Length of diaphragm pin on Pentax M42 lens? My Helios pin seems long...

[Trask]

I hoping some kind person who has access to one or more Pentax screw mount lenses could provide me with the measured length of the lens diaphragm pin that sticks out of the back of the lens. I'm asking because I have a Russian Helios-44M M42 lens that I'm using on a Pentax MX body using an original Pentax M42-to-K adapter, and when I screw the lens into the adapter when on the body, the diaphragm pin eventually hits the front of the upper box structure that holds the focusing screen where the foam mirror bumper is usually mounted. What concerns me is that if may be scraping the front of the box structure on the second-to-last spin, and maybe even fully running into the box structure on the final spin -- for all I know, it's actually being hit by the mirror as it swings upward.

So if my measurement is much longer than what Pentax used, perhaps I can file it down. The Helios A/M switch does nothing and I don't know how to repair it, wouldn't make a difference on the MX anyway, so I'm not really worried about future compatibility with other cameras. I must say, the Helios pin looks long to me.

Thanks, everyone.

 

[mmerig]

On a 55 mm f2 Super-Takumar, the pin is 3.5 mm long.

 

[drkhalsa]

3.5mm for the 55mm f1.8 SMC Takumar

 

[Xmas]

well given the pin penetration from the registration depth is your problem and not all lenses will have the same thread length

Pentacon Electronic /1.8 8.6mm
Fujica 55mm /1.8 7.8 mm

the mirror on the Cosina CSL is deeper than the mirror box but the mirror box ls only 8.23 mm deep.

So I have a witness mark I.e. paint gouge on the box and the Fugi does not have an auto manual switch

The M42 standard is not really a standard

eg. I've removed the open aperture index lug from the Fugi by swiss file

the Pentacon does have an auto manual switch but the pin is not withdrawn you need to push it in

The CSLs diaphragm pin pushing plate has a lead in to avoid bending pins on mount or unmount and the lead ins and plate surface also have witness marks...

 

[Excalibur2]

Well I think you have answered my problem I've had for years in that I couldn't screw my Helios44m lens onto a M42 Pentax S3...and no one could give me an answer till now.

 

[Trask]

Thanks, everyone. The pin length on my Heliar is probably 3.5 - 3.7mm long, and I appreciate the idea that the rear of the lens enters a bit further into the mirror box than some other lenses, making the pin hit the internal. It's not a deal-breaker, but I wish it weren't happening.

 

[Xmas]

With a k mount camera Id use the bayonet adapter rather than the M42 threads or use lenses with A-M option and push in the pin and don't use A option.

A Sharpie is useful for touching up gouge marks.

 

[Trask]

Well, this is the second reference to pushing in the pin. Is that what happens with M42 lenses with A/M switches -- that you can push in the pin and, in presumably the M position, the pin will remain retracted into the lens body? I never knew that, despite having M42 lenses. My Helios has the A/M switch, but it seems disconnected internally somehow, as in neither position does the aperture work in automatic; it seems stuck in manual. And the pin will not stay retracted with the switch in either position. I've been presuming the lens will never work in the automatic position, so only using it on manual.

 

[Xmas]

Well my Pentagon M42 lens does not retract the pin when set to manual but it can then be pushed freely in and stays in

My Helious is out on loan & I can't recall any details beyond its automatic diaphragm is automatic.