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HP41C nuts - how tight ? - Printable Version

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HP41C nuts - how tight ? - John Robinson - 05-01-2010

Hi all, just wanting to know how tight to tighten the two nuts in an early 41c, i.e. number of turns. I don't want to over tighten and strip the thread on the screw posts.


Re: HP41C nuts - how tight ? - db (martinez, ca.) - 05-01-2010

someone must know a best way but here's what i know:
the threads are fairly steep so when the case pulls together, that's about it.
start by spinning the screw backwards and when it falls/drops at the end of the threads; start spinning it the right way; that'll start it in the old threads
if the threads are in good shape and you haven't crossed them; it will screw easily till you get all the way in and then get tight all of a sudden.
you can use a screw driver with a small handle or grip the shaft only, so you don't easily over-torque it
if you find a funky screw; do the other three first to practice getting the range.
one should be able to screw those things in and out a hundred times carefully, or not quite once if you reef on it so it's good you are thinking ahead.
i've never seen one stripped but i've seen several split - you probably knew that.
i've never broke one, so neither will you.
and last but not least: lefty loosey, rightsy tightsy. but you knew that one too.

Re: HP41C nuts - how tight ? - John Robinson - 05-03-2010

Hi DB,

Thanks for the response. I'm actually not concerned about the four case screws, but the two nuts (approx 1/4" thread) inside the early 41c units that is used to hold the CPU board to the keyboard, hence applying pressure to the connectors between the two boards.

Any suggestions in these ?


Re: HP41C nuts - how tight ? - Geoff Quickfall - 05-03-2010

Very difficult to get this right.

The pca posts were not tapped in the first place. The nut was simply added and it self tapped! If it doesn't line up then you will cross thread and poof, no more lock down.

The best you can do is try but only until just tight. Close it up and test the unit. Tighten some more if it did not power up due to a bad connection. Repeat.

If, according to the manual, the nuts do not tighten then switch to a smaller diameter nut. Not a very helpful suggestion. If smaller nuts are not available then take some vinyl tubing of the same type everyone uses for the pinch roller in the card readers. Cut a section slightly shorter then the amount of post that sticks out the PCA when it is in place (with the nuts removed). Place the tube over the post when the PCA is in place. Replace the back and screw together the case. This may take a few attempts to get the tube length correct. So cut it long, then shave off some. You are trying to make a pressure fit that relies on the bottom case posts without over stressing the case posts themselves.

What you have created is a pressure fit that relys on the bottom case screws to press against the vinyl tubes which then press down on the PCA (what the nuts used to do) and complete the connection.

It should look similar to this:

Hope this helps.


p.s. In repair attempts on two of these types I have resorted to the above repair. I am either clumsy or this really was a bad design not conducive to more then one or two removals. Thus necessitating the newer design about one year later.

Edited: 3 May 2010, 11:15 a.m.

Re: HP41C nuts - how tight ? - John Robinson - 05-04-2010

Thanks Geoff for the suggestion. I presume I can try a CV case back as well to apply the pressure, to at least test that everything else is ok. The only caveat is thickness of the logic boards.

I am aware that the thickness of the 41 logic boards varied over the production run, do you have guidance on when the thickness changed. My understanding is, later fullnuts CXs (and maybe CVs) had thinner logic boards, hence the use of the thin nylon washers in some models.


Re: HP41C nuts - how tight ? - Geoff Quickfall - 05-04-2010

Hi John,

Don't have the specs on the thickness but I will measure some up as I have 6 different boards.

The advantage of the tubing is that it keeps the original halves together, will never strip again and retains the original serial number.

I have to check, but if memory serves, it was that the later boards with out the hex nuts did not fit the HP 41C case back. I may be mistaken though.

Cheers, Geoff

Re: HP41C nuts - how tight ? - John Robinson - 05-05-2010

Thanks Geoff