Page 1 of 2 12 LastLast
Results 1 to 10 of 17
  1. #1
    Join Date
    Feb 2004
    Location
    Boat (now back in) the Clyde
    Posts
    5,218

    Default Tapping thread in alloy mast

    Need to attach a 316 stainless steel fitting to a mast - fitting has 8mm holes so rather too big for Monel rivets, so proposing to tap threads for s/s machine screws.
    Should anything be used on threads to reduce corrosion between screws and mast?

  2. #2
    Join Date
    May 2016
    Location
    Suffolk
    Posts
    7,454

    Default Re: Tapping thread in alloy mast

    Yes, Duralac is the usual stuff.
    Rainbow Marine.
    www.rainbowmarine.co.uk

  3. #3
    Join Date
    Jul 2002
    Location
    Home: Kent. Boat: Chichester
    Posts
    43,052

    Default Re: Tapping thread in alloy mast

    Quote Originally Posted by dunedin View Post
    Need to attach a 316 stainless steel fitting to a mast - fitting has 8mm holes so rather too big for Monel rivets, so proposing to tap threads for s/s machine screws.
    Should anything be used on threads to reduce corrosion between screws and mast?
    For something close fitting like a screw thread Tef-gel is a lot less messy to use than the common Duralac ( barium chromate) paste. There is also another newer "Duralac Green" which is chromate free.

    Consider an insulating gasket between the stainless steel fitting and the mast

    Also consider a finer thread than the usual coarse M8 x 1.25mm if the wall thicknes is a bit thin for a 1.25 mm thread (there are 1.0 and 0.75 mm alternatives)

    Also consider Rivnuts or similar rather than tapping a thread in the mast wall

  4. #4
    Join Date
    Feb 2010
    Location
    South UK
    Posts
    565

    Default Re: Tapping thread in alloy mast

    You won't get much thread into the wall thickness of an ally mast. Rivnuts are the way to go.

    Plank
    Hang the Rich.

  5. #5
    Join Date
    Jan 2004
    Location
    Bricks & mortar: Italy. Boat: Aegean
    Posts
    10,415

    Default Re: Tapping thread in alloy mast

    Quote Originally Posted by PlankWalker View Post
    You won't get much thread into the wall thickness of an ally mast. Rivnuts are the way to go.
    If the location is in an extruded section, +1.
    All epigrams are false

  6. #6
    Join Date
    Jun 2001
    Location
    Oban
    Posts
    751

    Default Re: Tapping thread in alloy mast

    Be extremely careful if using the original yellow Duralac. It stains clothes very effectively. It's also pricey so you don't want any of it to go other than where it's needed. On the other hand, IME one tube lasts a lifetime.

    Derek

  7. #7
    Join Date
    Nov 2001
    Location
    Me: Johannesburg South Africa Yacht: Richards Bay East Coast Africa
    Posts
    6,776

    Default Re: Tapping thread in alloy mast

    Quote Originally Posted by dunedin View Post
    Need to attach a 316 stainless steel fitting to a mast - fitting has 8mm holes so rather too big for Monel rivets, so proposing to tap threads for s/s machine screws.
    Should anything be used on threads to reduce corrosion between screws and mast?
    Another approach that I have used is to rivet a stainless steel plate onto the mast using lots of smaller rivets then tapping the stainless plate to mount the fitting.

    The stainless plate would need a plastic gasket between the mast and the plate.

    Generally I would use a larger pitch thread in softer material like aluminium.

    BTW I have fixed mast steps onto mu mast with 4 M6 stainless crews tapped directly into the my mast which I consider is better than rivets.
    Life is too short not to have a sea view
    Distantshaws tinyurl.com/yclt6l68

  8. #8
    Join Date
    Feb 2004
    Location
    Boat (now back in) the Clyde
    Posts
    5,218

    Default Re: Tapping thread in alloy mast

    Thanks all. Tef-Gel or Duralec sounds like what I was thinking of.

    Was thinking to mount on thin layer of Sikaflex to insulate. This partly because also very slight curvature of mast behind flat plate.
    Interestingly none of the original fittings attached by the spar maker seem to have any gasket underneath. And like Roger Shaw the handful of mast steps are fitted with machine screws.

    Rivnuts are interesting. But would I need Monel ones? Where could one source a small quantity (only 4 needed) at the right metal spec? Also it was suggested that if mast wall thicker these might be too shallow, as designed for thin metal?

    Thanks again

  9. #9
    Join Date
    May 2007
    Posts
    35,803

    Default Re: Tapping thread in alloy mast

    You can buy small quantities of Ali rivnuts on ebay.
    They come in varied lengths for metal thickness.
    What is the fitting?
    If it has M8 fixings for good reason, you need some serious backing plate in the mast.
    If it's under not great load, you could bush the fixings down to M5 and just tap the mast wall.

  10. #10
    Join Date
    Feb 2015
    Location
    Erith YAcht Club
    Posts
    390

    Default Re: Tapping thread in alloy mast

    i need to add a new main sail guide gate to my zspar mast to stop the runners/cars falling out when raising or lowering the main. there are just burred threads where one used to be. the bolts are steel though but i would have to re-thread the holes.

    are rivnuts put into a hole then you screw into those? thanks
    Current boat: 1988 Jeanneau Sundream (lifting center board)

Page 1 of 2 12 LastLast

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  

Latest YBW News

Find Boats For Sale

to
to