EF 400mm F2.8L IS USM Bayonet Mount Tapped Hole Stripped

[inkjunkie]

My lens has seen some rough times. I purchased it used from someone who shot with it for a living. It is ugly. But was working well. The weather seal on the mount was shot, so I wanted to replace it. Sadly one of the 4 threaded tabs on the lens is stripped. The screw falls right into the once threaded hole. Thread on the screw are fine, the brass threads on the lens are not. Have a friend that has an idea or two on how to fix it...at least for the short term. Problem is we need to know what threads are on the bayonet mount screws so I can try and find a tap. Anyone have any idea? Spoke with a machinist friend, he was going to call a friend that owns a tool & die company to see if he can help me out but it may take a while for the calls to happen.

[shadowsports]

Hi InkJunkie,

Its been a while.  Your lens is about 24 years old.  Sounds like you've had a good run with it.  Hope you'll find someone willing to work on it.  This sounds more like a repair.     

[inkjunkie]

There is only 1 camera repair shop in our area that I know of. Last I used him I was less then thrilled. He will not look for parts. Customer has to. I have hunted around for the lens part that the bayonet attaches to, no luck so far. I really should just start looking for a replacement lens. I really like the 400 at the drag strips I shoot out. Really missed having it.

[jrhoffman75]

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@inkjunkie wrote:

My lens has seen some rough times. I purchased it used from someone who shot with it for a living. It is ugly. But was working well. The weather seal on the mount was shot, so I wanted to replace it. Sadly one of the 4 threaded tabs on the lens is stripped. The screw falls right into the once threaded hole. Thread on the screw are fine, the brass threads on the lens are not. Have a friend that has an idea or two on how to fix it...at least for the short term. Problem is we need to know what threads are on the bayonet mount screws so I can try and find a tap. Anyone have any idea? Spoke with a machinist friend, he was going to call a friend that owns a tool & die company to see if he can help me out but it may take a while for the calls to happen.

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If you have the screw a machinist should be able to determine the size and thread configuration.

You could also try US Camera for parts. 

[wq9nsc]

I agree with John that any shop can determine the thread spec of the current screw AND determine whether tapping to a slightly larger size or drilling to install an insert works best.  If the lens is otherwise still in good working condition, I wouldn't dump it over what it a type of issue that gets solved daily in dealing with stripped thread situations.

Rodger

[ebiggs1]

I agree with the two replies above. Any machinist worth his salt should be able to do this repair. 

[ebiggs1]

"It is ugly. " 🤔

The outside doesn't matter. Its the insides that take the picture.  😊

[normadel]

If the screw is falling through the formerly-threaded hole, tapping the hole calls for a bigger screw. You have to be sure the head of a larger screw does not interfere with the lens-mounting surface flatness.

[inkjunkie]

Which is exactly what I have run into. I measured the original screw. Spent some time on the McMaster Carr site looking at screws. A 2-56 is slightly larger then the original one. Problem is the head thickness is a bit much. Looked at JIS screws. Pretty much the same thing. Thing that stops me, at this point, from going to see a machinist is the last time I used this lens the images were a bit soft. Would hate to put $$ out to fix the screw issue only to have a focus issue. Heading into LR to find the last images it took when I get a chance.

[wq9nsc]

A 2-56 has a slightly larger major diameter (2.1844mm) vs a M2 (2mm) so your original screw is probably a M2 but it might be a M1.8 so check with calipers or a chart.

This may be a good application for a heli-coil insert, those are available standard order down to M2 and 1-64 sizes. These are pretty easy to work with if you are used to this kind of work, otherwise it is a simple task for a machinist willing to take on a tiny project.

This is the McMaster Carr link to Heli-Coil inserts for reference, M C is a great resource for all sorts of hardware and sells to individuals and does not require a commercial account:  https://www.mcmaster.com/products/helicoils/

Rodger