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Help 3LN3 welded sprocket nut

Discussion in 'Yamaha 250cc In-Line 4's' started by Dropdead53, Nov 3, 2018.

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  1. Dropdead53

    Dropdead53 Active Member

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    So I just discovered this monstrosity .
    I was going to change the chain and sprockets while I had the bike apart as I have no idea how old they are and now I’m in the ****. Any ideas? How should I attack this?
    The sprocket is quite loose behind the nut.
    The bike is a 3LN3.
    Help me!
    BE37F4E2-58AB-470D-9A49-01A046760FF6.jpeg
     
    • Bummer Bummer x 1
    Dropdead53, Nov 3, 2018
    #1
  2. risky

    risky risky

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    angle grinder slowly and carefully.
     
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    risky, Nov 3, 2018
    #2
  3. Dropdead53

    Dropdead53 Active Member

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    It’s really the only way isn’t it.
    Does anyone know what the thread size and pitch is so I can buy a die nut to run down it when (if) I get it off??
     
    Dropdead53, Nov 3, 2018
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  4. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    The problem may go deeper than that, because the sprocket is loose on the shaft - the shaft may be kaput, I don't want to scare you as you'll know more once you've removed the nut - be prepared though
     
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    ruckusman, Nov 3, 2018
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  5. GreyImport

    GreyImport Administrator Staff Member The Chief Contributing Member

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    GreyImport, Nov 4, 2018
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  6. 2valve

    2valve Well-Known Member

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    You could try a dremel if you have one , using one of the very small cut off wheel's.Just buy the quality cut off wheel's if your going to try this way , iv'e had the cheap ones explode on me , and yes they do hurt.
     
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    Last edited: Nov 4, 2018
    2valve, Nov 4, 2018
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  7. Murdo

    Murdo The Good Doctor Staff Member Contributing Member Ride and Events Crew

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    Carefully grinding the weld away is going to be the only way and there will be some damage to the thread from the weld.
    The sprocket doesn't look that bad and would go for a bit longer if you don't want to touch it, just fit a new chain and ride until sprocket is worn out.
     
    Murdo, Nov 4, 2018
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  8. Dropdead53

    Dropdead53 Active Member

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    Yes I agree, I’m worried that maybe the thread on the shaft may be stripped and that’s why some moron has welded it.

    I considered this but I don’t think I could ride confidently knowing I had this problem under my ass. Plus the sprocket is loose behind the nut also there is a big washer behind the sprocket that is loose also.

    Can I replace the whole shaft if I need too?? Or am I better off buying another whole gearbox??
     
    Dropdead53, Nov 4, 2018
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  9. GreyImport

    GreyImport Administrator Staff Member The Chief Contributing Member

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    If u read the Litetek link I posted above u will see it will surely be the wrong sprocket anyway .... all the commercial sellers sell a generic '3LN' front sprocket , which is only suitable for the 3LN1

    I wouldnt be riding that anywhere the way it is

    https://www.2fiftycc.com/index.php?threads/c-shaft-sprocket-for-fzr-3ln3-5-6-7-zeal.3220/
     
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    GreyImport, Nov 4, 2018
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  10. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    When I had a similar issue with my 3LN1 - the sprocket nut came loose and popped off into the sprocket cover - the sprocket made it's way onto the threads and basically wore them off - I had purchased a thread file in preparation to no avail, from memory it's 15mm or 16mm 1.0mm pitch.

    Unfortunately I couldn't salvage my countershaft because in addition to the threads being annihilated the splines were showing serious wear not unlike the pics in this thread.

    It was my only transport at the time, so I welded the nut back on and rode it until the splines on the shaft gave out.

    https://2fiftycc.com/index.php?threads/no-final-drive.9924/page-2#post-110940

    That is why I mentioned being prepared for the worst if there is a lot of free play with the sprocket.

    If you're splines are OK and the threads salvageable you may be able to get a thread chasing die which doesn't remove material to clean up the start.

    There looks to be little actual weld attaching the nut to the shaft, which may not impinge on the threads very far at all hopefully.
     
