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Screws for HP Calculator
Message #1 Posted by Spice_Man on 22 May 2002, 12:24 a.m.

Has anyone found any good replacement screws for your HP calculators? I'm missing some screws on some of my calcs (Classics, Spices, Woodstocks, and 41Cs). Can the right size be found at local hardware stores?

Thank you!

Re: Screws for HP Calculator
Message #2 Posted by db(martinez,california) on 22 May 2002, 12:49 a.m.,
in response to message #1 by Spice_Man

try the fourms search function. about a year or so ago someone wrote the numbers/name for the 41's screws. the bad news is that he said he could only find them in huge lots.

Re: Screws for HP Calculator
Message #3 Posted by Ellis Easley on 22 May 2002, 8:22 a.m.,
in response to message #1 by Spice_Man

The tri-lobed "Plastite" screws made to cut threads in plastic will probably be hard to find in a regular hardware store. Is there anyone in the Denver area who can look in McGuckin's? I went to one of their stores while visiting in the late 1980's, it was amazing. They had a large selection of orange juicers (for some reason that is my most striking memory!) and I think they sold HP calculators.

I looked in the other day and they don't have screws made for plastic.

Since the threads are already cut, the important thing from a replacement point of view is to find a screw with the same thread pitch (and diameter, of course). The worst thing you can do is to start cutting a new thread, even of the same pitch. This is what leads to the situation where a small, spiral cut tube of plastic comes out of the hole, then the threads are destroyed and you will have to start over with a larger screw. This is why I am always extremely careful when replacing a screw - even the original one - in a plastic hole. I always rotate the screw backwards to let it fall into the original thread, then slowly start turning forwards. I'm in the habit of doing this with all screws, but threads cut by a self-tapping screw, in plastic or metal, are the easiest to mess up.

Sheet metal screws are the closest commonly available type and I've noticed they come in two very different thread pitches for the same diameter, often mixed in the same bin. I've never heard an explanation for this. If you cut off a sheet metal screw to get the right length, be sure to de-burr the leading edge of the thread or you might wipe out the threads in the workpiece the first time you drive the screw in.

McGuckins Was: Re: Screws for HP Calculator
Message #4 Posted by Ren on 22 May 2002, 10:08 a.m.,
in response to message #3 by Ellis Easley

I no longer live in Colorado, so I can't pop over to McGuckins (Boulder)during my lunch hour. It is a shame that Traylor Hardware in Longmont is long closed, they were almost as good as McGuckins when it came to stocking screws and bolts. I wonder how McGuckins is doing now with competition from Home Depot and other hardware superstores?

Re: McGuckin / Screws for HP Calculator
Message #5 Posted by W. Bruce Maguire II on 22 May 2002, 11:39 a.m.,
in response to message #3 by Ellis Easley

Wow! Lucky you! ;-)

I have not been keeping up with the Forum lately (months, actually!), but I looked yesterday and today...AND...I live in Longmont!

I'll go look at McGuckin's, but you have to help me out a little---can you give me specific names/types? What screws are we looking for? I have some classics and 41s, but no Spice or Woodstock calc.s. If you're looking for a classic or 41 screw, I can take sample into the store. Otherwise, I'll need a name or _really_ good description (so that I don't look like the lost fool that I really am in the hardware store!).


My McGuckin story...
Message #6 Posted by W. Bruce Maguire II on 22 May 2002, 12:01 p.m.,
in response to message #3 by Ellis Easley


You know, McGuckin claims to be "World Famous", but after seeing their name mentioned HERE, in this most-international of forums, I'm a true believer!

Here's my story (to match your "juicer-selection" story):

In 1988 I was an undergrad at CU. My pride and joy (and sink of all my income) was my 1986 Toyota MR2. My dad lived in Idaho, and that was where I bought the car, so I had Idaho plates. They were very nice commemorative plates: red, white, and blue. Well, one morning I went out to my car, and found that someone had stolen my plates! No doubt for some sort of plate collection. Well, even worse than that (in my mind), they weren't considerate enough to leave the screws! Not being hardware-inclined, I figured I had a big problem---how to get screws to fit the rear license-plate-holder, when I had no SAMPLE to show anyone!

