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Heat gun precautions?
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PostPosted: Wed Jul 20, 2022 5:21 am    Post subject: Heat gun precautions? Reply with quote

My Pentax F 135mm f/2.8 has haze under one of the front elements and I'm trying to remove the top retaining ring.
So far, isopropyl alcohol, acetone, MEK and hair dryer heat haven't been enough to loosen the ring.
Even soaking in alcohol for three weeks didn't help.

It seems likely the retainer is being held in place by thread locker. And I believe the easiest way to break the bond is with high heat. So I'm considering the purchase of a heat gun.

I've never used / needed a heat gun before, so can anyone offer advice or suggestions? In particular, I'm concerned about possibly damaging the glass and coatings.
The spanner slots have already been marred by my carelessness.



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PostPosted: Wed Jul 20, 2022 7:56 am    Post subject: Reply with quote

This very likely won't come as a surprise, but using a heat gun on a lens assembly is really not recommended.

If you do decide to use a heat gun, pay a little more and make sure is has a digital temperature control, so you can set it on a low temperature (usually you can set the digital ones between 50 and 600 degree centigrade or so in 10 degree steps.) With these digital heat guns you can manually gradually increase the temperature such that the glass is not subjected to a large sudden temperature shock.

The regular cheap heat guns tend to have only one full heat setting or one high and one low setting, both of which will still be too big a temperature shock for a lens.

But, if as you say the hair drier didn't do it, I think you're not going to have much luck with the heat gun either.

If the lens retainer ring is made from aluminium like the rest of the cell mount, it may have simply seized/galled after all these years, which will make it it nigh impossible to unscrew it.

A few hints that may help:

- Many thread-lock compounds do not soften in isopropyl alcohol, so acetone or MEK are your best bet
- Thread-lock is slow to soften at best, and acetone is very quick to evaporate. Thus keep applying acetone several times and do give it enough time to work
- Prevent deformation of the cell mount as you grab it; any force that pushes it slightly out of perfect round will simply clamp the retainer ring in place. Easiest way to hold on to the cell mount in my experience is to have a set of those rubber beauty ring removal tools and hopefully find one of just the right inside diameter to tightly clamp around the cell mount. This will spread the clamping force and give less deformation of the mount as you hold on to it.

If the retainer ring has really seized, you can try and break the cold-fused or oxide bond by placing the tip of a thin flat screwdriver in the narrow gap between the retainer ring and cell mount (square on, at a 90 degree angle) and lightly (!!!) tap it with a hammer, and thus proceed all the way around the ring in a clockwise direction at 30 degree intervals or so (clockwise as seen from the front.) If there is any lateral play of the retainer ring in the mount, then by going clockwise you will encourage the retainer ring to gradually "walk" itself out of the cell mount thread. But don't tap too hard!; you don't want to damage or deform the thread. Once you have gone around the entire circumference one or two times, try again to unscrew the retainer ring. This works best if there is no thread-lock, but a small amount of old brittle thread-lock will also sometimes release using this procedure.


PostPosted: Wed Jul 20, 2022 3:22 pm    Post subject: Reply with quote

Thank you for the reply, Mark!
I've been reluctant to use more than a few drops of acetone or MEK, but I'll try bigger doses.

Indeed, the retainer appears to be aluminium. But I hadn't even considered the possibility of galling. I don't think I've ever encountered that before.

The lens appears to have been worked on previously, so I've been assuming the issue is an adhesive.
But I'll follow your suggestions before thinking more about a heat gun.

Thanks again!


PostPosted: Wed Jul 20, 2022 7:05 pm    Post subject: Reply with quote

55 wrote:
Thank you for the reply, Mark!
I've been reluctant to use more than a few drops of acetone or MEK, but I'll try bigger doses.

Indeed, the retainer appears to be aluminium. But I hadn't even considered the possibility of galling. I don't think I've ever encountered that before.

The lens appears to have been worked on previously, so I've been assuming the issue is an adhesive.
But I'll follow your suggestions before thinking more about a heat gun.

Thanks again!


If it has been worked on before I would guess it has been over-tightened, which should never be done with aluminium fasteners in aluminium threads (at least not without an appropriate anti-seize compound.)

I've had a galled mounting ring once in a Minolta FISHEYE ROKKOR 16mm/2.8. The older versions of those lenses had that ring manufactured of aluminium. It had been over-tightened in the factory; I had to cut it out and once out there was clear evidence of galling. It must have become a known issue to Minolta, as later samples of the same lens model had that retainer ring manufactured of brass, which eliminates the galling problem.


PostPosted: Sun Jul 31, 2022 4:48 am    Post subject: Reply with quote

RokkorDoctor wrote:

If it has been worked on before I would guess it has been over-tightened, which should never be done with aluminium fasteners in aluminium threads (at least not without an appropriate anti-seize compound.)

. . .


That may indeed be the case with my lens. Since my previous post I've soaked the group in MEK, tried your screwdriver tapping method and, yes, used a heat gun. No success.

For now I'm going to set this project aside. Maybe in a month (or two or . . . ) I'll think about it again.

In the meantime I have other lenses waiting in line for attention.
And I hope the next one will be more cooperative!