Chronograph Runner "skips" (Omega 865)

[watchfellow]

Hi everyone,

As you all know the Omega 865 movements have a single pusher for the chrono. On this one when I press and hold the pusher a second time in order to read and return the chrono hand, the hand "skips" for lack of a better word around like it is loose. However, once the pusher is released the hand always returns to zero, so I am thinking the problem is not the chrono hand itself (ie. it's not loose). Looking at the movement I can see that the chronograph runner indeed skips around when the pusher is held down. Would this be an issue with the chronograph runner, or perhaps an issue with the friction spring? Photo is attached. Thanks in advance for any help.

[HSL]

33 minutes ago, watchfellow said:

However, once the pusher is released the hand always returns to zero, so I am thinking the problem is not the chrono hand itself (ie. it's not loose). 

I'm a bit confused, if this is your problem, I think this is how this type of mono pusher is supposed to work.
I sense an opportunity to learn something again!

Edit: Sorry, when read the post again I think you can’t read the seconds elapsed before it returns to zero?.

Edited May 14, 2021 by HSL

[clockboy]

I am with HSL a bit confused with the issue. I have little experience with chronographs but what I do know is the chrono hand has to be on tight or it will slip on its pinion.

[watchfellow]

Hi guys,

Thanks for the quick answers, and sorry for the confusion. So what happens is when the pusher button is pressed to stop the chronograph, the seconds chronograph hand slips and moves around as if loose.

I know this is usually an issue with the chrono hand being on too loose on the pinion, but when that is the case then the hand does not reset back to zero, and in this case it always does reset back to zero! That's why I was thinking it might be another issue?

[HSL]

If my memory doesn’t fail me again the mono pusher doesn’t have the usual stuff like a minute jumper spring but rather relies on a friction spring underneath the chronograph runner. Check that you still have the correct tension on this one, and no oil accidently found its way, otherwise the chronograph runner will just move around when stopped.
The zero operation then are performed by a hammer pushing on the flat surface on the heart shaped bit on the chronograph runner.
If you have nice friction hold from the chronograph runner friction spring then the next option would be a loose chronograph hand.

[watchfellow]

Thank you HSL! Will check the friction spring and report back.

[watchfellow]

Well I ended up cleaning the friction spring again and replaced it and the seconds hand now resets to zero without skipping around, however now it doesn't hold at all while the pusher is depressed. Meaning as soon as I press on the pusher it jumps immediately to zero. As was mentioned earlier since this is a mono-pusher you are supposed to be able to hold it down and read the seconds before letting go and having it reset. Any suggestions on where to look? Would this still be an issue with the friction spring or possibly an issue with the pusher assembly?

[rodabod]

I’ve not worked on the 865, but is there not a stop-piece of some description which rests on the chronograph runner to fix it in place? My understanding was that the friction spring is purely there to take up “slop” as it’s like any other indirect drive centre-seconds. 

[HSL]

This is  a very simple chronograph construction without too many parts, no fancy minute wheel jumpers or so to keep things in place.

An operating lever is pushed against a connecting lever. The connecting lever is operating the lower cam. On top of the lower cam is the small upper cam, these has to be fitted together so a small nob on the upper cam is aligned with the hole in the lower cam.

There is an operating lever yoke which is fitted to the connecting lever. Underneath the operating lever yoke you find the operating lever yoke spring.

The cam jumper spring assures you get a snappy action. On top of this spring, you find the hammer spring. This is putting tension on the hammer.

There is a hammer bolt which should keep the hammer away during chronograph operation. The hammer bolt spring is located under the operating lever.

So, if the hammer bolt isn’t operating correctly the hammer will just go on and reset the chronograph immediately. Check if the hammer bolt has tension from the hammer bolt spring. If it is nice and springy you just have to see why it can’t catch and hold the hammer, maybe try to disassemble the cam part again and see if something is misaligned.

[watchfellow]

Thank you HSL, for your excellent explanation and good advice, you were right on the money. The spring wasn't set correctly on the hammer bolt. Once I fixed that it holds the seconds when the pusher is depressed. The seconds hand was still loose however, but I adjusted the play on the chronograph runner friction and now it does its job. The seconds hand holds correctly and resets properly! Thank you again.