Well, I went to McGuckin, figuring that it was a real long shot... I went in. I was directed to the "screw" isle, and found a green-smocked employee there. I sheepishly told him that I needed four screws to fit the rear license plate holder on a Toyota MR2.

Without a hesitation, he said:

"Toyota MR2, huh? What year?"

Being _REALLY_ impressed, I said: "1986." He went to a little drawer in a vast sea of little drawers, and brought me back four screws! And if there was any doubt, it was quickly erased, as the screws fit perfectly.

So, to this day, I don't know if this was an easy question or not, but it _really_ impressed me! It also cemented the fact that McGukin is the LAST WORD in hard-to-find hardware (and more!).


Re: My McGuckin story...
Message #7 Posted by Ellis Easley on 22 May 2002, 5:29 p.m.,
in response to message #6 by W. Bruce Maguire II

It was in Boulder that I went to McGuckin's. I forget what year. I had the impression there were more stores, is that correct? I remember the scale model of the solar system on the University campus. Also we went to the National Bureau of Standards, watched an orientation film starring Gomez Adams (it was John Astin, but he was smoking a cigar and grinning), looked at the atomic clock, learned that the UTC people (is that the right order? Coordinated Universal Time) use HP atomic clocks to keep all the national clocks in agreement, and how the NBS engineers had been trying for years to prove that a clock with a resonant cavity twice as long should work better, and how WWV uses a local TV signal to synchronize their clock in Fort Collins with the big clock in Boulder (that last one might be in the ARRL Handbook). Also went to NCAR, watched deer from the lunchroom window, saw the two Crays that they had, recognized the model number of IBM computer the Crays used for a disc controller as the same model Tandy used for their corporate mainframe, and saw for myself why those big disks are called washing machines.

Of course, there was a fire burning in the forest.

I pulled one screw from each of a 35, 25, 67, and 34C. They are all the same. I tried swapping them and all combinations I tried worked easily except one of the case screws in my 67 seems to be slightly smaller in diameter, and a bit longer, than the others. What is special about them is that they are made to be self-tapping in plastic and have a three-lobed cross section, one trademark for which is "Plastite". I thought they might be #4 but I found some "store-bought" #4 sheet metal screws which measure .110 inch diameter while the calculator screws overall outer diameters measure from .087 to .091 inches (that is the spacing I can set my dial caliper to, and the screws turn between the jaws without binding). The 97 service manual says the 97 case screws are "#4-20" (they are a lot bigger than the screws in the handhelds) and the screws that thread into the card reader plastic are "#2-28", which does appear to be the same as the handheld case screws. The length is 5/16 inch. The finish is called (I think) "black oxide".

Here's what I would do to avoid losing a screw or risking any damage to a calculator: drill some 1/16 inch holes in a piece of plastic about 1/4 inch thick. It should a similar kind of plastic to a calulator case, not like polyethylene at the rubbery end of the scale or Lucite at the very hard end. The first source that comes to mind is a piece of a TV cabinet. Thread a screw from your calculator into these holes. If you've never turned one of these screws into a fresh hole, you'll be amazed how easily the screw stays upright and cuts a clean thread (if 1/16 is the right size hole!) Then just take the piece of plastic to the store, along with a post-it that says "5/16" "round head" "philips (or is it Posi-Driv?)" "black oxide" and any other things you need from the store while you're out (save a trip!) and try the candidate screws you find in the threads that the HP screw cut. It's normal for Plastite screws to turn in slight fits and starts in a previously cut thread, because of the "tri-lobe" triangular cross section. While you're at it, it would be intersting to see if any sheet metal screws fit in the HP screw threads.

