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I have this new 8410 and as it is now the perfect time to play with locks I mounted it. Unfortunately it is rather pigheaded. Turning the knob CW is easy. When I successfully unlock it, turning the knob back is also quite easy. When the wheels are not right, turning the knob back is extremely difficult or even nearly impossible. It is also very difficult to move the brass part by hand, but I can squeeze together both moving parts. Any idea, what is wrong there? Unfortunately this is my only 8400 series lock and I can't compare it with a working species.

The S&G 8400 series hasnt been used in US Fed Gov since the mid 1980's and due to its special market, I wonder how old your "new" lock is. Check the casting mound between the bolt and the wheels in the case and see if it has a date code stamped.

The cam itself was not made to be disassembled for service but it may just need wiped out good and new grease applied.

If you are opening the cam with the back cover removed, the cam will tend to over extend and make it hard to retract back to the closed postion.

Those locks normally also had the "Centi-Spline" dial where the spline was adjustable so if its a new mount that could be part of the issue.

This.

The behavior is with cover. Just the photo is without. The sticker on the lock says it was born November 2018. So it is really new and unused. I don't see any grease.

What is a Centi-Spline dial? Is this referring to the hook? Or has it to do with the spline key?

The Centi Spline dial allows you to change the spline setting for the lock mount (VU,VD,RH,LH,) for your application.

With the dial removed you should see two set screws near the dial bearing. (Older models only had one screw) By loosening the set screw you can unscrew the brass hub and see the dial spline setting. There should be a reference to this in your installation instructions.

Yes, it is possible to disassemble the dial. I assume, that default is RH, because now in RH mounting the cam can be opened at 0.

Its been awhile since I did one of these but if the cam is opening at zero, its doing what it should.

Just found the dial settings for the D041 dial packed in your lock

RH=39
LH=89
VU=64
VD=14

I dont see a mention of default out of the box setting, but RH would make sense.

I used to carry the list with me years ago since these dials would loosen up and fall off before they added the second set screw.

It is opening very fine at 0. Just the closing is the problem.