Copal 3 mounting diameter - seems over sized

[SMBooth]

Whats the story with Copal 3 mounting diameter, Wikiped gives a diameter of 65mm as does Grimes but the thread diameter is 62mm. That's a big difference especially when the mount lock ring on my new Nikkor 450 is 65mm dia and can screw through the hole. I can work around it by flipping the lock ring and cutting a couple of slots to allow tightening but still why is the hole so big?

[Leigh]

The 65mm hole diameter is correct, as confirmed by the Copal instruction manual for that shutter.

The locking ring is supposed to go through the hole.
On the two smaller shutters there's a ridge on the ring that fits into the mounting hole.

Click on the image for a larger version.

Note the retaining ring at the right side of the drawing. This is the less-common short ring.
It has ridges on both sides, with a very accurate diameter of 64mm.
The 65mm mounting hole provides adequate clearance, and accommodates slight size errors.

It appears the intent is to protect the fine shutter threads from damage that might occur if the shutter was pushed through a hole only slightly larger than its threads.

- Leigh

[SMBooth]

I see what you mean the ring rebate protects the thread and ensures the lens is centered in the hole. My locking ring is perhaps 6 or 7mm deep and the flange does not contact the lens board surface unless i flip it over. I think my ring must be for a timber (thicker) lens board, not a 2mm aluminum one. Perhaps i should get a thinner one.

Thanks for the explanation.

[Leigh]

My locking ring is perhaps 6 or 7mm deep and the flange does not contact the lens board surface unless i flip it over. I think my ring must be for a timber (thicker) lens board, not a 2mm aluminum one. Perhaps i should get a thinner one.

Hi Shane,

The standard Copal #3 shutter comes with a spacer that goes over the long locking ring.

It's a simple cylinder, 65mm ID, 70mm OD, and 7mm long.
You could duplicate that or buy one. They are sold separately.

Alternatively, you could try to find one of those short retaining rings like the one in the drawing.
That might be a problem. I've never seen one in many years of using this equipment.
Of course you could have one made if you know a good machinist.

- Leigh

[SMBooth]

Thank Leigh, I brought the len 2nd hand and obviously it didn't come with the ring. I hunt around for something that works now I know what suppose to be happening.
ATM the flipped ring works fine as the locking pin stops the lens spinning, but its something that needs to be address in the medium term. I will drop a rubber O ring in around the thread to stop any short term rubbing.