(slightly OT) Trouble identifying thread

Gentlefolk,
This is not exactly a model-engineering issue, but I know some of you
are involved with larger machines, so I'll ask anyway...
I have a threaded hole (in an old tractor, as it happens) which is 7/8"
in diameter and threaded 9 TPI. Looking at various tables leads me to
believe it's either Whitworth or UNC. A UNC tap will go in, but I don't
have a Whitworth tap to try. My questions are :-
1) Will a 7/8" UNC tap screw easily into a 7/8" Whitworth thread ?
2) If so, how else can I work-out which thread it is ?
TIA
Reply to
Bob Unitt
Loading thread data ...
Fairly easily
Look at the rest of the threads on the tractor, or the spanner sizes. Do the bolts generally take Whit spanners, or AF spanners? It's no guarantee, but a good guide. If yo're not sure & it's not a critical strength application, use what you've got or can get & don't worry about it
Tim Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service
Reply to
Tim Leech
There is a slight difference between thread angles on UNC, 60 degree's, and Whit at 55 but in the real world production tolerances are never that close to affect the swopping of the two.
And before some purist points out, the exception is 1/2" where UNC is 13 and whit is 12 tpi.
-- Regards,
John Stevenson Nottingham, England.
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Reply to
John Stevenson
Bob Unitt said
Depends upon the tolerance to which the thread was cut and the wear to which it as been subjected. If it was screw-cut - rather than tapped - then anything is possible.
What you really need to know is the thread form. You could always melt some sealing wax on to one side of the hole and knock it out when set (or even chewing gum) from the resulting 'cast' you should easily be able to determine whether it is 55° (Whit) or 60° (UNC)
JG
Reply to
JG
In article , Tim Leech writes
Unfortunately that won't work - this is a (presumably) stripped-thread hole which someone has drilled-out and tapped to a larger size.
Reply to
Bob Unitt
In article , John Stevenson writes
O.K. - I'll fit whatever will go in...
Reply to
Bob Unitt
would help to know what make model and age of tractor?!. steve
Reply to
roundtooit
Your answer is simple. The Whitworth thread has a 55 degree between the flanks of the threads and rounded thread crests and the UNC has a 60 degree angle and sharper thread crests. If you've already screwed a UNC tap into the hole then it is now a UNC thread because the UNC tap must take a little material out of the Whitworth thread form.
You want a 7/8" UNC bolt.
Reply to
Dave Baker
In article , Bob Unitt writes
Now resolved.
Thank you for your comments and suggestions. I've now made a bolt which works - I first screwcut it with a 55 degree tool and, when that was too tight, went over it again with a 60 degree tool instead - giving a bolt that fits a bit loosely, but well enough for the job it has to do.
Reply to
Bob Unitt
The solution you have reached seems perfectly sound; pernickety types like myself might be tempted to take an impression of the thread with plasticene, or as someone else mentioned, paraffin wax, as this would allow you to eyeball the threadform. In practise, the very large size of the thread in question means that any loss of strength due to compromised threadform is inconsequential unless the joint is being subjected to cyclic loads approaching the maximum for the size of fastener, which is probably of the order of 10 tonnes or so.
cheers, David.
Reply to
penfold
In article , penfold writes
The bolt is one of six retaining the frame for a front-end loader on a Mid-60's Ford 3000. The original bolt would have been considerably smaller (probably 5/8" at most), as this is hole which has been drilled-out and re-tapped, presumably after having been stripped by an over-enthusiastic spanner-wielder. As far as I can judge, there's little or no tension load on the thread itself, all the load being in shear across the bolt diameter. I don't know what the maximum load will be, but I should think it would be well short of 10 tonnes.
There's a good chance that there hasn't even been a bolt in this hole for the past twenty years anyway, so anything I've done will be an improvement - that's the great thing about old tractors, they're actually designed to be repairable by bodgers like me...
Reply to
Bob Unitt
Most, if not all, BSW threads are coarser than UNC for diameters greater than 2"
Bob
Reply to
BobKellock
OK, when did you last use a BSW (or UNC) thread greater than 2" dia?
Cheers Tim
Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service
Reply to
Tim Leech
Half joint bolts on most of the HP and IP cylinders in most of the power station turbines in the UK :-)
Mark Rand RTFM
Reply to
Mark Rand

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