A thread question

alahol2

Well-known member
Joined
22 Apr 2004
Messages
5,787
Location
Portchester, Solent
www.troppo.co.uk
I have a mast gate held in position on the mast by a screw with a knurled head. The thread has stripped in the mast so I want to either get a rivnut to insert or drill a new hole and tap it. Problem is I can't identify the thread. The diameter of the thread is near as dammit 6mm (measured with a 6mm spanner which just fits over the threads). The pitch appears to be 1.25mm measured by taking a photo and exploding it to calculate the pitch from the diameter.
The boat is 1978 vintage. Anybody got any ideas please.
 

VicS

Well-known member
Joined
13 Jul 2002
Messages
48,307
Visit site
M6 has a pitch of 1.0 mm

1.25 mm works out as 20 tpi

1/4" Whitworth and 1/4" UNC are both 20 tpi
 

colhel

Well-known member
Joined
9 Jan 2011
Messages
3,983
Location
Gillingham(Dorset) Boat Weymuff
Visit site
Unless you got a better way of measuring the O.D all we can do is guess.
But, if you can geta screw or bolt which you know to be M6 you can lay this on top of your knurled bolt to see if the thread matches. as above said M6 is 1mm pitch.
But, a 1/4" is 6.35 mm so measuring the OD your way aint really going to help.
In my view you have two choices.
(a) find out exactly what the thread is on your knurled bolt and helicoil (a common thread insert) the mast.
(b) drill and tap to, say, M8 and find a bolt to suit.

Regards
Colin
 

Ubergeekian

New member
Joined
23 Jun 2004
Messages
9,904
Location
Me: Castle Douglas, SW Scotland. Boats: Kirkcudbri
www.drmegaphone.com
M6 has a pitch of 1.0 mm

1.25 mm works out as 20 tpi

1/4" Whitworth and 1/4" UNC are both 20 tpi

Camera tripod mounts are 1/4" Whitworth, which can make an easy check. I have just today confirmed that the missing screw which holds down the drum of my 1976 Gibb winch is 1/4 BSW.

Incidentally, OP, the Tap and Die Company is brilliant for these things - the quality is way above the rest, their range is astonishing (though they can't do an M5 x 0.8 for old Citroëns) and their prices are fine. A 1/4" BSW tap is £4.50 for taper, second or bottom.
 

FullCircle

Well-known member
Joined
19 Nov 2003
Messages
28,220
Visit site
I have a mast gate held in position on the mast by a screw with a knurled head. The thread has stripped in the mast so I want to either get a rivnut to insert or drill a new hole and tap it. Problem is I can't identify the thread. The diameter of the thread is near as dammit 6mm (measured with a 6mm spanner which just fits over the threads). The pitch appears to be 1.25mm measured by taking a photo and exploding it to calculate the pitch from the diameter.
The boat is 1978 vintage. Anybody got any ideas please.

If I understand correctly, you have stripped the thread IN the mast, whioch means that the current screw will not fit, as if you recut the thread it is likely to be larger.
So, it is irrelevant what the current thread is.
New M8 Stainless thumb screw here:

http://www.automotioncomponents.co....nurled-thumb-screws-stainless-male-DIN464.pdf

Then use an M8 tap, using a 6.75mm drill


http://www.fullermetric.com/technical/information/tech_recommended_tapping_drill_size.htm

Easy peasy.
Can send the tap and wrench to you in the post if you want to borrow them. You will have to buy a drill bit though.
Send a PM with your address, and I can do it at the weekend.
 

alahol2

Well-known member
Joined
22 Apr 2004
Messages
5,787
Location
Portchester, Solent
www.troppo.co.uk
If I understand correctly, you have stripped the thread IN the mast, whioch means that the current screw will not fit, as if you recut the thread it is likely to be larger.
So, it is irrelevant what the current thread is.
New M8 Stainless thumb screw here:

http://www.automotioncomponents.co....nurled-thumb-screws-stainless-male-DIN464.pdf

Then use an M8 tap, using a 6.75mm drill


http://www.fullermetric.com/technical/information/tech_recommended_tapping_drill_size.htm

Easy peasy.
Can send the tap and wrench to you in the post if you want to borrow them. You will have to buy a drill bit though.
Send a PM with your address, and I can do it at the weekend.

Your understanding is correct. I had hoped to get a rivnut which would allow me to use the current thumb screw but it looks unlikely that I will get a 1/4BSW rivnut. Plan 2 was to drill another hole alongside the original, tap it to 1/4BSW and extend the slot in the mast gate plate so I could again use the same thumb screw.

Thinking about it, your suggestion of an 8mm thumbscrew is probably the better option. Thanks for your very kind offer but I have a colleague with well equipped workshop who will be able to loan me a drill and tap.
 

boguing

Active member
Joined
5 Nov 2005
Messages
2,227
Location
Styx Marina
Visit site
When you do come to tapping the new larger hole, just use the 'first cut' tap in thinnish aluminium alloy. New thread will last longer.
 
Top