Tap size?

[George Poynter]

The starting threads on my power steering cylinder (1967) are flawed and I can't start the male fitting. I want to run a tap in but I'm not sure what tap to ask for the 7/16 threads are not your 7/16"-20 nf. Were they metric back then? Any help would be appriciated.

[John D.]

Re: Tap size?

Geroge, I assume you don't have a thread gauge? Worth having in the tool box. You see a lot of 7/16-24 on compression fittings, etc. If you buy a tap set get a bottom tap as well. I usually will cheat and force a little o'ring down in the blind hole so I don't hurt the seat on FI stuff.
But I am only guessing for your cylinder. You said it's not 7/16-20 so the 24 is the next size up. Pick up the thread gauge and then there is no guessing. John

[George Poynter]

Re: Tap size?

John thanks, I was unable to find my tread gage, I'm sure it will show up after I buy another one. In the McMaster-Car catalog the next is a 7/16"-28 NEF, can't find 7/16"-24. The size I'm trying to find is the same as most of the brake lines that's why I thought someone out in NCRS-land would pop back with the correct tap size.

[Greg L.]

Re: Tap size?

George

I'd be carefull and make sure you have the correct tap size before trying what you may think is the right one. I don't know for sure but I'll bet that it is a NPS size and not something that a regular tap and die set will have. Those sets often have a 1/4" NPT tap but I have never seen one with a NPS tap.

What I would do is wait for someone that knows for sure what size it is to chime in or take a hose to your local specialty bolt/fastener store and see if they can get a tap size for you.

I went through all this a year ago when I repaired the A.I.R. tube threads on my exhaust manifolds which are also NPS. It was quite the headache finding the proper tap to say the least. Due to my threads being shot I ended up getting a NPT repair kit as recommended by a forum member. Yes the original was straight and I used a taper repair kit so I had to be carefull to not go to deep or the hole would be too big. Otherwise it worked like a charm. That being said if all you are concerned about is cleaning up the first thread or two, you might get away with a correct size NPT tap...I have never done this before though so I can't really can't say how well it would work.

[Dick W.]

Re: Tap size?

The thread is 7/16-24. I purchased a set of thread chasing taps for tubing several years ago. The bottom of the tap is counter sunk. I will post photo of tap/die combination when I get back home.

[George Poynter]

Re: Tap size?

I have since found my thread gage and yes it is a "24" thread. Now the problem is where to buy one. I've Googled all over the place and still can't find a supplier.

[Joe L.]

Re: Tap size?

George-----

The 7/16-24 thread size is among the thread sizes known as "national special". You won't find these in most, if any, hardware stores. You should be able to find them from sources like Grainger or McMaster-Carr. I don't know, for sure, if either of these carry this size, but I strongly expect that one or both will have this size.

[Terry M.]

Re: Tap size?

George,
Note that Dick used the term "thread CHASING tap." A thread CHASING tap or die is different than a tap or die that is designed to CUT threads. The standard thread cutting tool will make a deeper cut than the CHASING tool. You don't want to cut the threads -- you want to move them around -- to be a little imprecise about it.
Given the unusual size of what you are working with, you may be limited in your choices of tools, but you and others working on cleaning up already cut threads should be aware of the difference and take advantage of using the proper tool if you can.

[Dick W.]

Re: Tap size?

Here is a picture of the tubing thread chasing die/tap. Notice that it is a combination tool. I purchased this from Snap On many moons ago. It is not in their online catalog now, maybe the dealer could get it for you. The other set I have was made by Bonney, whom I am not sure is still in business.

[George Poynter]

Re: Tap size?

Good point on tap Vs chaser. My bad threads are at the top and I went thru "McMaster-Carr" again and found a tap under "special-thread hand taps". Now the fun part of getting it done, on top of everything else it goes into the cylinder at an angle! Thanks for everyones input.

[Clem Z.]

since the PS cylinder is most likely cast iron

i would try and just use a correct steel fitting with vertical grooves filed into the threads to act as a thread chaser. the fittings can be pick up at the auto parts

[Joe L.]

Re: since the PS cylinder is most likely cast iron

clem-----

The section of the power steering cylinder that includes the threaded fittings is, I believe, manufactured from steel rather than cast iron. However, your suggestion might still work.

[Clem Z.]

Re: since the PS cylinder is most likely cast iron

i have used this on items i did not have a proper tap for and also have use nuts the same with with grooves filed inside in place of dies to straghten out buggered up threads. make sure you use some lube like moly kote to prevent any gauling since the thread chaser is not hardened. if you have a torch and some case hardening compound you could harden the piece

[Terry M.]

since the PS cylinder is most likely cast iron

Good suggestion Clem. I used this technique (file a groove or two on the male threads) on the left hand threads on the threaded holes in a ring gear after I got the bolts out without any threads on them. I left the bolt threads in the tapped blind holes of the ring gear.

Of course the ring gear is harder than hammered (*&& so no to minor damage to the threads there, but finding a left hand NF thread chaser when the differential is all apart was not in the cards. I was fortunate the local Chevrolet dealer had a LOT of the required bolts on hand -- and we all know why.

I'm sure I wasn't the first to learn these fasteners are left hand thread the hard way, and I won't be the last. On the other hand, I'll remember next time -- never mind that the service manual warns they are left hand thread. Us experiential learners (latest educational buzz-word there) have a good deal of fun on the way to knowledge.

[Gerard F.]

67 PS cylinder- I thought the threads

on mine were all bunged up when I was working with the cylinder on the car. But I was wrong, look at the angle of the tap in the picture:


The view is from the underside, if the ram is on the car. It must be at least 15 degrees off the axis of the cylinder.

The little brass fitting from a copper tubing compression fitting (water plumbing) was used to run the threads, and get the right angle. At the right angle the fittings from the lines threaded right in. (But after the cylinder was taken off the car

Check to see if you are not mistaken like I was. If they are really bunged up, follow the tapping or running advice of others who posted.

Having fun,

Jerry Fuccillo
#42179