JHZR2
Staff member
This thread is related to frustration and issues with my Mercedes exhaust manifold project, but is another that I think will be universally useful.
Let’s get one thing straight upfront. When doing helicoils, you drill a hole and use an STI (Screw Thread Interface) tap. That tap isn’t the same size as a regular tap for the thread you’re cutting. It’s bigger to account for the insert. So, for example an M8x1.25 STI will be larger, around 10mm. It does not cut the standard thread and diameter.
On the aforementioned Mercedes project, which is documented on this site, I had multiple issues.
1) Helicoil branded Chinese coo tap wouldn’t start so couldn’t cut threads
2) Helicoil branded USA coo bottoming tap from reputable source wouldn’t cut threads that were made with another M8x1.25 STI tap that is known to be correct to allow Helicoil installation.
#1 may be as ridiculous as a metric/imperial misalignment. The M8x1.25 STI taps say to drill with a 21/64” bit. 21/64=0.328”. 0.328x25.4=8.331mm.
But some tables Say that for iron/steel, to drill with an 8.4mm bit! Only use 8.3 for aluminum or soft metals.
So is that it for #1? I needed to drill 8.4mm instead of 21/64?
I don’t know that it’s all of the story because a different Chinese tap with more of a taper started immediately and cut perfectly.
Ok, now the really interesting one, #2.
After the Chinese Helicoil tap wouldn’t start and did not work whatsoever, my wife went to AAP, and got their brand. Helicoil had a pointy Chinese tap and coils made in USA. The AAP kit had a tap with more of a taper, and coils made in “AU” (Austria? Australia?), with the (IMO nicer and better) Chinese tap cutting easily and well.
So here’s the confusion with #2 and why it took so long with the story. One of the holes is blind. Not a ton of depth. It will be holding a turbo on to the manifold. So I wanted an ideal repair at ideal blind depth to take the helicoil a bit below the surface.
Imagine my surprise when the bottoming tap to cut the final threads would not thread in. When I tried it wouldn’t go. When I looked at the threads, the properly labeled tap looked different in pitch. But why? Aren’t these things universal?
Here’s the case in point. Top one is Chinese Helicoil branded that would not start under any case. Middle is US made Helicoil branded bottoming tap. Bottom is the Chinese AAP store branded tap.
Here are the threads of the two kit taps:
I’d say more or less even.
Here’s the Helicoil bottoming (right) vs a regular plug chamfer (left) that I used to cut the threads I this project:
Note the thread pitch doesn’t line up!
Thread gauge shows similar. Tap I cut the threads with:
New helicoil bottoming tap in same M8x1.25 STI
Way off and similarly not working at 1.00 pitch. Yet the plug chamfer M6x1.0 helicoill branded that fit the 1.0 gauge perfect.
Could my actual Helicoil tap also be wrong?
I don’t know. The helicoils seem to mate the threads with the bottoming tap fine.
Perhaps easier/better than the kits:
So what’s right or wrong? I don’t dare put that bottoming tap into the blind hole because if I ruin the threads I’m done for.
I’m tempted to buy a US-made helicoil branded basic plug chamfer tap to verify how much difference there is.
But seriously, what gives? Why is Helicoil giving the bottoming tap an oddball and different pitch than anything, and causing a big issue with interchangeability? Who is right and wrong here? What lessons can we learn?
Is Helicoil and parent Stanley screwing up both the US and Chinese taps?
Let’s get one thing straight upfront. When doing helicoils, you drill a hole and use an STI (Screw Thread Interface) tap. That tap isn’t the same size as a regular tap for the thread you’re cutting. It’s bigger to account for the insert. So, for example an M8x1.25 STI will be larger, around 10mm. It does not cut the standard thread and diameter.
On the aforementioned Mercedes project, which is documented on this site, I had multiple issues.
1) Helicoil branded Chinese coo tap wouldn’t start so couldn’t cut threads
2) Helicoil branded USA coo bottoming tap from reputable source wouldn’t cut threads that were made with another M8x1.25 STI tap that is known to be correct to allow Helicoil installation.
#1 may be as ridiculous as a metric/imperial misalignment. The M8x1.25 STI taps say to drill with a 21/64” bit. 21/64=0.328”. 0.328x25.4=8.331mm.
But some tables Say that for iron/steel, to drill with an 8.4mm bit! Only use 8.3 for aluminum or soft metals.
So is that it for #1? I needed to drill 8.4mm instead of 21/64?
I don’t know that it’s all of the story because a different Chinese tap with more of a taper started immediately and cut perfectly.
Ok, now the really interesting one, #2.
After the Chinese Helicoil tap wouldn’t start and did not work whatsoever, my wife went to AAP, and got their brand. Helicoil had a pointy Chinese tap and coils made in USA. The AAP kit had a tap with more of a taper, and coils made in “AU” (Austria? Australia?), with the (IMO nicer and better) Chinese tap cutting easily and well.
So here’s the confusion with #2 and why it took so long with the story. One of the holes is blind. Not a ton of depth. It will be holding a turbo on to the manifold. So I wanted an ideal repair at ideal blind depth to take the helicoil a bit below the surface.
Imagine my surprise when the bottoming tap to cut the final threads would not thread in. When I tried it wouldn’t go. When I looked at the threads, the properly labeled tap looked different in pitch. But why? Aren’t these things universal?
Here’s the case in point. Top one is Chinese Helicoil branded that would not start under any case. Middle is US made Helicoil branded bottoming tap. Bottom is the Chinese AAP store branded tap.
Here are the threads of the two kit taps:
I’d say more or less even.
Here’s the Helicoil bottoming (right) vs a regular plug chamfer (left) that I used to cut the threads I this project:
Note the thread pitch doesn’t line up!
Thread gauge shows similar. Tap I cut the threads with:
New helicoil bottoming tap in same M8x1.25 STI
Way off and similarly not working at 1.00 pitch. Yet the plug chamfer M6x1.0 helicoill branded that fit the 1.0 gauge perfect.
Could my actual Helicoil tap also be wrong?
I don’t know. The helicoils seem to mate the threads with the bottoming tap fine.
Perhaps easier/better than the kits:
So what’s right or wrong? I don’t dare put that bottoming tap into the blind hole because if I ruin the threads I’m done for.
I’m tempted to buy a US-made helicoil branded basic plug chamfer tap to verify how much difference there is.
But seriously, what gives? Why is Helicoil giving the bottoming tap an oddball and different pitch than anything, and causing a big issue with interchangeability? Who is right and wrong here? What lessons can we learn?
Is Helicoil and parent Stanley screwing up both the US and Chinese taps?
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