Jerry Feather

The taps I ordered are the RD20x1/8 for the later 928s. I am holding off anything about the smaller taps for the early cars, and I'm now glad that I did.

I brought the Tow Eye in from my 79 to measure and now find that it is not 14mm or even 15mm, but looks more like 16mm. The OD of the male thread mesures .610 inch and that translates to very close to 15.5mm. Since this style of thread has some clearance between the outside diameter of the male thread and the outside diameter of the female thread, for me that translates to an RD16x1/8" tap. Any other discussion about this?

jpitman2

I make my small eye thread 15.8mm, which is pretty much spot on for 5/8".
jp 83 Euro S AT 57k

Jerry Feather

Thread in the hole or thread on the tow eye?

jpitman2

On the male thread.
jp

Jerry Feather

Thanks JP, but what is your significance for "spot on for 5/8"? My male thread measures a bit smaller than yours, but for a Knuckle thread the female thread is slightly larger than the male; I suppose to account for the debri allowance that this kind of thread is noted for. Too, that is probably why the thread for the early smaller thread Tow Eye is most likely 16mm by 1/8" pitch.
All the listed specs for this thread are in metric, except for the thread pitch; although I see that there is a later specification which uses metric pitch spacing.

CraigL

I see that you've already ordered the larger taps.
Several years ago I purchased one from these folks:

Cheboygan Tap and Tool
1306 Higgins Drive
Cheboygan, MI 49721
Phone: (800) 633-3133
http://www.cheboygantap.com/

As I recall, the tap cost around $125. Service was great.
I'm putting this in the thread for informational purposes.
At the time, it wasn't easy to find a source for a Knuckle thread tap:
>
> Thread Size & Pitch: 20mm x 8 tpi
> Thread Form: Knuckle Thread
> Right or Left Hand: Right
>
It worked like a champ. I used a 17mm drill to clean out the aluminum plug.
I still have the tap and drill. I loan it out a few times each year when someone needs it.

jpitman2

Jerry,
I dont believe its a coincidence that my tow eye thread is 5/8" - it doesnt make sense that threads with 8tpi would be made in odd metric ODs - note that all the Knuckle threads have imperial pitches. I would bet that the originals were made in imperial sizes, and they have been adopted by Europeans. Did you know that there are NO, NONE, ZERO metric size threads in refrigeration equipment? Told to me by people who make such stuff. Same as there are no metric sized tyre/wheel diameters, after one failed attempt - Michelin, 400mm.
jp 83 Euro S AT 57k

Jerry Feather

Actually, JP, I am pretty sure that it is. In the case of the early smaller Tow Eye, although your's measures 15.8mm, mine measures only 15.44mm, so mine isn't quite "spot on 5/8". Then, these measurements are only for the male thread and it appears pretty clear that the female thread is a bit larger, so even in your case that matching female thread is not going to be spot on 5/8" either. (Edit) Then too, as to the other taps called out in the catalog, such as 8, 10, 12, 14, 16, 18, 20, and larger, which are all in millimeters, what do you think they are "spot on to" in inches?

As to car wheels, it may seem logical to you that they are in inches, but then even a wheel such as a 16 inch wheel that is 7 and 1/2 inches wide is likely to have a tire put on it that is 16 inches but then its profile is likely to be 225 which is in millimeters. Too, that wheel, if on a Porsche, is going to have a lug pattern that is 5 by 130, which is also in millimeters. Go figure.

jpitman2

Jerry,
I wasnt implying there was any logic involved in wheel sizes, or thread sizes, just that they might both be due to inheriting something that works, is available in large numbers, and is cheaper than inventing something new.

Why do you find in model aero engines that the 2 ball races are frequently a mix of metric and imperial sizes? Because the lightly loaded front bearing (probably 1/4"x3/4") is made in huge numbers that makes it VERY cheap.

Why are Saturn rocket boosters the diameter they are - because they have to fit on existing rail cars for transport from manufacture to launch site.
Why are rail cars the size they are - because they fit on roadbeds the same size as roads used for carriages and wagons.
Why are the roadbeds the size they are - because they are the same as the excellent roads the Romans built all over Europe and England.
Why are Roman roads the size they are - because they allowed for armies to march 4 men abreast, and take supply wagons.

And so on.
jp 83 Euro S AT 57k

GlenL

"Why?"

Tevye knows.

"Tradition!"

jpitman2

Exactly!
jp

Jerry Feather

All this says now is that they are what they are, which is just what I have been saying, I think. However I think, before, you were, and may still be, suggesting that because your one example of 15.8mm being very close to "spot on" to 5/8 inch, that means that the knuckle threads originated in some kind of Imperial measurement setting. There is no evidence to support that; and your one example is in fact a mere coincidence.

