes the thread depth shallower/weaker and able to hold less torque before failure.

I work more with metric fasters so I can give this example easier. An M8 threaded hole requires a 6.7mm drill bit. The tap finishes the hole to the correct size and thread depth which is 66% of the pitch of the fastener if I remember right. The reason for dilling the hole undersize is to leave enough material to get the correct thread depth.

Does that make sense?

edit: Read your post somemore and no a thread chaser and a thread tap are not the same. Its hard to explain, but a thread chaser is not a sharp if you will as a tap. Thats why it dosent remove any material. Its easy to see the difference if you have both in your hands. I can see the difference in pictures but I know already what it is. A tap has sharp edges to create the threads. A chaser is more rounded. If you have a local tool supply they should carry both and you could see the difference in aggressiveness between the tap and a chaser.

Yea that's the part that gets me. I realize they are different, but when you say "retap a hole with a normal tap it will again remove some material from the threads" is what I can't really see. How can a tap that is only the diameter of the original tapped hole, remove any "more" metal than the original tap did ? For it to do that on one side it would have to leave the other side uncut becuase it ain't that wide. I understand it will remove some crude and rust but how can something of the same size as the original tap cut any "more" than the original ? That part is blowing my mind. Are you saying that only one side of the threads could be rusted so as to force the tap against the other side only ?

 

[bluex]

Ok I think I understand your question a little better

A thread cutting tap is designed to cut threads. It will never engage the already cut threads in the same manner every time. You can take a new drilled and tapped hole and run the tap back through it and it will remove more metal. It won’t only remove material from one side; it will remove it from the entire circumference of the hole. It is almost impossible (by hand) to get the tap lined up in the exact same orientation to retap a hole.

In a poor example that I can do like this the first time you cut the threads they will look like this > <, every subsequent time you run a thread cutting tap through the hole will remove the point of the threads and round them off, reducing your thread depth making them look like this ] [. This is from the tap being slightly out of alignment with the original threads and it is cutting the threads in the alignment it is at now. That is the material that is getting removed.

What are you wanting to clean out? If its a non stressed part then using a normal tap wont be a really big deal. I would not use a normal tap for any critical fastener. Something like and intake or accessory bolt holes is no big deal. But definately use a thread chaser for something like main bearing caps or head bolt holes. Just bear in mind that threads cleaned with a normal tap wont be able to take the same torque load due to the reduced thread depth and contact area.....

 

[R_W_B]

Ok I think I understand your question a little better

A thread cutting tap is designed to cut threads. It will never engage the already cut threads in the same manner every time. You can take a new drilled and tapped hole and run the tap back through it and it will remove more metal. It won’t only remove material from one side; it will remove it from the entire circumference of the hole. It is almost impossible (by hand) to get the tap lined up in the exact same orientation to retap a hole.

In a poor example that I can do like this the first time you cut the threads they will look like this > <, every subsequent time you run a thread cutting tap through the hole will remove the point of the threads and round them off, reducing your thread depth making them look like this ] [. This is from the tap being slightly out of alignment with the original threads and it is cutting the threads in the alignment it is at now. That is the material that is getting removed.

What are you wanting to clean out? If its a non stressed part then using a normal tap wont be a really big deal. I would not use a normal tap for any critical fastener. Something like and intake or accessory bolt holes is no big deal. But definately use a thread chaser for something like main bearing caps or head bolt holes. Just bear in mind that threads cleaned with a normal tap wont be able to take the same torque load due to the reduced thread depth and contact area.....

Ok you reply is very good. I'm still not totally sold but since I'm no expert, you do make a good case. As to why I need to clean the threads out, they are rusted and full of old loctite etc and are for cylinder head, intake manifold and exhaust manifold bolts.

 

[ChrisAU]

Well I would think that in almost all applications the tap would follow the path of least resistance, whether that be air or dirt, old loctite, etc. And for a critical application, I would be afraid that a chaser would leave behind a small amount of foreign material and not make for a precise torque reading.

 

[R_W_B]

I got this off a Machinist forum.
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If the threads are really cruddy and rusty then you will damage the tap, not the thread. The tap acts as it's own lead screw and once it is in one full revolution it is not going to cut anything that is where it should be. If the thread is deformed and material is in front of the cutting edge it will be removed and should be. If they finding threads looser after cleaning the rust and such out the remember that the rust is part iron and yes it will be looser, but not because of the tap, but because metal has been removed by rusting.

I am in the power generation industry. When we disassemble a turbine we clean every thread. We stard with a wire brush and if that doesn't do it we run a tap or die. It's the last resort because rust and grit will ruin a 500 dollar tap in one hole. Small holes we tap.
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