MTFCA Forum

I have a few rear spring perches with good threads, some with their original nuts and some without. The nut thread size is 11/16” – 16 tpi. Having bought a couple of new perch nuts from the vendors, I find these replacement nuts cannot be threaded on the perches. Some rudimentary measurements are as follows:

Original Perch thread OD = 0.685”
Original Nut ID = 0.629”
Repo Nut ID = 0.622”

The new nuts are too “tight” to thread on the perches.

Has anyone found this situation too? Do I buy an 11/16-16 tap and run it through the repo nuts? And if I can find such a tap will it be sized for the original nut threads or for the modern replacement nuts? I haven’t been successful in finding the specifications regarding the major and minor dimensions for 11/16-16 taps threads, so I don’t know what is considered standard.

I know I can ask the vendors to confirm their rear perch nuts are correctly sized, and I probably will, however, I’d like to try and fix the problem myself first.

Thank you for your experiences.

Rich C.

however, I’d like to try and fix the problem myself first.

I know that's why I like the hobby myself too

Sorry, I have no technical info on this, but just wanted to say that I like your approach

Cheers, Adrian

.625 is the correct tap drill.

You're holding the parts, not me, so I of course cannot be sure, but I'd sure suspect that you're dealing with a minor bur on the thread and little else. Taps are taps. Larger drill or smaller drill simply leaves lesser % or greater % of thread engagement. At .625, the thread of the nut is well less than a point, so cannot be the source of the bind on the perch. With that size of a thread, a few thou one way or another is not the source of the problem of whether or not it threads onto a bolt.

Now, if this was threaded in a lathe, there are a dozen ways to do it wrong and you may unfortunately be dealing with such a situation. The vendor surely should be made aware of this and will make you whole...that is an expensive tap to just go buy to repair something that should be right in the first place.

My guess is that it was made with a thread mill and depthing was off by a few thou and their quality system was not robust enough to catch the error. If yours are truely wrong, they're all wrong.

Scott C is probably right on the money!
Those nuts are an old used-to-be standard, that hasn't been used significantly in several decades. For that reason, nuts are made in small runs by specialty manufacturing companies, and if their quality control is a bit lacking? Poor fit is a likely result. The odd size also means that taps are short run, and specialty made! And therefore a bit pricey! I don't recall what the price was for the tap, but the last time I priced the nuts they were over twelve bucks apiece!

Yes, you should make the parts dealer aware of the fit problem. A good tap would probably fix this issue, but given the cost for a one-time use? The dealer may have such a tap, or not. Give 'em a call.

Years ago, I knew I had several, somewhere, but I couldn't find them. After finding the cost, I spent another hour looking and found ones I had. I don't have many (if any?) extras, but I was sure glad I bought a bunch of cheap junk many years ago and didn't have to pay the price for new ones.

Wayne, your search for the $$ tap made me chuckle. They say money cannot buy you happiness, but I have found that it does a fine job of financing the illusion.

The lack of money is frequently my inspiration to look harder in dark corners for stuff I know I have but cannot find. We are kindred spirits!

And for completeness for the OP, the entire range of acceptable tap drill for this part is .6200 to .6284, though going toward the small end means more work/HP to make the thread with negligable increase in thread strength. So again, it isn't the ID that is the problem.

Your nuts are under by .007, option A) redrill to 5/8 and re tap, or B) try another supplier. Also they could be metric by error. The 16 and 17 mm are very close and easily mixed up. And there are possibly 10 sizes in between. Some metric sizes are supplied as 16.1, 16.2, 16.3 etc. Some will go on 3-4 turns and bind up and others will not start. With air power they can be forced on and some time are. There are some bolt nut assemblies that are designed with limited clearance for a design tightness. Some metric and imperial look identical with only a few thou difference or a slight thread pitch difference. You may not have a mixture of metric and imperial but here in Canada it can be an issue and suppliers can get them mixed.

Rich,

I believe I have two used serviceable originals you could have.

Hank

Thanks to everyone for your thoughts and offers. I’ve been playing with my rear perch & nut pieces this morning. What I did was lightly file the tops of the treads on the perch and nuts to remove any burrs and slightly reduce the height of the threads. Then, using some valve grinding compound, little by little screw on the nut constantly using a back and forth motion until the resistance subsided. That worked well, and after a good cleaning I’ve had excellent success. 11/16-16 taps and dies are not common Wholesale Tool items for home use, so I’m happy I didn’t have to locate any.

Rich C.