300mm f2.8 spareparts

[Bjørn Solberg]

Bought an optically fine 300mm, but two screws are missing/destroyed. Can probably manage to make some odd replacement, but if original spares are to be obtained for a fair price, it is to prefer.
Any advice would be welcomed.

[richardHaw]

are you referring to the knobs or just screws? 

The knobs are pretty much unique to every lens so the only way for you to get parts is to salvage them from a junk.

if you can take that to a machinist, the can estimate or measure the pitch and radius of the thread for him to make you a female thread with accompanying knob.

that silver one looks like a button. if it is what i think it is its just a cylinder with neck that fits to a small leaf spring (slot) inside

[Seapy]

Looking at my example, the larger knob is captive, the leatherette cover conceals the detent screw I suspect.  Am loath to explore, I agree the best way would probably to get a salvage knob.

The smaller silver screw is also a knob, it secures the focus preset detent and is also captive, probably requires the removal of the front element to fit a new one and retain the captive feature.  It looks like 3.5mm diameter and finer than standard ISO pitch but without disassembly would be difficult to measure.  Depending on your needs, the smaller screw is probably less important than the collar locking knob.  I use the focus detent for motor racing to lock onto a fixed point on the track but otherwise it's pretty redundant.  I suppose for birding it could be used to lock focus on a branch where birds regularly perch?

[richardHaw]

Looking at my example, the larger knob is captive, the leatherette cover conceals the detent screw I suspect.  Am loath to explore, I agree the best way would probably to get a salvage knob.

The smaller silver screw is also a knob, it secures the focus preset detent and is also captive, probably requires the removal of the front element to fit a new one and retain the captive feature.  It looks like 3.5mm diameter and finer than standard ISO pitch but without disassembly would be difficult to measure.  Depending on your needs, the smaller screw is probably less important than the collar locking knob.  I use the focus detent for motor racing to lock onto a fixed point on the track but otherwise it's pretty redundant.  I suppose for birding it could be used to lock focus on a branch where birds regularly perch?

yes. there should be a small ball bearing inside it with a screw and spring

[Bjørn Solberg]

Yes, probably engineering is needed. The big knob goes trough the "cylinder" on the picture, probably just to get the broken one out, and replace it with an unoriginal. The small silver one, the threads are probably worn out, where indicated, due to very light alloy. But original parts still to prefer ; -))

[Bjørn Rørslett]

The critical part to get into working order is the locking screw for the tripod collar. As long as the part fits and has the appropriate size,  any non-original item will do. Most long lenses of that era have similar knobs.

The smaller screw is for setting a "pre-focus" spot and something I always secure at the position corresponding beyond infinity focus, then forget it in practice. Once locked it "rides" along with the focusing collar. You could epoxy it into place I presume.

[Bjørn Solberg]

Probably epoxy on the small one, and then the broken screw on the picture has to be prolonged and a head added. I have a local farmer that is handy, he used his lathe to make me a EL Nikkor 62mm f3.5 with infinity focus. The adapter with focus, tree times in the lathe, finally infinity and tight fit, then glue.

[Seapy]

Wonderful!   What will do a lot will do a little, with care.

Might be worth giving it a day or two in case someone here has a dead lens with the required bits?

[Bjørn Solberg]

Yes Robert, that would be great. I paid 360 GBP for it, still a good price I think.

[Seapy]

A bit less than I paid for mine about seven years ago?  The glass could do with cleaning in mine again, it was serviced when I got it but it's got a few little patches on some of the elements, been out in some pretty wet weather over the years, photographing car and motorbike racing.