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Ktm 640 LC4 auto decomp pin repair

Discussion in 'Thumpers' started by thanvg, Feb 8, 2017.

  1. thanvg

    thanvg Adventurer

    Joined:
    Dec 27, 2011
    Oddometer:
    27
    Hi to all,

    had an issue with a friend's 640 auto decomp, starter could not crank engine at all, no clicks while kicking it etc. All the usual symptoms as described here (thanks for all the advice) led to a bad auto decomp.
    Took the head cover apart, and checked the cam. Indeed the pin was pretty worn. The female part was also worn, but I had a better one as a spare. (sorry for the bad pic)

    IMG_20170203_204926_small.jpg

    Well, after some setting up and (only) one broken end mill I milled the original pin out. Here it is along with the replacement pin:

    IMG_20170203_214724.jpg

    After some thought, since this part will not receive real impact loads but more like wedging activity I opted for HSS for the new pin. Worst thing would be a fracture that would result in the auto comp getting out of order again. In that case, I might try another hard material for the pin or just get a new cam.

    Anyhow, this is the repaired cam on the left, compared to an older cam on the right (pin might have worked on the older but lobe wear was much worse).

    IMG_20170203_221850small.jpg

    Put it on, works like a charm. Opted for a bit oversize pin, could be milled or stoned to size if auto decompression operation was found a bit sensitive, but it is ok.

    Posting this as a feasibility study in case someone is interested, since I had read that a replacement cam was the only solution for this issue.

    BR,
    Thanos
    #1
    NitroAcres likes this.
  2. NevBlu

    NevBlu Been here awhile

    Joined:
    Feb 1, 2014
    Oddometer:
    312
    Location:
    Carson City, NV
    thanvg, very interesting.

    When rebuilding my 2001 KTM LC4 640, I looked at this decompressor pin and did not know how this pin could ever be removed or replaced when damaged. Not being a machinist, it kind of baffled me, so your post makes it more clear ... machinery and knowledge were the keys. Very nice work too; thanks for this helpful post.

    1. If known, will you state the year of your friends KTM if know?
    2. Was there a part number for this pin or did you create it yourself?
    3. Were the cams standard or an after market substitute?
    #2
  3. clintnz

    clintnz Trans-Global Chook Chaser

    Joined:
    May 17, 2004
    Oddometer:
    5,066
    Location:
    Rotoiti, North Is, New Zealand
    Good work! How hard was the original pin material to machine out? I have an old 640 cam I have thought about refurbishing - the lobes need doing also - I had considered trying to TIG weld something on to that pin so it could be pulled out & replaced with a piece of HSS drill.

    Cheers
    Clint
    #3
  4. thanvg

    thanvg Adventurer

    Joined:
    Dec 27, 2011
    Oddometer:
    27
    Hi guys,

    answers to your questions:

    1. If known, will you state the year of your friends KTM if know?
    It seems like a 2000-2001, black engine.
    2. Was there a part number for this pin or did you create it yourself?
    Pin milled, ground and honed to size. Material: shank of a 4mm HSS drill.
    3. Were the cams standard or an after market substitute?
    Standard cam.

    How hard was the original pin material to machine out? I have an old 640 cam I have thought about refurbishing - the lobes need doing also - I had considered trying to TIG weld something on to that pin so it could be pulled out & replaced with a piece of HSS drill.


    Pin was fairly easy to machine out, used a 3.8 mm carbide end mill which had no issues milling the pin. Centered the thing properly and managed not to harm the hole. However, I did manage to break the endmill, I think when I reached the bottom end of the pin. Not sure if it could be pulled out, but since the cam is already a mess, won't harm to try.

    Forgot to say that I used some bearing loctite when I pushed the new pin back inside. Also, as I mentioned in the original post, I am not 100% sure that HSS is the best material for this application, probably not. However I had no other hardened 4 mm pin and thought it would not harm to try. If it breaks, no damage seems very possible, so I could just repeat the repair with something more suitable.

    BR,
    Thanos
    #4
    clintnz likes this.
  5. fata moutra

    fata moutra Adventurer

    Joined:
    Jun 4, 2007
    Oddometer:
    32
    Hello iconia

    I am a friend of thanvg and a great fan of his machinist skills :)
    The bike is still running. No problem so far with the decompressor mod..
    I do not know how many kilometers it had since the repair however.
    #5