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    ruckusman, Nov 4, 2018
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  11. Dropdead53

    Dropdead53 Active Member

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    Ok, so I got the front sprocket off sarvo, the thread on the end of the shaft is definitely rooted but the spline doesn’t look too bad, what’s everyone recommendations from here? Obviously buy a new front sprocket but will I just weld the new one onto the shaft or is that dodgy? Is there a way to salvage or retread the end of the shaft??

    110D9127-EB2A-485C-B879-123754985D8D.jpeg

    -slow n steady
    E7D64C91-5D69-4007-9E1D-2D2722A2BAD4.jpeg

    The shaft after getting the nut off, the spline looks like it will be alright but obviously the thread on the end is rooted.
    72C34EB5-DFC5-40BA-A0E3-E5CCC788F871.jpeg

    -this is the washer, sprocket and nut that came off it
    D30039B6-BCA2-4BCC-A02B-95D0F63DD866.jpeg
     
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    Dropdead53, Nov 4, 2018
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  12. Linkin

    Linkin The Mechanic Premium Member Contributing Member Dirty Wheel Club

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    Linkin, Nov 4, 2018
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  13. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    That looks so similar to how mine looked it's uncanny - I'd say the shaft is kaput because any operation that you could attempt would require removal of the shaft - welding then into a lathe to trim then cut a thread & redo heat treatment - or just cutting another thread which wouldn't jeopardise the heat treatment on the splines and find another nut.

    I welded the nut to the shaft, unfortunately as the weld shrinks it draws the nut away from the sprocket.

    To do it over I'd consider the shaft kaput; tig weld a new sprocket to the shaft and then weld the nut to the sprocket prior welding the nut to the shaft and be prepared to disassemble the engine to replace the lot come the time.
     
    ruckusman, Nov 4, 2018
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  14. Murdo

    Murdo The Good Doctor Staff Member Contributing Member Ride and Events Crew

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    The spline will go a bit longer if you use the correct sprocket.
    For the thread, measure what diameter is left and see if you can find a thread to match dia (pitch doesn't matter but don't go whitworth or UNC if possible) and get a die and nut to suit and rethread.
    Only reweld as a last desperate attempt.
     
    Murdo, Nov 4, 2018
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  15. Dropdead53

    Dropdead53 Active Member

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    I just bought a new front sprocket from litetek, should I buy a new nut aswel? If so where from? I’m just going to weld it as ruckus explained in option number 2. I bought the bike for $600 and it’s nothing special, I’ve already spent more money on it then I planned so can’t really justify the time and money involved to replace the whole shaft, either that or abandon the project and cut my loses.
     
    Dropdead53, Nov 4, 2018
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  16. Dropdead53

    Dropdead53 Active Member

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    Murdo, the shaft is about 15mm diameter at its minimum and about 15.1mm at its max
     
    Dropdead53, Nov 4, 2018
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  17. Linkin

    Linkin The Mechanic Premium Member Contributing Member Dirty Wheel Club

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    You will probably have to use a 14mm die nut on that. Once the thread is cut, you will need a new clamping nut to suit.
     
    Linkin, Nov 4, 2018
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  18. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    Given that your splines are in good nick and the shaft is nicelt round I'd go the rethread as Murdo suggested, I'm saying this because I also just found this

    https://www.ebay.com.au/itm/M15-x-1...=item41e4958de9:g:lHQAAOSwbEZbIgsn:rk:11:pf:0

    and this

    https://www.ebay.com.au/itm/Select-...pidpXStTNr02BKWtRJAA:rk:1:pf:1&frcectupt=true

    M15 1.25mm pitch option available
     
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    ruckusman, Nov 4, 2018
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  19. my67xr

    my67xr Bike Enthusiast Staff Member Premium Member Contributing Member

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    The new sprocket clamping nut will need a 3mm deep recess in it too as the shaft stick's out 2.5mm past the face of the sprocket so it might be worth finding another oem nut and just welding that onto the shaft

    You'll need to be careful with the heat generated from welding the nut onto the shaft, you don't want it to cook the oil seal for the output shaft
    Pic's of my spare engine, this one had a JT sprocket with a JT sprocket retainer sitting behind it to give it the correct spacing


    20181104_185744.jpg
     
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    Last edited: Nov 4, 2018
    my67xr, Nov 4, 2018
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  20. Wozza

    Wozza Active Member

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    ......
     
    Last edited: Nov 4, 2018
    Wozza, Nov 4, 2018
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