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03-16-2016, 11:22 PM   #16
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QuoteOriginally posted by TomB_tx Quote
OK, I've re-checked the gears that adjust the shutter curtain tension on my MX examples. Since the serial numbers are on the bottom plate, which can be swapped, and the film box frame is on the back, which is removable and could be swapped, I'm not sure what conclusions to draw.
The "white" plastic gears are on my example serial 4390xxx, which also has the plastic film frame on the back. The gears look like this:
Attachment 302963
I have three in the 918xxxx to 932xxxx serial range, and all these have the metal film frame, and gears that look like this:
Attachment 302964
And finally my black MX has serial 9479xxx has a plastic film frame, and gears like this:
Attachment 302965
The darker gears of the middle three look like steel or maybe heat treated brass. The "brass" gears on the last are...brass.
Often on high volume production items there are many running changes in internal parts that are not specified, if deemed interchangeable. That may be the case here.
My 9229354 has black cogs as shown in the centre. One would expect that it would be the later models that went to plastic. Obviously not a good move.

03-17-2016, 04:10 AM   #17
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Not only did the material change, but the thread form changed with the plastic type. The metal gears use a modified acme thread with a more square shape, where the load on the thread flank is more in the direction of thrust, which is better for high loads. The triangular shape of the threads used with the plastic gears will apply more force tending to separate the gear from the worm - making it more likely to jump teeth and slip.
03-17-2016, 03:20 PM   #18
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QuoteOriginally posted by TomB_tx Quote
Not only did the material change, but the thread form changed with the plastic type. The metal gears use a modified acme thread with a more square shape, where the load on the thread flank is more in the direction of thrust, which is better for high loads. The triangular shape of the threads used with the plastic gears will apply more force tending to separate the gear from the worm - making it more likely to jump teeth and slip.
Just to add to the information, I checked my 4096519 MX and it has the brass colour metal cogs. I wonder if the plastic was used with the last run, perhaps substituting components from another line of models.
03-17-2016, 11:26 PM   #19
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QuoteOriginally posted by arnold Quote
Just to add to the information, I checked my 4096519 MX and it has the brass colour metal cogs. I wonder if the plastic was used with the last run, perhaps substituting components from another line of models.
Might not be serial number dependent running a production line one has several sources and select the cheapest bidder which might vary between production batches.

The manual for my Auto calls out a French or Yugoslavian manufacture for the alternator. But there may be generic ones as well fitted on failure.

03-18-2016, 03:00 PM   #20
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The quality of many Japanese manufactured items began to suffer in the mid-1980's.
I suppose they were trying to hold the line on price despite rising manufacturing costs.

It's a little disappointing that Pentax applied these economies to an existing model.

A number of brands began employing nylon/plastic gears sometimes with near disastrous results,
e.g. some of the later Minolta XG SLR models.

Ultimately many Japanese manufacturers moved production overseas,
with only high profit and flagship models still being made in Japan.

Chris

Last edited by ChrisPlatt; 03-22-2016 at 04:49 PM.
03-18-2016, 07:25 PM   #21
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Well, I found this thead to be interesting and informative. I'm new to the MX -- have had mine for only a few days and I'm still running my first roll of film through it. Mine's s/n is 416-somethng, so it's a later model. But I got kinda concerned about the nylon gears so I removed the bottom plate. Mine has brass gears. So I guess it wasn't too late in the product run. It has the plastic film memo holder too, so most likely the original back. That little rubber thingie on the shutter button lock collar is missing on mine. Anybody know where I can get one? I can live without it, but I'm just interested in completeness.

I'm growing quite fond of this little guy. Looking forward to years more use out of it.
03-18-2016, 09:21 PM   #22
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QuoteOriginally posted by cooltouch Quote
That little rubber thingie on the shutter button lock collar is missing on mine. Anybody know where I can get one?
Perhaps you can find a bit of suitable black plastic and shape it to make a tight pressure fit.
03-20-2016, 06:18 PM   #23
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Great discussion. I thought I had accidentally landed in the Leica Forum! Non-the-less, nice to see people serious about their Pentax's.

03-20-2016, 06:37 PM   #24
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I've already taken the bottoms off both my 4* and 9* s/n bodies twice since I started following this thread. No more, I've tempted the fates too much, I'm NOT going to risk losing one of those tiny screws
03-23-2016, 08:02 PM   #25
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QuoteOriginally posted by arnold Quote
Perhaps you can find a bit of suitable black plastic and shape it to make a tight pressure fit.
This stuff might be what you're looking for...

https://sugru.com/

Although it may be a little pricey
03-26-2016, 05:30 PM   #26
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Wow, cool stuff. And I think I could probably find plenty of uses for it. But the price does put one off a bit, doesn't it? I bought some silicone seal the other day. Chances are it will work just as well on my MX.
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