Bellhousing threads (block side) of my recently acquired LS engine are not the greatest. A couple of the through holes threads are not good. One of the bolt holes were really bad so I ran my chinesium HF tap in there (m10x1.5). That did a world of damage and I'll likely have to drill and tap that hole now. Great.
Is it a bad idea to use a tap to chase threads? I see there are dedicated chasers online but there big $$$ and 8 days way via shipping. What's the GRM to clean up threads?
Good quality sharp taps do not damage holes when thread chasing. HF taps are a consistent fail in that regard. That said many damagd holes cannot be chased back to use. Keensert or Timesert are the only answer.
If they are not too damaged and mainly just rusty/dirty you can make a thread chaser by cutting some slots in a screw the correct size, preferably the highest grade you have available. It isn't going to do much if the threads are mangled.
Slots like this:
What EvanB said.
If the threads are boogered up it takes a thread chaser or a good tap to clean them. Luber helps.
A full set of quality thread chasers need not be too expensive.
Virtually every single thread chase tool set sold today is made by Lang Tools. SnapOn, Mac, Matco, Cornwell, Craftsman, Kstar, etc....All are made in USA by Lang Tools and then rebranded and upcharged by others.
The deluxe set, Lang part number 971 can be bought for around 70 dollars delivered.
Here is where I got mine. Free shipping too.
I have used mine for years. It's invaluable.
There are class fit taps and dies such as 1A, 2A, and 3A for external threads and 1B, 2B and 3B for internal threads with the higher number being the tighter fit. So using a 1 in a 3 could damage a thread. I suspect for most automotive applications the class 1 fit is most common.
When I am chasing threads I always use either a starter or plug tap rather then a bottom tap. The taper on the starter/plug tap insures the tap enters the existing threads straight and square without damaging the threads. I also use a lubricant. I use good quality taps and dies even though they are expensive. They are made of better quality tool steel and ground to tighter tolerances and stay sharper then some of the discount brands.
Internal threads that can't saved can be repaired using a Heli-Coil however they require a special tap after drilling the oversized hole and a insertion tool.
Chasing boogered threads takes some finesse. Lots of lube and multiple passes to clean the chips out of the tap. I never had much luck until I watched a guy who was really good at it.
Is it just one bolt hole that is nackered? A lot of guys are only running the 4 or 5 out of the 6 bell housing bolts without any issue.
If it's in aluminum, helicoil it.
It just sounds to me like you need more practice using taps. Square, trying to engage what's already there, lube, not letting chips build up, etc. Majority of the threads Ive ever cleaned have been with taps. There's nothing inherently wrong with it, but the worse the existing threads are, the more careful you have to be. At some point you should just start over with new threads.
Interesting set, Eddie. I need to look at the metric taps I use for my Toyota work. Toyota has some somewhat oddball thread pitches, but I think they use the 1.25.
I usually use my HF taps for thread chasing. I don't have a problem using them. You do have to be careful when you do it. You need to very carefully get it started in what's left of the existing threads, go slow, use Tap Magic. The cheapest HF taps are even more useful because they are dull and tend to reform instead of cut the threads. And you have to be very careful and stop when you hit bottom on a blind hole, otherwise you pull all the threads out. If you just jam a tap in the hole and crank, you're going to have problems (and no threads.) I saw a mechanic do that once to the plug on my primary chain case on my bike when I told him to use a tap to clean up the threads. Jam, crank, no threads at all now. Uh, that's not what I meant. He pulled the primary, TIG'ed up the hole, drilled and tapped it. Fixed. Eventually, I got one of the other mechanics to work on my bike instead of him. He was a good guy and my friend, and he could get anything running with nothing but gum and bailing wire from the side of the road, but he fixed everything that way even when he had the right tools and parts. At his funeral, no one would ride his bike, and there were some serious hardcore Bandits there, but the bike was too dangerous for anyone to ride. RIP, Gypsy.
It's my understanding that the reason to use a thread chaser instead of a tap is because it's not designed to cut. If your subject has rolled threads, they stay stronger if you chase them instead of running a tap through them. The chasers are designed to move metal back where it belongs, not cut it away.
I've got a set of each (not HF), and I do find the chasers feel smoother when you're using them. 90% of the time, there's probably no difference - but once in a while, when the thread is particularly boogered and/or strength is a factor, I'm glad I have the chasers. They also come with thread files, which are a very useful repair tool.
