A Question For Machinists on Rethread (Chaser) Taps

[CCI]

I've got a good set of rethread taps and dies AKA thread chasers, and I understand why it's not a good idea to use a regular tap to clean up an existing threaded hole.

The set only goes down to M6 and I need to clean up an M5 x 0.8 hole. I have had no luck finding an M5 thread chaser.

Would there be any benefit in tapping an M5 x .8 hole in some steel, then chucking the tap into a lathe, stoning down the major diameter a few thousandths, and then running the tap into the previously cut hole with a little lapping compound in order to make a slightly undersized tap? And then using that tap as a rethread tool?

While we're on it, how much smaller (proportionally) is a rethread tap from a cutting tap?

 

[spasm3]

EDIT: I'm not a machinist!

Take an M5 bolt and cut a couple of vertical slots in the threads. Run a nut up and down it once and the clean up with a wire brush on a bench grinder or on a drill.

Do it your self thread chaser.

 

[P10crew]

I would recommend getting an Allen head bolt of proper length and cutting a slot vertically up the threads with a 1/16” thick cut off wheel in a die grinder. That little slot will pickup a lot of trash and not damage the original threads.
When I was working we had to thread stainless bolts into newly galvanized blind tapped holes. They protected the holes when galvanizing but the rinse operations prior to galvanizing used acid and the holes were loaded with rust. Running one of these cut bolts in and out worked real well to clean them out prior to spraying with cold galv.
I have never cut a bolt that small but it should be worth a try.

 

[P10crew]

Treed by the spaz!

 

[atikovi]

I've got a good set of rethread taps and dies AKA thread chasers, and I understand why it's not a good idea to use a regular tap to clean up an existing threaded hole.

How bad are the threads. Sometimes the flat end of a chaser can't get a bite on the threads if they are very rusty while the tapered end of the tap works much better. Can't image anything with M5 diameter would be tightening something critical like a cylinder head.

 

[Trav]

I've got a good set of rethread taps and dies AKA thread chasers, and I understand why it's not a good idea to use a regular tap to clean up an existing threaded hole.

The set only goes down to M6 and I need to clean up an M5 x 0.8 hole. I have had no luck finding an M5 thread chaser.

Would there be any benefit in tapping an M5 x .8 hole in some steel, then chucking the tap into a lathe, stoning down the major diameter a few thousandths, and then running the tap into the previously cut hole with a little lapping compound in order to make a slightly undersized tap? And then using that tap as a rethread tool?

While we're on it, how much smaller (proportionally) is a rethread tap from a cutting tap?

You can either make your own or use a cheap 2B tap they are not as precise as a high quality HSS that is best for cutting new threads.
This is a cheap 2B tap, good for cutting warm Swiss cheese also.

Irwin Hanson High Carbon Steel Metric Plug Tap 5 - 0.80 mm 1 pc - Ace Hardware

IRWIN Hanson Metric Thread Taps (HCS) is ideal for auto repair and machinery where metric threads are specifiedFind the TAP CARDED 5MM-.8MM at Ace.

www.acehardware.com

 

[atikovi]

Have always used regular taps to clean up threads but after this thread I ordered a basic set of chasers. https://www.amazon.com/dp/B000AEX8DW/ref=pe_386300_442618370_TE_dp_i1

 

[D60]

I'm a machinist and of the opinion - based solely upon my personal experience - that thread chasers are 50% effective and 50% "urban legend" that people love to point out to others to try to sound really knowledgeable about the trades...

They're a "you're not supposed to...." people use to try to sound helpful but really just want to impress others.

I've got the typical KD/Lang/SnapOn kit and they're soooo loose that in several instances (for me) they just don't do enough.

If you have threads that are way tweaked....yeah, maybe they'll shove 'em back into place. But it might still put up a fight going back together.

If you're trying to clean dirt and rust out of the thread roots, good luck -- you really need a hex die (or split die if you want and allows for ultimate adjustability)

I've had the "restorer" dies spin over rusty, crudded fasteners by hand with ease, then I ran a hex die over it and got a pile of dust out of the roots.

TL;DR: the restorer kits are a useful tool in your arsenal but BY NO MEANS a panacea....and honestly they're not as awesome as your friends will tell you at a cocktail party when they start with, "You're not supposed to...."

 

[paulo57509]

I'm not a machinist; one question I always ask myself when confronted with buggered threads is, how critical is the fastener that's going to go into the repaired threaded hole?

 

[clinebarger]

I'd run a M5 tap just to see what happens first. Chasers are good for cleaning out Dirt/Debris/Paint/Thread Locker & making minor thread corrections.

If that doesn't work & you have space allowed....I'd tap it to M6x1.00 after drilling out the M5 treads. I don't always follow the recommended drill size as a slightly smaller size can result in a better cut.

There's always the TimeSert route.

Not a machinist....Just deal with F'ed up stuff on a daily basis.

 

[HangFire]

TL;DR: the restorer kits are a useful tool in your arsenal but BY NO MEANS a panacea....and honestly they're not as awesome as your friends will tell you at a cocktail party when they start with, "You're not supposed to...."

I have the Lang kit as well, and yeah. Sometimes it's awesome, like when it reformed the mangled crankshaft end threads on my Kohler V-Twin perfectly. Other times they seem to do nothing useful and I reach for the gun, tapered, or even the plug tags. TL;DR for me - keep trying until you find what works. Like you say, keep more than one tool in your arsenal.