Removing flange from a Pentac 8 inch f2.9

[adelorenzo]

I got one of the old WWII Pentac lenses today, the 8-inch f2.9. I'm having difficulty removing the flange, it will thread off a few turns (with some difficulty) and then seems to be stuck. There was a small screw in the barrel that looks like it might have been intended to keep it from unthreading but I removed that.

I'm assuming it is paint or something else that is binding the threads, but I wanted to check to see if there is anyone can thing of some thing I might be overlooking before I go all brute force on it or suggest any techniques to get it free. I have squirted some penetrating lubricant in there.

[EdSawyer]

Be careful with lubricant, it may get inside the lens. Post a pic if you can. It sounds like you have a dallmeyer version with removable flange? Some of the other versions have non-removable flanges. Is it brass or aluminum? In disassembling some of the aluminum pentacs, I found it necessary to use a pair of strap wrenches to loosen things, as the threads had corroded a small amount. Once apart though they cleaned up rather well.

[Jason Greenberg Motamedi]

Yes, the aluminum on most of those ex-MOD Pentac lenses is pretty bad. You are lucky that you can move it at all. Follow Ed's advice, and examine and clean the threads very carefully.

[Dan Fromm]

I had a similar problem with a 12"/4 TTH telephoto in an F.139 lens cone. Unlike yours, my beastie's mount threads were inaccessible. I sent it to Steve Grimes, who was really good with a hacksaw, with a request to cut the cone off the lens. Instead he handed it to Adam Dau, then a new hire, and told poor Adam to squirt some lapping compound into the set screw holes in the cone and work the lens out. Doing this took Adam a day. He's still mad at the lens.

[adelorenzo]

The lens is not labeled Dallmeyer but the flange does come off. Like I said I have been able to work it a few turns back and forth, but it just won't go any further. And now it is pretty much stuck in one position. Normally if you break the threads free I would expect things to start moving easier rather than becoming more difficult.

It won't affect my use of the lens but I am trying to disassemble it as much as possible to clean it up. I'll try some strap wrenches and see if I have any better luck.

Does anyone know if the front element threads off? There appears to be a set screw for that as well.

I'll post some pics when I get home this evening.

[c.d.ewen]

I wouldn't try to force the threads, as that might just cause it to bind permanently. If you can move the flange at all, then it isn't seized, and you're way ahead in the game.

Use some solvent, rather than a lubricant, on the threads. There may be some muck stuck in there. There's a possibility that the threads are damaged at the point where rotation stops. If you can't see any damage, then what I'd recommend is to rotate the flange back and forth through as many turns as possible, while injecting solvent. Make an index mark on lens and flange where the binding happens, then rotate a few dozen times and see if you've made any progress passing the mark. Heat and cold can be useful, but without knowing your level of risk aversion, I won't recommend hitting the flange with a torch. You might stick the lens in the refrigerator for a while and see if anything good happens after that.

Charley

[EdSawyer]

heat can definitely help. I have used a heat gun to unstick threads on some aerial lenses, which were probably put together with threadlocker from the looks of it. I wouldn't use a flame/torch, but a heat gun can be just as good, especially when combined with strap wrenches.

Usually the front element can be removed also. I have 2 pentacs (both with integral flanges) and with some effort was able to fully disassemble both of them for cleaning. The nice part about pentacs is the 2 cemented groups never seem to have any separation or haze between the elements, unlike some other aerial lenses I have. Once you get all 3 groups out, they usually clean up super nice and clear.

Good luck, keep us posted
-Ed

[adelorenzo]

Thanks for all the help. I tried strap wrenches tonight and nothing, even though I had it moving originally I can now now longer budge it in either direction. I can't get huge amounts of torque as the wrench tends to want to slip off the narrow flange but I was giving it a pretty good effort.

Like I said, the lens is in pretty good shape as it is but I'd still feel better if I could get the flange off at some point. I've made a lens board out of a piece of aluminium clipboard but haven't had a chance to shoot anything yet.

Here are a few pics (sorry about the quality). It is probably hard to tell but right now the lens is stuck partway unthreaded from the flange.

[adelorenzo]

The front element just un-threaded quite happily. Glass is in great shape with only minor dust inside the elements, aperture works well. I seem to have jammed the flange (will try heat next) but all in all pretty happy with this lens. I think for $150 I paid a good price.

Can't wait to try shooting it!

[Jody_S]

Since there are screw holes in the flange, you can fasten it to a larger piece of wood and get a good grip with your strap wrench on just the barrel. I've found it helpful in the past to use brass or steel bristle brushes (hand powered or circular ones in a Dremel tool) to clean aluminum threads. Aluminum quickly forms a coating of aluminum oxide on any exposed part, and if it's been exposed to humidity this layer can be quite thick and thus bind threaded parts. I don't know of any solvent that would break this down without requiring a Haz-mat suit and respirator to use, so I generally limit myself to mechanical means.