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1968 Bultaco Matador m26

Discussion in '2 smokers' started by blaine.hale, Mar 20, 2011.

  1. lrutt

    lrutt SILENCE.....i kill you

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    Jul 25, 2005
    Oddometer:
    2,070
    Location:
    Deltona, FL
    New bore? if so, did you chamfer the ports and also position the rings so they don't hang up in a port. VERY VERY important on the 2 smokes.
  2. blaine.hale

    blaine.hale Long timer

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    Oddometer:
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    Nah, not a new bore. Just honed it as it was and put in the new piston and rings.
    JUST in case, do you have an article on doing that? I need to cover all bases, heh.
  3. lrutt

    lrutt SILENCE.....i kill you

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    Jul 25, 2005
    Oddometer:
    2,070
    Location:
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    all 2 strokes I've worked on have a steel pin in the ring groove to keep rings from rotating. But I've never worked on a Bul before.

    As for chamfering, if you just honed it you should not have a problem as the ports likely already have a small chamfer to them.
  4. Twin-shocker

    Twin-shocker Long timer

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    1,779
    Unless you have used an OE Mahle piston, your problems may have been caused by very poor quality replacement piston. There is small part fitted into the LH case, which takes the kickstart ratchet out of engagement whens returned to forward position by spring, if this is loose or worn it might be the reason the ratchet is noisy.
  5. Jeffn

    Jeffn Been here awhile

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    May 27, 2005
    Oddometer:
    118
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    East Central FL
    My thought also about the ring stops...
  6. blaine.hale

    blaine.hale Long timer

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    Think you guys are right.
    Pulled the motor, freed the piston, checked the crank bearings and primary. All was good so it pointed to the piston. Pulled the jug, re-honed it (there were some not so perfect spots) and noticed that the piston had some side to side slop on the con rod.
    Decided to put it all back together and back in the bike because everything was buttery smooth again. Started up in one crank so I began adjusting the carb for a proper idle. Locked up again.
    At this point, I'm 90% sure it's the wrong piston size and the guys I ordered from didn't recommend the right one for the bore measurements I gave them. No big deal, I guess. I'm going to pull it again and measure the new piston vs. the old piston and the bore then post the numbers up here to see what you guys recommend. All I know of the top of my head is that it's a 250cc jug bored 1st over.

    Really glad it takes all of 20 minutes to pull the engine from the frame ;)
  7. PSchrauber

    PSchrauber Long timer

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    I would recommen to measure the cylinder now after you have re-honed the cylinder a second time and then figure out which oversize piston you need. The oversize pistons come in +0.25mm steps. Normally the diameter of the cylinder is recorded on a lable on the box where the piston is delivered:

    Here the first oversize piston for my Bultaco with 85.25mm in diameter and a given diameter for the cylinder of 85.28mm so there is 0.03mm clearance between cylinder and piston. (Sherpa model 199b 340cc ultrashort stroke engine)

    Just for comparison the piston of my KTM, (a 500cc two stroke with steel sleeve), has 90.17mm in diameter and the clearance is given with 0.06mm between piston and cylinder. (standard stroke).

    I would not take any rule of thump clearances get a standard oversize piston, get the cylinder honed until the clearance fit to your piston and you will get the engine running trouble free. (BTW: never had problems with Wiseco pistons so far ..., I can recommend Wössner pistons they even do custom made pistons for a very good price here in Germany)

    [​IMG]
  8. lrutt

    lrutt SILENCE.....i kill you

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    Oddometer:
    2,070
    Location:
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    I've never seen ordering a piston to the bore. You always buy the first oversize that will fit then bore to match the piston wtih recommended clearances. Sure the piston will fit but if there's only .001 clearance on a 2 smoke she will lock.

    Need to take the piston (if it's still servicable) with the cylinder to someone who know how to do 2 strokes. Just any old shop won't do 2 strokes properly. Cast pistons require a different clearance than forged. You need to have the recomended clearances to provide to the machinist. You may also need new rings given the 2 soft siezures you've experienced.
  9. Domiken

    Domiken Been here awhile

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    Great job man, can't wait to see the finished product. That is truly a work of art.
  10. PSchrauber

    PSchrauber Long timer

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    As the TO has now honed the cylinder twice and is still using the same piston there might be an issue, maybe he has now too much clearance between piston and cylinder and have to get the second or third oversize piston? Sadly we don't know the diameter of the piston or the cylinder.

    Normally when you "overhaul" the top of an engine with an steel sleeve ... (my bad english ...) you get a new piston with has the next oversize. Then you normally give the piston and cylinder away for a rebore. The spec's for the clearance (as you see in the pic above) between piston and cylinder are given by the manufactor of the piston at least we in Germany do so. You should not only use new rings also a new piston pin the bearing for the piston pin and the clamps that hold the piston pin should also be replaced. (All these parts - at least in Germany, France and Spain - you get new when you buy a new piston).
  11. blaine.hale

    blaine.hale Long timer

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    It may help everything if I post the cylinder measurements and both pistons I have tonight or tomorrow.
    Thanks for all the knowledge, guys! I'm eating it up :)
  12. Solo Lobo

    Solo Lobo airhead or nothing Supporter

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    Mar 16, 2004
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    11,681
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    Shoreline, WA
    About time for a bump on this one I think...
  13. blaine.hale

    blaine.hale Long timer

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    Sheesh, forgot about this. I still have it. I put the motor back together a year or so ago and the crank didn't rotate smoothly, maybe crank bearing installed crooked so I put it in the shame corner and moved to other projects. Now that I have a dedicated shop and finished a couple projects AND just ordered a vapor hone cabinet, it may be next up on the block.