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Please bear with this rank amateur. Being "the mechanical one" I was asked to look at a clock by my brother, who had just spent £200 with a clockmaker to get his old Irish pendulum mantle clock going, but which was now losing time very badly.

On inspection there was no nut on the pendulum rod, so the weight had been able to work gently down the rod, although it was still a reasonable interference fit. The nut was lying in the bottom of the case and is a standard looking rating nut, but had two slots cut in it axially at 180°, which could create a friction locknut, but not if the slots have spread, as here.

The rod and nut look and feel like brass. The rod is square section in its upper half and this rides in a groove in the weight which is held in place by a bracket. The bottom half of the rod is threaded round. The threads (something like 2 or 3 BA to my eye) were very worn and loose. I nipped the brass nut with some pliers to close the slot and this has tightened the nut up on its threads, but I suspect the slots will give over time and the problem will recur.

My question is are these slots a normal design for a rating nut or are they a bodge to avoid renewing the rod and nut? I don't want to start down the track of renewing the rod and nut if I'm missing something obvious.

Many thanks for any advice.

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Hi Colin,

The slots in the nut are something normal. The nut is only for fine adjustment of the pendulum length.  There is no pressure or force there. Once it is adjusted, no more touching and turning is needed, so it is very strange that the thread is worn. Is it possible that the nut has been replaced with one with bigger size of thread? If now the nut stays in place and needs some force to be rotated, You don’t have to worry about it – all is perfect.

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Thanks for that. The nut has not moved since I adjusted it and my brother is now getting it to keep good time, so I think we're sorted!

Good to know the slots are a normal feature, but it seems odd that a clockmaker would have left the nut slack so that it just fell off. Anyway, i have learnt something so that's good!

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