Thread pitch of altimeter adjustment shaft?

birdus

birdus

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Jay Williams
I have a Cessna altimeter and another one out of a Luscombe. They appear to have the same pitch of the shaft onto which the adjustment knob fits. A .5mm thread pitch gage fits pretty well. Someone over on Practical Machinist suggested it might be 4-48, and that I try a 24 TPI gage to see if it fits (every other thread). That appears too coarse. The shaft diameter is about .113".

By the way, 4-40 is definitely too coarse.

So, what is the pitch of the shaft onto which the knob goes for adjustment of the altimeter?
 
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0.5mm would work out to 50.8tpi which is unlikely. It still could be 4-48, that's a standard UNF size, diameter .112, so it's likely it's a #4. It could be M3x0.5, but it's very unlikely that metric threads would have been used on an older instrument. Non standard thread pitches do exist in some specialty areas, though. First thing I'd try is to obtain a 4-48 nut and see if it fits. Both 4-48 and M3x0.5 nuts are readily available.
 
Unified Fine threads (UNF):

upload_2021-4-12_17-48-15.gif

If it was fine thread it would be #4-48. Pretty fine.

If it was coarse (not course) thread it would be #4-40.

upload_2021-4-12_17-49-35.gif

And that's assuming it's not a metric thread.

Might be 3mm, 0.5 mm pitch, too. 0.113" is really close to 3mm.
 
Dana said:
0.5mm would work out to 50.8tpi which is unlikely. It still could be 4-48, that's a standard UNF size, diameter .112, so it's likely it's a #4. It could be M3x0.5, but it's very unlikely that metric threads would have been used on an older instrument. Non standard thread pitches do exist in some specialty areas, though. First thing I'd try is to obtain a 4-48 nut and see if it fits. Both 4-48 and M3x0.5 nuts are readily available.
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Metric shows up in the strangest places. Many ball bearings are metric. The small 6xxx-series are in inches.
 
Ok...Ive been restraining myself, but I’ve just gotta say it...the thread pitch for an altimeter screw is 1000 feet per inch. :D

now back to something useful (hopefully).
 
MauleSkinner said:
Ok...Ive been restraining myself, but I’ve just gotta say it...the thread pitch for an altimeter screw is 1000 feet per inch. :D

now back to something useful (hopefully).
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Oh, we need to address the metric possibility along with that: 100 metres per 1.2 kPa.
 
For anyone who's interested...

The Cessna altimeter is labeled:

Cessna
United Instruments, Inc.
Wichita, Kansas
Made in Japan

The thread of its adjustment shaft is M3 x .5.

The altimeter that's out of my Luscombe (the instrument in question) is labeled:

Type MB-2
U.S.A.F Stock No. 6080-A80MB
Aerosonic Instrument Corp
Cincinnati, Ohio

The thread of its adjustment shaft is 5-44. Figuring it out felt like solving a mystery. Feels good to have succeeded.

Here's the new knob I made for the Luscombe. After adding a backlight to the instrument, not much shaft sticks out from the panel. For that reason, I couldn't use the locknut that was on it before, so I used set screws (I used 2 for reduced pressure on the shaft, as its tip is slotted and thus somewhat fragile). Also, I added a boss on the rear of the knob for added thread contact.

AltimeterKnob-1.jpg

AltimeterKnob-2.jpg

AltimeterKnob-3.jpg
 
Snowmass said:
Nice workmanship birdus. I love DIY. Of course that knob isn't certified so you may soon go to prison.
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“What’re you in for?”
“Uncertified knob. You?”
“I reported clear of the runway .379 seconds before I was actually clear. You wanna make something’ of it, punk?”
 
Snowmass said:
Nice workmanship birdus. I love DIY. Of course that knob isn't certified so you may soon go to prison.
Click to expand...

That is simultaneously both funny and not funny.
 
birdus said:
For anyone who's interested...
Here's the new knob I made for the Luscombe. After adding a backlight to the instrument, not much shaft sticks out from the panel. For that reason, I couldn't use the locknut that was on it before, so I used set screws (I used 2 for reduced pressure on the shaft, as its tip is slotted and thus somewhat fragile).
Click to expand...
That's a nice looking knurl... what pitch knurling tool did you use?
 
I'm seriously impressed. That looks very nice.

Nauga,
who is also an uncertified knob
 
What's the blue color on the knob.... from heat treat, or cold bluing? Or is it aluminum and you anodized it? Looks nice, either way.
 
Dana said:
What's the blue color on the knob.... from heat treat, or cold bluing? Or is it aluminum and you anodized it? Looks nice, either way.
Click to expand...

On all my monitors (3 of them color corrected, 1 not), it looks black (although I didn't color correct the photo, so maybe the color isn't perfect). It's painted (and baked) aluminum. The paint is black.
 
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