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

inkjunkie
Enthusiast

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.

15 REPLIES 15

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.

EB
EOS 1DX and 1D Mk IV and less lenses then before!

inkjunkie
Enthusiast

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.

ebiggs1
Legend
Legend

Some magnified safety glasses will be a good idea.

EB
EOS 1DX and 1D Mk IV and less lenses then before!

inkjunkie
Enthusiast

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.

😁

EB
EOS 1DX and 1D Mk IV and less lenses then before!

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. 

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