BTW, while I was looking for my #4 sheet metal screws, I found what looked like a regular #6-32 machine screw, but on closer inspection, had a similar triangular cross section, and I remembered that this is called a "thread roller". It is a self-tapping screw that is just slightly tapered at the leading end, doesn't have the deep "drill bit" notch that some other self-tapping machine screws have. At Tandy we used them in "extruded holes" in sheet metal, where some metal is pushed out when a hole is punched, surrounding the hole and providing a region that has room for more thread turns than the sheet metal thickness alone.

Re: My McGuckin story...
Message #8 Posted by W. Bruce Maguire II on 23 May 2002, 11:51 a.m.,
in response to message #7 by Ellis Easley


I'm really busy at the moment, but I'll try to get in to McGuckin this weekend. The store in Boulder is the only one! My wife's brother is good friends with the family that started/owned (I don't know if they still do...) the store.

From what David wrote below, exact replacements may not exist, but I'll see what they've got.

So, "Spice_Man", cross your fingers! ;-)


Re: My McGuckin story...
Message #9 Posted by Spice_Man on 28 May 2002, 4:42 a.m.,
in response to message #8 by W. Bruce Maguire II

My fingers are getting tired from being crossed for so long... :-)

Re: My McGuckin story...
Message #10 Posted by W. Bruce Maguire II on 28 May 2002, 4:15 p.m.,
in response to message #9 by Spice_Man

My fingers are getting tired from being crossed for so long... :-)

Sorry Spice_Man!

To tell you the truth, I thought the issue was dead, given the two follow-on posts that seemed to authoritatively say that the screws were not available (especially the one that mentioned that he looked a year ago, and even checked with McGuckin's supplier!).

So, I have not made the trip in.

But, since you have crossed your fingers for so long, I'll give it a shot.

Do you think you could wait until this time next week? I don't know if I'll be going into Boulder during the week---I'll try on the weekend if not earlier. OK?

I'll let you know.


Re: My McGuckin story...
Message #11 Posted by Spice_Man on 29 May 2002, 5:06 a.m.,
in response to message #10 by W. Bruce Maguire II

Bruce: Please don't go out of your way to try and find it... I don't think that an identical screw is available.

Something close may be, as posted on the other branches of this thread, at some other hardware websites...and I'm crossing my (tired) fingers that someone will report back and say they found a screw that works just right! Let's hear it for optimism! :-)


Re: My McGuckin story...
Message #12 Posted by Erik Wahlin on 23 May 2002, 10:42 p.m.,
in response to message #6 by W. Bruce Maguire II

I grew up in Boulder and checked McGuckins a year ago for the screws. They are (if my memory serves me right) a # 3 sheet metal and not available. I checked with Timberline fasteners (A&I Bolt) in the area (huge distributor that McGuckins get's their screws from)and they told me the only way to get them is a special order of 10,000 screws or more. I think I mentioned this in the forum a year or two ago.

Re: Screws for HP Calculator
Message #13 Posted by David Smith on 22 May 2002, 5:40 p.m.,
in response to message #1 by Spice_Man

The sacred screws are #3 self tapping sheet metal screws with a type "B" point. You won't be able to find them... anywhere... For $2000 bucks plus shipping a guy in Taiwan will crank out 250,000 of them though... or so it was several years ago when I last quested for the holy grail.

Re: Screws for HP Calculator
Message #14 Posted by CJ on 22 May 2002, 6:18 p.m.,
in response to message #13 by David Smith

I know nothing about these screws but after looking at some descriptions here, one thought came to mind.

Since I fly RC Helis I thought maybe a hobby shop might be another place to look.

Some of the parts on helis are plastic and have self tapping screws.

Just a thought. Good luck.


Re: Screws for HP Calculator
Message #15 Posted by David Smith on 23 May 2002, 5:41 p.m.,
in response to message #14 by CJ

Been there, checked that. Lots of stuff that is sorta similar but nothing that is proper.