A little tech: https://www.autoserviceprofessional.com/article/94859/Thread-chasing-Restoring-male-or-female-fastener-threads?Page=2
In a sampling of roughly 10-12, my error rate in chasing threads with a tap is approximately 100%. Aluminum is boogered before I even stick the tap in. Maybe you guys are way better mechanics than me (probably), but I won't even try it any more.
burdickjp said:If it's in aluminum, helicoil it.
If it's in aluminum Time-Sert it.
In reply to Keith Tanner :
My instructors made such a huge deal of this during school 15 years ago it still sticks with me. The tap runs the chance of hogging out the threads, supposedly the chaser will not.
I appreciate the female chasers a bit more personally. They're great for clearing Loctite off threads without having to wirewheel or brush them.
Lots of good info here, thanks. Mostly that my thread chasing skills are poor at best.
I have that same chaser set, but it didn't have 10mm x1.25. I didn't have the tap either technically, but I could buy the tap at the local hardware store.
I think I was just fine in this case however. I made sure to be very careful getting the tap into the existing threads, and then turned it all the way in by hand (with lube). I only put the handle on to spin it out quickly. Did two holes and now they both seem much better.
Also, I could have bought a bolt and made a chaser by cutting the slot as mentioned, but the tap was $8 and the right size bolt was $3.40!
Robbie said:
I have that same chaser set, but it didn't have 10mm x1.25. I didn't have the tap either technically, but I could buy the tap at the local hardware store.
I think I was just fine in this case however. I made sure to be very careful getting the tap into the existing threads, and then turned it all the way in by hand (with lube). I only put the handle on to spin it out quickly. Did two holes and now they both seem much better.
Also, I could have bought a bolt and made a chaser by cutting the slot as mentioned, but the tap was $8 and the right size bolt was $3.40!
You must have a different set if it doesn't have 10x1.25
In reply to Cousin_Eddie :
Apparently it is different! Looks pretty gd similar though...
In reply to Robbie :
I've got that set! Have had it so long that the exterior of the box is pink now.
I have the metric only set. It's pretty useful, just used some of the dies to clean off rtv and threadlocker from fasteners for engine rebuild. I too had issues destroying threads using cutting taps previously.
You can get some inexpensive but effective tap guides that increase your chance of success greatly.
The critical point is initially engaging the tool threads and the existing threads,
and keeping the tool straight and true is really important.
Google tap guide for a tour of what's out there
But if you need something right away, and the surface surrounding the hole is flat, you can use a plain nut.
Obviously, the size should be the same. Thread the nut onto the tool until the tip just pokes out.
Press the assembly against the opening so that the nut face sits flat. You may need to deburr the hole if anything is sticking up above the flat surface, use a deburr tool or countersink, even a flat file can be used carefully.
Hold the nut loosely, and rotate the assembly until you feel the threads engage. Don't be afraid to back off and try again.
Hold the nut steady, and rotate the tool by hand to ensure that you have caught the threads. Not holding the nut steady could result in pulling the threads out, so take care and feel what's going on with your fingertips.
Once engaged, thread the nut up the tool to get it out of harms way, you can apply more force to the tool to chase the threads,
A nut driver handle is better than a T-handle, as it limits the torque.
Taught to me by an old grizzled machinist who cut his teeth building Liberty boat motors during WW II.
HTH
Carter
I know this may sound elementary to most but for those who don't have a lot of experience make sure you are matching the tap or thread chaser pitch to the threads pitch you are trying to chase. Most thread systems have both fine and course threads. For instance 1/4 inch has both 20 and 28 threads per inch and there are differences with metric also.
I make thread chasers out of bolts all the time, to clean gick out of threaded holes. The key in making a chaser is to cut the flute much deeper than the thread root, so the gick has a place to go. When I do a head gasket and am installing new bolts, one of the old bolts gets a flute cut in it, and then I cut the head off and grind three flats on that end so I can chuck it into a light duty drill. Because cleaning 10-30 head bolt holes by hand is tiring, and air ratchets might wreck the threads if you're not careful. (Impact guns are right out)
I have a collection of homemade 9x1.25 and 11x1.50 and other oddball sizes as a result 
Another good tactic for fixing damaged threads is to thread a bolt in BY HAND until it becomes tight. Tap the head in and sideways all around, thread it in some more, tap tap tap, thread in some more. This way you are hammering the threads back into place instead of just ramming new threads through. Sometimes it even works.
I beat this into my apprentices regularly.
Taps CUT metal.
Thread chasers move the metal back into place.
If the threads are chewed up, exactly how are you making them better by removing metal?
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