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15/32 -18 ??? Thread size

Froneck

Titanium
Joined
Dec 4, 2010
Location
McClure, PA 17059
A friend of mine purchased a BMW. I'm not sure what the parts do but got a new bracket to mount the strut. The nut supplied will not fit the rod on the strut. In checking the nut it is near 12mm but not 1.75 or 1.25 mm thread. I screwed the nut onto a piece of plastic. Using metric thread gauge it's not 1.5mm either. I used my inch thread gauge and 18TPI (.0555) is perfect. That is dam near 1.4mm (.0551). Gauge pin put in the nut (tap drill size) is .412" snug fit.
I tried threading a rod 15/32-18 with a .412" pilot. 60° threading tool. When I got near the pilot diameter I ran a 60° file in the threads to remove any burrs and kept testing the size with the nut as I took light cuts. I continued until any increase in thread depth reduced the OD. Nut still will not screw on. Any one know what the thread size might be and the thread specs so I can thread this strut rod. Possibly not 60°??
 

Kyle Smith

Stainless
Joined
Apr 25, 2008
Location
Helmer, Indiana, USA
I know nothing about BMW struts, but are you sure it isn't M12 X 1.25? Pitch is only 0.006" different.

Edit: After comparing 1.25mm, 1.5mm, and 18TPI thread gauges it would be clear they are different, never mind. 1.5 is even closer at 0.004".
 

tdmidget

Diamond
Joined
Aug 13, 2005
Location
Tucson AZ
What do these parts do? A "strut" is a common name for a critical suspension part. Who made the parts? That might reveal a clue.
 

fciron

Stainless
Joined
Oct 14, 2009
Location
Louisville, KY, USA
If the outside of the thread has a sharp point, which your descriptions suggests it does, that could intervene with the for in the mating part. Most thread standards call for some shortening of both the thread root and the peak.
 

Froneck

Titanium
Joined
Dec 4, 2010
Location
McClure, PA 17059
Definitely not 12mm X 1.75 or 1.25. I have both taps. I can see it's not 1.25 as it is like looking at 1/2-13 and 1/2-20. Visually I can see it's not typical 1.25 thread size but did try the taps, neither would start. I also see 12mm X 1.5mm taps are available. But after screwing the nut on plastic as mentioned ruled out 1.5mm. I seen that first thread aligned but second was slightly farther than the grove in plastic and fifth was almost in line with the peak of the thread. Also as I said 18TPI fit the plastic I made by using the nut as a die. No it's not 7/16 X 18, I have a tap and it's sloppy loose in the nut.
So yes I assume it to be a British special. Anyone have the specs on 15/32-18, there is a tap in Australia on ebay 15/31-18 BSF. Don't need a tap and can thread on a lathe both inch and metric, I simply need to know thread information. Is it also 55°?
 

Limy Sami

Diamond
Joined
Jan 7, 2007
Location
Norfolk, UK
IME&O Unlikely to be 55deg.

But f it's 55deg, it will show a little light through an 18tpi 60 deg thread gauge

As for the numbers, when stuck I find a thread pitch that matches, so in your case I'd use the data for 5/8 - 18 UNF and alter the diameters.

What I would do is first make a male copy of the thread you have to use as a gauge, using the 3 wire measuring technique.

YMMV
 

Paolo_MD

Stainless
Joined
Apr 6, 2013
Location
Damascus, MD
I think that what Liny Sami meant is that it doesn't appear to be a standard British thread.

Two comments. First, although Froneck is far from being a beginner, I would be careful on the outside diameter of a cut thread growing as the crest get pointy.
Second, especially if it is for an old BMW, it does not surprise me of being imperial thread. It's always a challenge to measure female threads, especially for the angle. I'd suggest you "massage" a little bit your test plug and, when it screws in, blue it and see which part of the thread profile rubs against the female thread: this should give you a hint if the original thread is indeed 55°.

Paolo
 

BT Fabrication

Stainless
Joined
Nov 3, 2019
I would almost Guarantee it is M12x1.5 . being a strut bolt, they ususally deform the first threads on purpose to act as a lock nut. I have also had shitty cheap parts come with oblong nuts due to too much deformation.
 

john.k

Diamond
Joined
Dec 21, 2012
Location
Brisbane Qld Australia
Cone lock nuts are deformed almost to a triangle in the coned part,and commonly used on shocks.Allow adjustmet of rubber mount deformation ,more secure(and probably cheaper) than a nylock.
 

Froneck

Titanium
Joined
Dec 4, 2010
Location
McClure, PA 17059
I would almost Guarantee it is M12x1.5 . being a strut bolt, they ususally deform the first threads on purpose to act as a lock nut. I have also had shitty cheap parts come with oblong nuts due to too much deformation.

I assumed the nut could have been deformed. In addition to the gauge pin, I used Starrett 123-6" vernier caliper and did not detect any out of roundness in as far as the inside measuring probes on the caliper could reach on both sides, when testing on my test screw I used the side opposite the cone. Also used Starrett 700a inside mic with same results. Using the nut as a Die I threaded a piece of plastic about 3/16" wide X .470" in height. I went on with out any change in rotational force and came off smooth. Plus as I said I used a metric thread gauge on the plastic, 1.5mm was not a match, 18TPI was. My Starrett metric thread gauge does not have 1.4mm, it does have 1.75, 1.5 and 1.25mm none as I mentioned matched. Being 1.5mm and 18TPI are only .0035" difference if the threads were 1.5 I'm thinking it would start about 1 turn and as I increased depth of cut I would get a greater number of turns but what I got was about 1/2 turn that didn't increase as depth of cut increased.
Today I'm thinking of using the nut as a die to thread something soft, maybe wood. I have an optical comparator and can measure thread width. If I had another nut I would cut it in half so that I cold measure the threads in width and angle.
 

Froneck

Titanium
Joined
Dec 4, 2010
Location
McClure, PA 17059
Up Date

I machined a piece of black Delrin. Used the nut as a die. Checked the threads with a thread gage, again 18TPI is perfect. The 1.5mm thread gauge has 10 "threads" as I mention the last thread is quite a bit off. Put the screw on my comparator and measured 10 threads, .556" so pitch is 18TPI. Though it's not the best thread form I can see a radius at the bottom of the V and the top looks like it's radiused too. Looking at Machinist handbook Whitworth threads are radiused and 55°. However no where in the text on Whitworth threads does it say which V is 55° as I assume the other V is 65°. Because the threads are not cut in the screw the surface is not quite clear however it looks like if cutting the screw the tool has to be ground 55°.
Is that correct??
 

awander

Stainless
Joined
Jun 11, 2012
Location
Eastern PA
I machined a piece of black Delrin. Used the nut as a die. Checked the threads with a thread gage, again 18TPI is perfect. The 1.5mm thread gauge has 10 "threads" as I mention the last thread is quite a bit off. Put the screw on my comparator and measured 10 threads, .556" so pitch is 18TPI. Though it's not the best thread form I can see a radius at the bottom of the V and the top looks like it's radiused too. Looking at Machinist handbook Whitworth threads are radiused and 55°. However no where in the text on Whitworth threads does it say which V is 55° as I assume the other V is 65°. Because the threads are not cut in the screw the surface is not quite clear however it looks like if cutting the screw the tool has to be ground 55°.
Is that correct??

There is no 65 degree V anywhere in a 55 degree thread.

Make your tool's included angle 55 degrees and thread away!
 








 
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