Morning quiz! Name that thread...

I received this from Keld yesterday.

Mark,
This is a very strange case. The nut earlier send to you is a UNC 3/4-10 and that is compatible with BSW/Whitworth.
Have you had your old nut on the shaft after removing the propeller? (to check if the thread has been slightly damaged when removing the old propeller) Have you used tools with the new nut, or just by hand?
Best regards,
Keld
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So I grumble a bit and finally dig out those thread gauges to compare the nut that our friends in Denmark have sent and my mild steel 'ebay special' so I can prove him wrong. The flipping Whitworth thread gauge and my callipers prove Keld to be right and me wrong!

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​
I'll go at the thread on the shaft tomorrow with the Dremel with the wirebrush attachment and try again, if that doesn't work I'll get hold of a UNC die and run it down the thread as you suggest Plevier. It has a 10:1 taper, and there is a locking grub screw, however, that grub screw has no loctite on it... I suppose I should listen to the manufacturer and clam up!
 
Just to finish this one off should any other hapless DIY prop changer come this way. I ordered a tap and die from Avon Tap and Die (http://www.avontapdie.co.uk/) at a cost of some £45, I ran the tap through the nut and that took some of the thread off, so I thought I'd cracked it and I wouldn't need the die. Came to fitting the nut and there was the same problem! I ran the die over the shaft thread, and then finally success.

You can see the last four threads have been put through the die:

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My advice should you have the same problem is to spend the £45 and get the tap and die first.
 
Thanks all, and it was pretty brutal (and I was worried), the but now the nut sits nice and tight with no wobble. Both nut and thread are set perfectly to BSW 10tpi so I'm pretty relaxed. What a palarva, nothing is ever straight forward (I guess that's why its so much fun messing about with old boats...!)
 
I've got a bit confused somewhere along the way.
As I understand it shaft thread was BSW new nut was UNC.
Unless the shaft thread was damaged, why did you need to run a UNC die over it if you were tapping the nut out BSW?
Or the converse if you were re-cutting the shaft thread to UNC why did the nut need re-tapping?
Best solution, wouldn't they make a BSW nut? Wasn't it their mistake to start with?
 
I think the shaft thread was damaged. If you look at the first photo with the plastic rule held against the threads it appears that measuring from crest to crest or trough to trough, ten threads are actually a bit less than the 25.4mm they should be. I think that in the past somebody has probably hammered on the shaft end whilst trying to free the prop, and compressed the last few threads a bit whilst spreading the diameter.
 
I got confused along the way too, for sure. The process went something like:

  • Nut sent to Flexofold to identify thread.
  • They identify the thread as 10 TPI UNC. - It doesn't fit the thread on the shaft when it arrives!
  • I order BSF and BSW threaded 3/4 nuts from ebay (both 10 tpi of course) in mild steel to check fit - the BSW threaded nut from EBAY fitted!
  • I put my thread gauges on both the nut from Flexofold and the nut from eBay, they look identical - I assume I must have screwed up and go back and try the nut on the shaft again. The Ebay nut fits, the Flexofold nut doesn't!
  • I take the nut to a big engineering outfit, they tap the nut to the closest they have to BSW (I never actually saw the tap). STILL DOESNT FIT.
  • I assume the thread must be damaged, order the tap and die, and re-cut the thread on nut (nut still doesn't fit) and studding, huzzah, success.

None of this explains why the BSW nut from ebay fitted the thread - I can only assume the company making the mild steel nut had different tolerances. It wasn't even like I ordered them from a fly by night company. Oh well, prop on, and is working splendidly! :D

Edit: Thanks Plevier, as I understand it, its almost impossible to distinguish UNC from BSW with thread gauges on a nut so it was likely a compound cock up. Thread damaged on the shaft and Flexofold mis-identified the thread :)
 
Last edited:
MagicalArmchair said:
Could it be 12TPI as opposed to 10TPI...?
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Don't think so, not from that picture. You need to put a thread guage on it to get an accurate pitch but I bet that's 10 tpi, given it's an inperial diameter. Threads do differ sometimes, I bought a lathe faceplate where the thread was made with a tap of the correct size but which still didn't fit. Maybe you should send the nut back and ask them to relieve it a bit ?

Boo2
 
Gosh what a saga!
I guess you mean you bought BSW and UNC nuts - you can't have 10tpi BSF. Presumably only the BSW went on.
I don't like the sound of "tap the nut to the closest they have to BSW"; BSW is BSW, but the closest thing to it is UNC!
I think it would indeed be almost impossible to distinguish the nuts with a thread gauge. You're looking for a 5 deg difference in angle on the profile. You could see that on a male thread -if your gauges are accurate enough - by looking at the light, but inside a nut you are only going to feel the pitch.
It is interesting that your BSW nut went on; that suggests insignificant thread damage unless it was a really lousy loose nut! I don't think I could diagnose any damage from the original photo but will look again.
Modifying a thread with a tap or die is never going to give a good result.
Were your tap and die UNC or BSW? In either case, one of them should not have done anything!
 
The tap and die set were both BSW and they BOTH cut and did something (albeit the tap through the nut didn't take much off, it took plenty off the shaft though), which had me scratching my head as to exactly what tap the engineer had actually put through my nut. I didn't think to wrestle the tap off him to scrutinise it before he put it through, assuming he was the engineer and employing trust (and besides all the big fun CNC machines around the shop were distracting me).

And yes Earlybird, it could have been all customed up by a local shop. Flexofold reported the original prop only fitted the taper at about 20% of its circumference!! So it all sounded like a bit of a lash up (hence I lapped the shaft/hub with grinding paste to ensure a good fit).

Glad that job is done! We live and we learn, thanks for the input one and all, it is a great source of comfort when battering ones head on the brick wall of the latest challenge/buggeration!
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earlybird said:
I'm wondering if the original shaft and nut were made as a matched pair by a local machine shop, to "near enough" tolerances.
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If both were made by a local shop then the odds are that the nut was made with their tap and the shaft made to fit the nut. So if the tap was worn or not the right size in the first place then that might be an explanation. But from the OP's posts it seems it was made to a Witworth thread standard so that is the real reason.


Boo2
 
MagicalArmchair said:
The tap and die set were both BSW and they BOTH cut and did something (albeit the tap through the nut didn't take much off, it took plenty off the shaft though), which had me scratching my head as to exactly what tap the engineer had actually put through my nut. I didn't think to wrestle the tap off him to scrutinise it before he put it through, assuming he was the engineer and employing trust (and besides all the big fun CNC machines around the shop were distracting me).

And yes Earlybird, it could have been all customed up by a local shop. Flexofold reported the original prop only fitted the taper at about 20% of its circumference!! So it all sounded like a bit of a lash up (hence I lapped the shaft/hub with grinding paste to ensure a good fit).

Glad that job is done! We live and we learn, thanks for the input one and all, it is a great source of comfort when battering ones head on the brick wall of the latest challenge/buggeration!
free-mad-smileys-264.gif
Click to expand...
Good to hear you're sorted now anyway,

Boo2
 
MagicalArmchair said:
The tap and die set were both BSW and they BOTH cut and did something (albeit the tap through the nut didn't take much off, it took plenty off the shaft though).....
Click to expand...

I am very happy to hear that all your investigation and hard work has paid off. Happy sailing for the rest of the season - you haven't missed much so far this year.

This post is mostly for my education:-
Given that 'plenty' was taken off the shaft, could it be that the very original shaft thread was 13/16" BSW as on mine? (Did you ever measure the O/D of the shaft before cutting?)

Cheers
Bob
 
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