Re: Screws for HP Calculator -- #3 x 3/8"? Pan Head?
Message #16 Posted by Paul Brogger on 24 May 2002, 2:24 p.m.,
in response to message #13 by David Smith

Here are some #3 x 3/8" pan heads. Will those work?

Re: Screws for HP Calculator -- #3 x 3/8"? Pan Head?
Message #17 Posted by David Smith on 24 May 2002, 5:03 p.m.,
in response to message #16 by Paul Brogger

It looks like they may be very close. I don't know if they have the type "B" tip or not. It may not matter, it is the little "V" shaped notch in the end that helps them cut threads in plastic. Also the regular HP screws are 5/16 inch long. Again it probably does not matter, you could probably grind the tip off. For less than five bucks a hundred they are certainly worth a try. They are also silver (like the early HP35 screws) and not regulation HP black. They could be black oxide coated at a metal shop if you are picky.

Re: Screws for HP Calculator -- #3 or #2?
Message #18 Posted by Ellis Easley on 25 May 2002, 12:32 a.m.,
in response to message #17 by David Smith

I was mistaken when I said I took a screw from my 35 yesterday, it was my 45! My type 2 35, which I have never opened, does have silver screws. (If it had HPIB, that would mean non-metric threads!) But otherwise it is identical to the screws in my other units.

You mention the V-shaped notch - all the calculators in my HP collection have screws without such a thread cutting notch. They have the blunt tip and the tri-lobe cross section which I recognize as similar to "Plastite" brand. As far as whether the pointed tip matters - in my opinion, if the overall length with the tip was no longer than the original, I wouldn't be concerned except that it means there will be a couple thread turns less hold. But if the tip made the screw longer than the original, I would be very concerned that if the tip hits the bottom of the hole, the screw will stop going in and will strip the threads if you try to tighten it any further - same with a longer blunt tip screw. If I checked the depth of the hole and it had more room, I guess it would be OK, except the circular cross section sheet metal threads won't cut a new thread in plastic as well as the tri-lobe thread will (of course, it would only be a thread or two).

Regarding the size #3 or #2 - I have no special experience here, I thought it might be #4 at the outset because that's the smallest size I usually see in common use. But here are the reasons I now think it is #2:

1. The handheld case screws I measured were from .087 to .091 inch in diameter. I got this measurement by finding the smallest setting of my dial calipers that allowed each screw to turn between the jaws without binding. This is necessary because of the tri-lobe cross section.

2. The HP97 service manual (on Museum CD 5) says on page 6-6 that the six screws (three 1/4 inch long and three 3/8 inch long) that thread into the plastic of the card reader are #2-28. The ones I have out for repair right now measure from .086 to .091 inch diameter. I held a 1/4 inch long one alongside the handheld case screws, crest to crest, and the thread pitch matches across 6 threads. Also, I tried the case screws in the card reader plastic and they fit just right.

3. has a page of stainless steel sheet metal screws which says the diameter of a #2 is .0891 inch. It doesn't say just how that is measured, but another page of stainless steel flat washers says the inside diameter of a #2 is .094 inch.

4. I have a little book "Pocket Ref", Second Edition, compiled by Thomas J. Glover. I dug it out because I noticed sells it - I got mine at Harbor Freight Tools. On page 277, it says the shank diameter of a #2 wood screw is .086 inch, a #3 is .099 inch, and a #4 is .112 inch. Then on page 278 it has some data on sheet metal screws starting with #4, it says the diameter of a #4 is .112 inch. Unfortunately, doesn't give the diameter of #4 and above sheet metal screws, and skips #3 altogether.

Even if a #2 sheet metal screw has the right diameter, I wouldn't try it in a calculator until I verified that the thread pitch was the same as the originals.

