topic EF 400mm F2.8L IS USM Bayonet Mount Tapped Hole Stripped in EF & RF Lenses

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

inkjunkie — Wed, 22 Nov 2023 13:40:43 GMT

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.

Re: Canon EF lens EF400mm F2.8L IS USM Bayonet Screws

shadowsports — Wed, 22 Nov 2023 03:25:04 GMT

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.

Re: Canon EF lens EF400mm F2.8L IS USM Bayonet Screws

inkjunkie — Wed, 22 Nov 2023 04:07:18 GMT

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.

Re: Canon EF lens EF400mm F2.8L IS USM Bayonet Screws

jrhoffman75 — Wed, 22 Nov 2023 12:18:15 GMT

@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.

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.

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

wq9nsc — Wed, 22 Nov 2023 14:26:19 GMT

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

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

ebiggs1 — Wed, 22 Nov 2023 15:59:05 GMT

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

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

ebiggs1 — Wed, 22 Nov 2023 16:00:55 GMT

"It is ugly. " 🤔

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

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

normadel — Wed, 22 Nov 2023 17:03:30 GMT

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.

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

inkjunkie — Fri, 24 Nov 2023 22:34:45 GMT

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.

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

wq9nsc — Sat, 25 Nov 2023 11:52:31 GMT

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

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

ebiggs1 — Sat, 25 Nov 2023 16:27:01 GMT

Rodger I was going to recommend a heli-coil insert but I was unsure if they came that small. At any rate any machinists should be fully capable of inserting it. Plus it should be of minimal cost and quickly done. I have used them many times but usually on 1/4x20 size ranges.

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

inkjunkie — Wed, 06 Dec 2023 23:24:45 GMT

Went and visited a machine shop, was not interested in looking at it. I found some 2-56 Ultra Low Profile Socket Head screws online. They fit into the recess perfectly. Purchased a 2-56 tap with the hopes that there is enough material in the stripped hole to be re-threaded. But just in case I purchased a 2-56 Heli Coil kit. Waiting until I am feeling better to fix it, have a pretty bad sinus infection right now. I appreciate everyone comments.

I looked thru a bunch of the images I shot last with this lens. From road course to drag strips to motocross, all looked sharp. The ones that were a bit soft were from when I was fighting with my remote radios, was not just this lens that was soft from that time frame so pretty confident nothing is wrong with the lens.

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

ebiggs1 — Thu, 07 Dec 2023 16:10:44 GMT

Some magnified safety glasses will be a good idea.

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

inkjunkie — Fri, 08 Dec 2023 20:06:42 GMT

I ran the 2-56 tap thru the stripped hole. There was more then enough material left. 2-56 Ultra Low Profile Button Heads fit into the recess of the mount with no "massaging" necessary. Lens is back together.

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

ebiggs1 — Sat, 09 Dec 2023 07:34:15 GMT

😁

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

sun573 — Wed, 13 Mar 2024 18:47:43 GMT

Hey OP, what length screw did you use for this? I think my 100mm F2.8L might have some stripped threads after it took a fall. Thanks in advance.