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#398314 - 10/11/11 1:07 am Gearbox drain plug thread size?  
Joined: Jun 2007
Posts: 10
TonyK74 Offline
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TonyK74  Offline
BritBike Forum member

Joined: Jun 2007
Posts: 10
Effingham, IL
I'm in the process of tearing down my '70 TR6 motor for a complete rebuild. After pulling everything from the gearbox I unscrewed the level tube drain plug (T3668). Upon inspection, the plug only has about two threads left near the head of the plug and the threads in the case appear to be stripped. Now the plug won't screw back into the case. My thought is to have the drain hole welded closed and then retap for a new level tube plug. Can anyone tell me what size tap I need for this?

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#398363 - 10/11/11 10:46 am Re: Gearbox drain plug thread size? [Re: TonyK74]  
Joined: Jun 2002
Posts: 9,295
Stuart Online content
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Stuart  Online Content
BritBike Forum member

Joined: Jun 2002
Posts: 9,295
Scotland
Hi,

Originally Posted By: tk74
'70 TR6
the threads in the case appear to be stripped.
My thought is to have the drain hole welded closed and then retap

Nah - Helicoil or Timesert the existing hole, the original thread is 1/2"UNC. If possible, try and find an individual or company locally that does 'em, or you'll be shelling out for ten inserts and the tap ... whistle

When selecting the helicoil/Timesert, be aware that they come in various lengths, each being a mutliple of the diameter - i.e. in this case, '1/2d' is 1/4" long, '1d' is 1/2" long, '1.5d' is 3/4" long, etc., etc. Measure the thickness of the casting and select the correct one. Whatever you do, don't do what the muppet who put my 500 engine together did - put in a longer helicoil protruding into the gearbox; crazy I couldn't work out why only about 200cc of gearbox oil came out ... 'til the rest spilled all over the garage floor when I took the cover off ... mad

Originally Posted By: tk74
gearbox
level tube drain plug (T3668).
new level tube plug.

I wouldn't replace with another 'level tube drain plug' - ime, the originals have a propensity for the level tube coming out of the drain plug; shocked if you're lucky, the tube just rolls around the bottom of the case; if you're unlucky, the loose tube was picked up between two gears, whereupon the engine seizes immediately, and breaks the gearbox case. sick Given the general standard of 'cheap bits' available today, I wouldn't trust a new one to be any better made. frown

Iirc, from '71, Triumph fitted an ordinary bolt and changed the gearbox inner case to incorporate a plastic dipstick.

On all my Triumphs, I've replaced each drain bolt with a socket cap (Allen) screw sized to protrude just into the case with, outside, a spring washer and plain washer compressing an 'O'-ring - Allen screw because the hex. key is shorter than a spanner so I'm less likely to strip the case thread, 'O'-ring 'cos it makes a better seal than a crappy fibre washer.

T160's have gearbox dipsticks as standard, the T100 I've changed the gearbox inner cover for one with a dipstick. I've a later gearbox cover for the T150 but, to be honest, I don't miss the level tube - I've long poured gearbox oil into a cheap enamel measuring jug (from a kitchen shop) before pouring it into a gearbox; the gearbox oil can't go anywhere unless there's a leak; if I want to know how much is in the 'box, it's hardly a chore to drain it into the aforementioned jug.

Hth.

Regards,

#398440 - 10/11/11 11:03 pm Re: Gearbox drain plug thread size? [Re: TonyK74]  
Joined: Jun 2007
Posts: 10
TonyK74 Offline
BritBike Forum member
TonyK74  Offline
BritBike Forum member

Joined: Jun 2007
Posts: 10
Effingham, IL
Stuart,
Thanks for the info. I think I'll take your advice and use an insert with an allen bolt. Like you, I think I would prefer to just measure the amount oil needed instead of waiting for it to dribble out of that level tube.

#398501 - 10/12/11 8:45 am Re: Gearbox drain plug thread size? [Re: TonyK74]  
Joined: Oct 2009
Posts: 264
Buckshot1 Online content
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Buckshot1  Online Content
BritBike Forum member

Joined: Oct 2009
Posts: 264
Old Hangtown, Kalifornia
Stuart,

My 72 has the metal tube arrangement. Perhaps the plastic tube came along later or? I do like the single bolt idea, and being a 4 decade machinist I have a thing for Allen bolts. A stainless bolt with an o-ring groove and drilled for safety wire might be a nice touch. The use of coarse threads in the castings to attempt the confinement of oil makes me cringe every time I change oil, adjust primary, etc.


Michael

currently owned by a 72 T120R
maker of plunger conversion jig

Moderated by  John Healy 


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