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New Lentar 28mm lens is nevertheless frozen
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PostPosted: Fri Oct 11, 2013 8:23 pm    Post subject: New Lentar 28mm lens is nevertheless frozen Reply with quote
I hope somebody has a suggestion on this. I'm out of ideas, at least for the moment:

Brought in a couple of NOS (new old stock) Lentars:

Click here to see on Ebay

Click here to see on Ebay

Both were manufactured a long time ago. On the first lens, the 200, the focus is quite stiff but it is OK and usable. I could re-lube the lens but, for now at least, I most likely will not do so.

It's the second lens, the 28 also NOS, that's giving me serious fits. The focus is completely locked. It is frozen!! Aside from that the lens itself is clearly brand new and unmarked. The glass is perfect in every respect, for example. There are zero scratches. There is no indication whatsoever of anyone ever having worked on the lens. It's new! But it's frozen.

So far:

So far I've tried the tricks I have in my kit. I was fortunate to be able easily to remove both the front and rear lens groups while doing no damage whatsoever to the lens. It's perfect glass and I set it carefully aside.

What's left is the all metal barrel (no plastic in these older lenses) but with helical absolutely frozen tight . . no motion whatsoever.

First I tried acetone . . lots of it. Nothing.

Then I went to the garage and grabbed my Bernz-O-Matic (propane) torch. I heated the barrel to a pretty high temperature (no damage done) in hope of something breaking free. Using cloth to protect my hands from the hot metal, I tried to unscrew the helical. Nothing. It remained frozen solid!

My next thought is to immerse the thing in gasoline (petrol), but I've not tried that yet as the lens barrel is likely forever to smell of gasoline even after thorough cleaning.

I really don't know what could have that helical so frozen, especially on a brand new (old) lens. I do note that it appears to be brass inside (what I think is) aluminum (aluminium). Perhaps galvanic action over the years has done the poor lens in.

Does anyone have any thoughts as to what I should try next?


PostPosted: Fri Oct 11, 2013 11:10 pm    Post subject: Reply with quote
If it's desperate measures, then I would heat the metal parts wit a hot air paint stripper until it's as hot as you can get it and throw it in iced water. The two metals will have a different coefficient of thermal expansion and should expand / contract at different rates.

If acetone hasn't seeped through the helical, then I doubt much else will. But how about a weak acid such as vinegar?

I had an old Range Rover for a few years and the galvanic corrosion was a nightmare. I think there are far more ways of preventing it than fixing it.


PostPosted: Sat Oct 12, 2013 12:35 am    Post subject: Reply with quote
Lloydy wrote:
If it's desperate measures, then I would heat the metal parts wit a hot air paint stripper until it's as hot as you can get it and throw it in iced water. The two metals will have a different coefficient of thermal expansion and should expand / contract at different rates.

If acetone hasn't seeped through the helical, then I doubt much else will. But how about a weak acid such as vinegar?

I had an old Range Rover for a few years and the galvanic corrosion was a nightmare. I think there are far more ways of preventing it than fixing it.


Thanks, Lloydy. I guess "great minds think alike" because I've been thinking about this and I had an idea similar to your own.

I can comment as follows:

First: cause. The helical is so frozen I can think of only two possibilities. First, and I believe most likely, is galvanic action over perhaps forty years, give or take. The second possibility is incorrect assembly. When helicals are mis-threaded but nevertheless twisted together through excessive force, they become locked and frozen. I won't tell you how I know that. Embarassed Embarassed

My only problem with the heat/cool solution is paint. The paint on the lens is in new, perfect, condition. But I am not able completely to remove the two rings which set aperture, and of course those rings include painted numbers. They cannot be removed and set aside because their removal requires that, first, the helical be separated. With too much heat I believe I will destroy, or at best deteriorate, the lovely existing paint.

All of that said, I agree with you a thermal cycling solution, taking advantage of the differing coefficients of expansion of the two metals, offers me the best hope to salvage the lens.

There is not a dull moment in this life. Smile


PostPosted: Sat Oct 12, 2013 4:42 pm    Post subject: Reply with quote
I don't think you will need enough heat to damage the paint, if you use a hot air gun be careful. If you're prepared to get it wet use a pan of boiling water and a pan of iced water and cycle it between the two. The important thing is to get the heat / cold through all the metal and not just the surface.


PostPosted: Thu Jun 12, 2014 7:18 pm    Post subject: Reply with quote
Here is an update on the above-described dilemma. Progress so far:

Epic fail Sad


I dismantled the lens, removing all glass including front and rear groups.

Inserted front of stripped lens into dish of ice water, thereby protecting paint.

Thoroughly heated rear (protruding) portion of lens with propane torch.

Inverted the (stripped) lens.

Gosh, that certainly did not work. I may try again, but this helical is really locked.

It's such a pity. The lens is literally brand new.

The good news (there's not much):

I don't seem to have destroyed the lens yet. Aside from a bit of harmless discoloration where I played the torch, all is OK.


PostPosted: Thu Jun 12, 2014 11:12 pm    Post subject: Reply with quote
time to get brutal......try 'gently' distorting the lens into an oval shape, just enough so it springs back to round, and keep doing all around the lens.
The other option is to put some sustained firm, but not damaging, end to end pressure on the helicoid and then give it the hot and cold treatment.


PostPosted: Fri Jun 13, 2014 11:24 am    Post subject: Reply with quote
Lloydy wrote:
time to get brutal......try 'gently' distorting the lens into an oval shape, just enough so it springs back to round, and keep doing all around the lens.
The other option is to put some sustained firm, but not damaging, end to end pressure on the helicoid and then give it the hot and cold treatment.


Thanks, Lloydy . . . . that's not a bad idea at all

I might try striking the lens vigorously with a (hard) rubber mallet. I do own a large press. But the many tons of pressure of which it is capable are difficult to calibrate with care.

I thoroughly cleaned up the mallet only a couple of days ago (it had been used previously in less delicate and persnickety situations than the striking of lenses). Used it in an effort to free my Angenieux lens large front element . . . . which needs cleaning but is thoroughly stuck in place . . . . or glued. You never know which. Striking the lens with the mallet did not damage the lens. But it also did not free the front element.

Between the glue and the corrosion, work on these old lenses is too often a trial . . . and not fun. It's never fun when a lens is damaged or destroyed.

To be honest, the best way I can conjure to free the Lentar would require me to fab up some fixtures which would allow application of large amounts of torque bidirectionally and repeatedly. The fixtures would hold the lens body in place against the applied torque, without damaging it. I wouldn't bother trying to repair the Lentar except it is otherwise brand new and, also, it is a preset lens and I'm a complete sucker for preset lenses.

Course, the fact it's a preset lens offers some inkling as to its age. I could be fighting fifty years of corrosion. Sad
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