Re: Screws for HP Calculator -- #3 x 3/8"? Pan Head?
Message #19 Posted by David Smith on 25 May 2002, 10:18 p.m.,
in response to message #17 by David Smith

At one time I put #2, #4, and HP screws under a microscope. It's definitely not either. Also Katie Wass also came to the same conclusion.

Re: Screws for HP Calculator -- #3 x 3/8"? Pan Head?
Message #20 Posted by Ellis Easley on 26 May 2002, 4:10 p.m.,
in response to message #19 by David Smith

Have you compared the handheld case screws to the ones that hold the 97 card reader together? There are three 1/4" that join the two plastic parts and three 3/8" that hold the PCB. The case screws fit into the threads they cut perfectly. The 97 service manual says they are #2-28.

I think we are dealing with something similar to pipes: there are a lot of different kinds of pipe that are all nominally 1/2 inch but are all very different in outside diameter. It has to do with the difference in material, wall thickness and the inside finish, but all 1/2 inch pipes will supply about the same amount of water at a given pressure. Likewise, the "#n" sizes are used for a lot of different kinds of screws and probably refer to something fundamental like "holding power" but it results in something slightly different for each kind of screw. The screws in my HP calculators are not sheet metal screws. Sheet metal screws have a circular cross section. The three-lobed screws that HP used are made especially to cut threads in plastic. As I understand it, sheet metal screws have a tendency to crack plastic posts because being circular, they are exerting the same force in all directions from the center. The tri-lobes screws stretch the plastic into a triangular shape. The extremes of the lobes cut a deep thread but then move on, relieving the pressure between the lobes. It is as if you cut a thread with a certain size tap and then used a slightly smaller screw. It works, and it improves the situation, because plastic is resilient, which metal is not.

I ordered the #2 X 1/2" "plastic thread cutting screws" from the website that Paul found. It's the first time I've seen anything by that description offered in a catalog. I've been trying to remember the name of the company that used to supply hardware to the factory where I worked. They were in Dallas and "Central" was in the name and the guy who we worked with was named Joe Brown. It might have been Central Industrial Supply Inc., they are listed under fasteners in the Fort Worth yellow pages with a 972- number. I might just call them next week for a reality check! Of course, whatever we got from them we bought in large quantities and were probably made to order.

That's an idea for a new thread - things you have, or had, access to at work that you can't find as a retail customer!

Re: black oxide finish
Message #21 Posted by Ellis Easley on 26 May 2002, 4:24 p.m.,
in response to message #17 by David Smith

This is something I discovered by accident, it's not the same as black oxide but it is dark and, hopefully, corrosion resistant. I found that after leaving some slightly rusty plated screws soaking in CLR for too long, it had dissolved all the zinc. After I rinsed them they started rusting immediately. So I put them in the oven at 400F for about a half hour, sitting on a piece of foil. Then I took them out and immediately dribbled some 3-In-One oil over them. The result is a dark brown finish much smoother than black oxide (maybe it is just a very thin black oxide). I don't know how much the soak in CLR had to do with the result. If it is iron oxide, it would probably be necessary to remove any zinc or cadmium plating first.

Re: Screws for HP Calculator -- #3 x 3/8"? Pan Head?
Message #22 Posted by Ellis Easley on 25 May 2002, 12:57 a.m.,
in response to message #16 by Paul Brogger

I'm still thinking they are #2, but I saw an interesting thing on another page of the site you linked - click on "Self tapping or Wood Screws" and among the choices are "Self Drilling Screws - Phillips" and "Thread Cutting Screws for Plastic - Phillips - Ptd". They use the same image for both, which I recognize as the self-drilling screws used for galvanized steel studs used in building construction as a replacement for wood. The screws literally have a short length of twist drill at the tip to drill a pilot hole. Now, I wonder what the "Thread Cutting Screws for Plastic - Phillips - Ptd" really look like? And they only have one size - #2 X 1/2. I may order a bag, after I've looked around this site for a while.

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