Do Merkur handles fit Muhle heads?

Anyone know for certain?

If your talking about a true 3 piece Merkur like the 23c classic the answer is yes, my 23c handle screws onto my Goodfella head which has the same metric threads as the Muhle and EJ.

Thanks Bob.

You'll probably find that any handle from a 3 piece razor made since around 1910 will fit any head!

Although the newer heads and European ones like Merkur and Muhle etc have metric threads, an old type handle will fit the metric head and the new handle will fit the old type head, so the threads must be nearly the same.

I recently found a load of Goodwill heads that I'd put aside because the original handles were all split, I then bought a load of modern razors that had really good handles but not very well-made heads, so I replated the Goodwill heads and married the heads with the new handles and made up some great razors.

I'm sure Bob will know the pitch of the metric and imperial threads used etc and why they are so near?

The imperial handles are #10-32 and the metric are M5 X 0.8.
The threads on many vintage Gillette handles are so worn that they will screw onto a metric.
How ever a in spec #10-32 will not screw onto a Metric head as the metric is about .006 of an inch bigger.

Does over tightening cause damage over the long run? Do you recommend having a backup of a favorite three piece razor?

The razors are identical. There may be tiny differences in blade angle from razor-to-razor, but it could occur in the same brand. The handles may be slightly different, but the real difference is the Muhle costs about twice as much as the Jagger and they're the same product.