Seamaster 1961 - cal 600 14759 SC-61

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Hey everybody, and for clarity, I'm living in the UK.
I purchased a Seamaster dated to 1961 a couple of years ago and all was good until........ I recently decided to replace the crystal which had tiny, feathery blemishes across most of it.
Now, I have done this sort of thing before but beyond that, and replacing batteries, I won't tinker with much of the workings. I demagnetised the watch and all tools (I always do) removed the case back, stem (using the release button) movement holding, popped the movement onto a pad, replaced the crystal and reversed the process.
The crystal now looks great! However, on reassembly the watch is running fast, about 8 mins in 24 hours, AND will only run when static and face down.
Any ideas? Even if I can't fix it myself it would be useful to have an idea of what a repairer would need to do.

 
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Why would you demagnetise the watch if it was running fine?

That could be the issue or you knocked it out of regulations somehow. If it was a front loader I’d say you put the crystal in too deep but this isn’t.
 
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8 minutes a day is a lot; it's more than out of regulation, as @TIMElyBehaviour mentioned.

I suspect something is preventing the balance from moving correctly. In that position the watch does work, the balance wheel moves with a low amplitude such its timekeeping is very poor. Do you have a manual dust blower? You could attempt to gently blow air on the movement and hope it helps (don't use your breath!).

 
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I have heard of watches running crazy after demagnetising when it’s not needed. Research that and see what’s what.