COMPASS, PRISMATIC, Mk3 (by Peter Laidler)

[Badger]

You can view the page at https://www.milsurps.com/content.php...Peter-Laidler)

Information

Warning: This is a relatively older thread This discussion is older than 360 days. Some information contained in it may no longer be current.

[bouletbill]

Concise and to the point, in laymans terms ... excellent .

Next project ! stripping and cleaning the bino's ?

[trademarklondon]

I am a compass maker by trade, and this article, while obviously written in good faith, gives fatally flawed information that will lead people to carry out work on beautiful, precision instruments, that will do such damage as to possibly render the compass beyond economical repair.

Over the years I have been asked so many times to repair bad servicing that I have lost count. I often wondered why on earth people would use putty or mastic instead of the 20pence purpose made o-ring that would be correct for the job. Mastic destroys the compass, it weeps into the fluid and coats the interior in such a way that it causes incorrect operation of the instrument. Basically it makes a mess and cam never provide a good seal, and for the compass maker who has to repair the repair it takes hours to remove. It is not just amateur, it is criminal. To seal the sealing glass you use a purpose made D profile o-ring that costs pennies, nothing more.

Special press to press down the sealing glass and mastic? Not really

Surgical spirits? Not really

PLEASE, anybody who has a G-150, and M-73, a MkIII or any other wonderful precision instrument, PLEASE do not follow Peter Laidler's instructions. I am sure he wrote them in good faith, but following those instructions would be like taking a Rolex watch to the butchers to be serviced... you WILL end up very disappointed and with an instrument that will probably then cost more to put right than the actual value of the instrument.

Peter, if you want me to re-write the article for you, and re-post it under your name in such a way not to unwittingly lead people to wreck their instruments then I would be happy to do so.

The average liquid prismatic compass comprises over 120 component parts. Working on these compasses is comparable to working on a wristwatch... you need special tools and to treat the instrument with respect and a delicate hand.

There is the QM's method of "repairing" compasses, and there is the compass maker's method... one leads to tears, the other leads to an instrument that will survive wars, arctic and desert conditions and that will last so long it will be passed from generation to generation as an heirloom.

I realise this is a fairly judgemental "first post" for a forum FNG, but the article will lead people to unwittingly destroy their compasses.

[Peter Laidler]

I didn't realise that there was a comments page or I'd have answered earlier!

Most people won't have the equipment that you or I have (had) access to nor the damping fluid - and that's all it is, a damping fluid. I'd like to say that the 'QM's' method you allude to is a bit, well, derogatory really because I was taught to repair compasses using the EMER method and have to say that so far as I am aware, have never butchered one yet although I have broken a few rotating bezels by not pressing them on with the flat press adaptor!

You mention that the article was akin to taking your Rolex to the butchers too. You'll probably have guessed that we were also taught wristwatches too and I've cleaned/repaired/rebuilt many of them. Obviously not Rolex's because we didn't have them but we did have and clean/repair/rebuild to EMER spec. the tat that we did have, such as Omegas, Longines, Jaeger le Coutre, Eterna, Record, Vertex plus some. Not Rolex I agree but.......... Well, the two Omegas that I did liberate plus the Longines that I gave to my son a couple of years ago are still running like, er.........., clockwork!

I've got a couple of calibrated compasses here and due to the lack of Isop, are now filled with degassed methylated spirit - and still going strong. So while it might not be to the original spec, it is as good as Joe Public can get and it works

[kjplzak]

... To seal the sealing glass you use a purpose made D profile o-ring that costs pennies, nothing more.

Where can I obtain such a D-profile o-ring? What size do I look for?

[grads]

Peter recommends gunge or putty, but you will need a special flat bench press and a suction pump - maybe the Doctor will bring it in his tardis

[puck finn]

Dear Sir:

I've had one of these old compasses laying around for awhile hoping one day I might learn how to get rid of the bubble. I loved the mother of pearl card and though I knew next to nothing about compasses, I figured the card composition meant quality. Now having stumbled across your article and this wonderful site, I spent a few hours in the morning disassembling far enough add isopropyl alcohol, work the bubble out, then carefully work the screw down enough the create a vacuum. I'll do the compression a few more times next week then we'll see if the gasket that came with it will do the trick. Is there anything to smear on the bowl to keep it from slowly seizing to the case?

Regards,

Tami Hansen

P. S. I'll hunt round this site some more and see if someone has done a similar tutorial for the U. S. mil regular issue soldier's compass. I have been in the army for just over four years and the intricacies of the compass are not understood by many of my comrades. I have about 5 of those compasses, starting with a radium painted dial imprinted 12-59 on the cover. The rest starting with one from 1972 through the late '80s, appear to have the tritium tubes (the early '70s one doesnt specify anything other than that it is radioactive.)

---------- Post added at 09:15 AM ---------- Previous post was at 09:12 AM ----------

Dear Sir:

I apologize, but I have one more question. Any notes on calibrating the prismatic or U. S. compass?

Thank you,

Tami Hansen

[nagamitsu]

Hi Peter,

It would be nice to have even more details just for curiositie's sake !!

Dear Sir:

I've had one of these old compasses laying around for awhile hoping one day I might learn how to get rid of the bubble. I loved the mother of pearl card and though I knew next to nothing about compasses, I figured the card composition meant quality. Now having stumbled across your article and this wonderful site, I spent a few hours in the morning disassembling far enough add isopropyl alcohol, work the bubble out, then carefully work the screw down enough the create a vacuum. I'll do the compression a few more times next week then we'll see if the gasket that came with it will do the trick. Is there anything to smear on the bowl to keep it from slowly seizing to the case?

Regards,

Tami Hansen

P. S. I'll hunt round this site some more and see if someone has done a similar tutorial for the U. S. mil regular issue soldier's compass. I have been in the army for just over four years and the intricacies of the compass are not understood by many of my comrades. I have about 5 of those compasses, starting with a radium painted dial imprinted 12-59 on the cover. The rest starting with one from 1972 through the late '80s, appear to have the tritium tubes (the early '70s one doesnt specify anything other than that it is radioactive.)

---------- Post added at 09:15 AM ---------- Previous post was at 09:12 AM ----------


Dear Sir:

I apologize, but I have one more question. Any notes on calibrating the prismatic or U. S. compass?

Thank you,

Tami Hansen

[puck finn]

Hi Nagamitsu,

What kind of details do you mean? I suppose a photo is worth a thousand words so I'll see what I can do about posting pics of these compasses.

Sincerely,

Tami

[nagamitsu]

The calibration of the Prismatic compasses.

Hi Nagamitsu,

What kind of details do you mean? I suppose a photo is worth a thousand words so I'll see what I can do about posting pics of these compasses.

Sincerely,

Tami