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Grand Piano Bolts - 6.6mm x 22tpi

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Mark Elen 117/11/2019 14:58:11
117 forum posts
201 photos

I was in the local nut and bolt merchant last week to overhear the proprietor telling a local piano restorer that he had never seen anything like the bolts the restorer had presented to him.


They basically hold a wooden frame into the top of the open part of the piano.

As I walked in, the owner of the nut and bolt shop was saying that the local engineering shop would be the best first port of call, but that they were flat out busy. What he needed was somebody with a lathe in the garage.... (the words were.... just the man.)

I agreed that if I could work out what thread was on the bolt, and also set my lathe up to cut it, I would oblige.

Measuring the bolt, over both the shank and the threads, it is 6.6 mm, with a set of thread gauges, it is 22 tpi. The only thing that I can find anywhere near that is an old German thread, from the 1850s, a Hamann Partonen 7mm x 22 tpi.

As the parts come from a 140 year old German Grand piano, it looks like a promising candidate.

My lathe (a Seig SC4) doesn't have a 22 tpi gear setting -


But looking at the above, as the 20 and 24 settings basically use the same first 3 gears and it is only the last gear that produces the 20 and 24, I used the logic that a 55 would be smack in the middle.

I roughed out the shanks to 6.6 mm and set to work screwcutting the thread. Amazingly, it fits:





3 made, I need to round off the heads and cut a slot, so I have ordered a Hemingway radius tool that I'm going to have to make. Once done, they can go in the mill and have the slots cut with a slitting saw.

An interesting little project.



Mark Elen 117/11/2019 15:08:51
117 forum posts
201 photos

I forgot to say that I got all excited when I found that 5/16 BSF is 22 tpi, and that I could get a die. Apparently the Germans used a lot of British Standard threads in those days. I thought great, just buy a die and cut the threads with that.

Only snag is, 5/16 BSF major diameter is 7.9375 mm



Ian Skeldon 217/11/2019 15:17:36
486 forum posts
37 photos

Well done Mark, I would have cheated and drilled it out and tapped it next size up in metric and let him buy the screws/bolts.

old mart17/11/2019 15:49:15
1819 forum posts
148 photos

It just shows that a bit of positive thinking can overcome all obstacles, well done.

Michael Gilligan17/11/2019 15:51:55
15853 forum posts
693 photos
Posted by Mark Elen 1 on 17/11/2019 15:08:51:


Only snag is, 5/16 BSF major diameter is 7.9375 mm


But a BSF chaser would be reasonably agnostic regarding diameter.



P.S. ... I must echo Ian’s congratulations !!

Clive Foster17/11/2019 16:06:10
2239 forum posts
73 photos

Closest listed standard thread would be 16-22 ASME which is 6.81 mm / 0.268" diameter.

Which is pretty unlikely for an old German piano. Finding a die would be almost as unlikely.

I'm convinced that ASME threads were devised as the American entry into the "weird thread size stakes". (Can't let those effete Europeans and Brits have it all their own way.)


Robert Butler17/11/2019 16:38:40
151 forum posts
6 photos

BSF 1850's ????? I doubt it?

Robert Butler

Mark Elen 117/11/2019 16:54:29
117 forum posts
201 photos

Many thanks for your kind comments gents.

Yes, Robert, after a bit of research, BSF was adopted by BESA in 1908. Whitworth was earlier.

The Hamann Patronen was a German Machine thread from 1850. Used on Mechanical and optical instruments



Edited By Mark Elen 1 on 17/11/2019 16:58:04

Edited By Mark Elen 1 on 17/11/2019 17:02:30

old mart17/11/2019 17:44:12
1819 forum posts
148 photos

I hope you get to see the result of the piano restorers work when he is finished, your special screws will still be in use in a hundred years time probably.

Mark Elen 108/12/2019 20:04:10
117 forum posts
201 photos

Hi Mart,

Yes, the restorer is going to give me a call when it is done to go and have a look. I finished off the radius tool last weekend, then finished off the bolts. The original is the one on the left. My new ones are the 3 on the right.







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