Weird Metric Thread on Air Compressor Switch

[SteveHGraham]

I ordered a 1/4-18 tap. Not sure if I'll use it. I don't want to wreck the threads on the switch housing. What would I do then?

The threads on the fitting seem to fit an 18 TPI gauge exactly, though.

[SteveHGraham]

I don't quite comprehend what happened here, but I succeeded in fixing it. I made an aluminum fitting with a 0.505" OD and 18 TPI threads. It went into the switch housing just fine, as far as I can tell. I took a $5 3/8" snubber hose I had lying around, cut it to length, immersed one end in boiling water, slipped it over the fitting, and clamped it. I just pumped the compressor up to full steam, and it's not leaking.

I do not understand why the Chinese would make an 18 TPI fitting with a slightly reduced OD, but it sure looks like that's what they did.

The original hose was an astounding piece of crap. As far as I can tell, it was doomed to repeated failure.

I hope it's okay to use pipe dope on aluminum. I figured it would be lube up the threads and help me out.

[Russ Hanscom]

Nothing wrong with using pipe dope on aluminum, in fact some form of lube or never seize is mandatory if you do not want galled threads.

[SteveHGraham]

I figured aluminum-to-aluminum was risky without something a little greasy.

[Harold_V]

I do not understand why the Chinese would make an 18 TPI fitting with a slightly reduced OD, but it sure looks like that's what they did.

Not all that uncommon, apparently. I had the same experience recently, when I installed Susan's Bosch dishwasher. I confess, I don't know where the elbow fitting was made, however.

Harold

[Torch]

I don't think anyone would bother making a thread virtually identical to a standard imperial size but slightly off.

Good point, but the Americans did anyway. Historically, British Standard Pipe Thread (which by definition is the truly "imperial" standard) is based on the Whitworth thread form, dating from 1841 and used worldwide. National Pipe Thread was developed 25 years later, c. 1864, and use is pretty much limited to North America.

[SteveHGraham]

Maybe an 18 TPI tap is something every compressor user should have.

[dly31]

Both "1/8 inch" and "1/4 inch" pipe thread taps and dies are pretty useful and are often included in tap and die sets.

[ken572]

Here are a few more resources.

Edit: Update.
- - -
Gewinde-Normen (The Best On Threads Info)
http://www.gewinde-normen.de/en/index.html
- - -
The International Fastenings Locator 1980 (116p)
https://archive.org/details/FasteningsLocator1980
- - -
Bolt Science
http://www.boltscience.com/pages/screw4.htm
- - -
history of screw threads
http://www.sizes.com/tools/thread_history.htm
- - -
British Tools & Fasteners, LLC
http://www.britishfasteners.com/?gclid= ... fgodgokALw
or
http://www.britishfasteners.com
- - -
British Standard Whitworth (BSW)
https://en.wikipedia.org/wiki/British_S ... _Whitworth
- - -
What Is a Screw Thread?
http://www.wisegeek.com/what-is-a-screw-thread.htm
- - -
Screws, Nuts & Bolts on IndustryNet®
http://www.industrynet.com/search.asp?headers=SC0720
- - -
Sems Screw .com
http://www.semsscrew.com/
- - -
PSM i
http://www.psminternational.com/en/prod ... nload.aspx
- - -
Ken.

[SteveHGraham]

To anyone else out there who has this problem, my advice is to turn an 18 TPI fitting with an OD of 0.505". It should screw right in.

I'm a little concerned about the internal diameter. I used a 1/4" bit, and that appears to be a little smaller than the original fitting, but I guess I can fix that later, and anyway, it will work fine for a blow gun and filling tires.

[JackF]

This has been another interesting and educational thread. Thanks Steve.

Jack.

[warmstrong1955]

Maybe an 18 TPI tap is something every compressor user should have.

Did you not get the memo Steve?
You're supposed to have at least one of everything!