Jerry Feather

I am making some progress with the Tap Kits and have several 15.25mm (.600") left twist drill bits that I am going to turn the drive end down to 3/8 inch so the user will be able to drill with them. I also have some 3/8 inch bits that are also left hand twist that can be used for a pilot hole. Then I have made some 1/2 inch pieces of cold rolled steel about 7 inches long that can be use as drive rods that I envision inserting into the 3/8 inch pilot hole before the larger bit is used and then it can be struck several times with a hammer to try to break up the corrosion that is going to be in there. I also have the number 6 easy outs for the kits and they might even be tried first in the 3/8 inch hole, although I am not too sure that that hole will be big enough, but that can be a first try at turning the aluminum plug out. If not, then the .600" bit will open it up and the easy out can be tried again.

What is likely to happen with the easy out in the larger hole, if the plug does not come out, is that the plug is likely to break in two just above the threaded part. Then the easy out will hopefully be put back into the stuck threaded part and hopefully it will come out.

A final step it it doesn't want to come out will be to drill it again with a 21/32" bit, but I have been able to find only one of the that is left hand twist; and it is a used one. At this point I am not planning on putting that large a bit in the kits, but we will see how my hoped for procedure works without it. I suppose I can redesign the kits and include a 21/32 inch bit that is right hand twist if it looks like that is what it is going to take.

Finally, except for the taps, I have several 3/8 inch drive extensions and some 14mm 3/8 inch drive 12 point sockets to use to turn both the easy out and the tap.

Then too I have some nice wood and hardware to make the packages for the kits. They will be made to just exactly fit the small USPS flat rate boxes.

The taps wont be here until mid January.

dr bob

Hi Jerry -

In my experience trying to easy-out soft metal like this, the easy-out expands the metal plug as it draws itself in. That seems to tighten the lock on the threads at least as fast as it improves the bite of the easy-out. Hole and easy-out need to be smallest-possible for the torque needed to break the plug threads loose without snapping the easy-out in the plug. In my car, drilling a series of small holes around the end of the impact-rated Allen driver I'd broken off in the plug proved the biggest bit of work effort. Then 5/8 and 9/16 regular drills hogged the lion's share of the remaining plug metal out. At that point it was good enough to use the drill clean to the threads. The tap made short work of the remaining aluminum in the threads at that point.

Knowing what I know now, I might be playing with a 1/2" or so left-hand-thread tap set, drilling and threading the plug itself as a first step. That's "first step before breaking the Allen driver in the plug"... Then a matching left-hand-threaded mandrel would go into the threaded hole, continue turning counter-clockwise to get the plug out. The plug depth isn't that great so tapping it isn't particularly dangerous to the fitting it's stuck in. No worry trying to scare up a 1/2-13 left-handed bottoming tap that way.

Might be a little out of the box, but...

Jerry Feather

The Taps arrived today from Poland. They look and feel outstanding. I think the difference between these and the tap made form an original Tow Eye is that these will clean the female thread which is larger than the male thread, by about .025 in. A tap made from a tow eye will be only about .780 in diameter and these taps measure .807 in diameter which accounts for the clearance this thread is noted for for use in dirty environments. I don't know just how that will affect the operation in use by any of us.

While I was waiting for these taps I used some of the other tooling I was gathering to try to remove one of my aluminum plugs. I had first tried to remove the plug with a hex wrench sized to fit snuggly, with no results; and then again after hammering on the plug a bit to try to crush some or any of the corrosion. Again with no results.

Then I drilled the plug out with a 3/8 inch left twist bit and tried the easy out, also with no success. The easy out will not fit far enough into the 3/8 inch hole to get a grip.

Next I drilled the plug out further with the .600 in. left twist bit and that actually was enough to cause the outer part of the plug to fall off. Then with the easy out I found that it locked itself into the hole very solidly, but with no movement of the remainder of the plug. What I think I determined is that at this point the easy out is simply locking itself into the steel of the threaded hole rather than the aluminum of what is left of the plug.

I also noticed that I had drilled off center some and that I have drilled into the female threads along the bottom some, so what is left of the plug is very thin there and very heavy along the top. So, I used a small rat tail file and filed a groove in the aluminum along the top down(or up) to the steel threads, but I haven't gone beyond that to see if I can pick any of the remaining aluminum out with a pick. I guess I will try that and then see just what results I get with the new taps.