CraigL

Hi,

Over the weekend I discovered the Aluminum Tow Hook plug that was screwed into the steel receiver on my GTS 18 years ago and promptly forgotten.

After trying several techniques to coax it out, I gave up and broke a 6mm allen socket (as was part of the factory design). Using a die grinder and several drills, I've got most of the plug removed. But I would like to clean it all up with a tap.

It turns out this is a weird non-standard thread. It is called a Knuckle Thread and I need a 20mm x 8 tpi tap. A 3/4" x 8 tpi tap would also be in the zipcode.

I cannot find a source for the tap. One of our RL members (JPitman2) had one made several years ago. Unfortunately, he's in Australia. So, what are you folks using and where did you buy it?

Whitesands

I need help finding a tap also, anyone have any idea of the cost of having a tap made?

hinchcliffe

Taps can cost $100 or so to make, especially if it is a one off. I'd look on-line in McMaster.com or MSCdirect.com. Call a few local machine shops to find where specialty taps can be made for you.

I don't see anything that is small TPI for 3/4 then the standard 3/4 - 10 tpi

dprantl

http://www.fastenal.com/ also has some obscure fasteners and taps, worth a look.

Dan
'91 928GT S/C 475hp/460lb.ft

CraigL

hinchcliffe: Both McMaster and MSCDirect have ACME threaded 3/4"-8 taps. These taps are priced in the nosebleed section.
dprantl: Tried Fastenal. No joy.
Whitesands: I've made contact with a company that makes custom taps. They are getting a quote.

Thanks for the suggestions. Please keep them coming!

nlneilson

I have never heard of a "Knuckle Thread" but that doesn't mean it does not exist or that it's not referred to by another name.

I tried the eye-bolt that came with the '79. It has been several years but IIRC that is just an Acme thread:
http://en.wikipedia.org/wiki/File:Acme_thread.jpg

White Lightnin'

Take the tow hook to a machinist and have him duplicate the threads in tool steel and then cut debris notches through the threads... just a thought.

danglerb

To clean threads you should not use a tap. Thread chasers are made to clean and reform the thread without cutting, where taps cut on each pass.

Good news is that its easy to make a thread chaser from a regular nut or bolt, you just cut a channel through the threads.

http://www.instructables.com/id/Inex...thread-chaser/

Chaser.

White Lightnin'

Yeah... that's what I meant!

Thanks Mike!

CraigL

Danglerb & White Lightnin': Excellent suggestion!

hinchcliffe

Craig I can hit up a local tap maker if you want. PM me the info (Diameter x tpi and overall length) I'll see what they say.

jpitman2

I am organising to send my tap over asap, probably to Alan, as I have sent him other stuff (blank gas cap pawls in steel) in the past. ACME thread is square section, where the knuckle is very much a rounded thread form, so I doubt it would fit at all well. I suspect its designed to be used in areas where debris is common, and it can tolerate rubbish . I have used my tap on a couple of cars to good effect so far. I also suspect that it wont be easy to find a bolt in 8tpi, 0.8" OD to convert to a thread chaser. in UNC, 8tpi is 1" OD, and in UNF the coarsest is 12tpi starting at 1" OD.
The tap is copied off a towing eye, so its not going to be as tight a fit as a proper tap IMHO. Also, looking in the hole, its not a thread cut into a solid piece of metal, I think its been made by stamping the panels around a threaded plug, and then cleaning it up a bit.
jp 83 Euro S AT 54k

inactiveuser1

Article at
http://jenniskens.livedsl.nl/Technic...5/MyTip545.htm

jpitman2

Thanks for the link to Theo's adventures. Such a pity that everybody refers to the thread by its metric measurements, when its plainly an imperial dimensioned item - 0.8" OD x 8tpi. I might have to see if our toolmakers can make me another one, if US people cant get one made.
jp 83 Euro S AAT 54k