2003 DTR 125 engine rebuild



  • I recently just top end rebuilt my 2003 DTR 125, power valve somehow shot out a bit to far one day and caugh the piston, but its firing up now and driving but when at low revs there's a knocking sound coming front the head? I changed the top end baring and it's still the same, any suggestions much appreciated thanks



  • @bobbym94 Just a common sense suggestion, but have you checked that all the bolts you loosened are tightened back up to the correct torque settings???


  • Administrator

    If the barrel has been rebored then the valve will need altering to stop it catching the piston. It is common practice on two strokes.



  • @CYBER-NINJA

    Yes pal, I had the head and barrel off the engine to change piston and gaskets, I changed the piston and top end baring etc hooked her back up and it had a knocking noise coming from it In bolted the barrel leaving the He's attached and changed the top end baring, the piston felt a little more tight on the con rod at this point, I put the head back on bolted her back up and still knocking noise coming from it 😕 double checked every bolt that had been loosend, they were all ok BUT they were not torqued up with a torque wrench
    Thanks for your reply mate



  • @Calum
    No mate the barrel is still the same as before, the powervalve has been set to open for the time being


  • Administrator

    @bobbym94 So the only reason why the valve would hit the piston in this instance is, the barrel has been rebored in its life and is now on an oversize piston. The piston is in the wrong way or the powervalve is not the right one, I.E. it's a valve out of a 3BN barrel and not a 3MB.

    I don't think it's possible to put it in wrong and it can spin around all it likes, I don't think it would hit the piston.



  • According to the previous owner it has not been skimmed or re bored, I also haven't done this to it, i had been running it fine the way it was for 7 months or so then one day it just went, like any 2 Stoke does when it pops. So took the head and barrel off, no scoring at all in the barrel but the piston definetly hit something and the power valve has a bit of scoring on it so it must had caught it, I have no idea how this happend but I have heard if it's not set properly it can spin right round and come to far for the piston to strike it.
    Thanks for your reply mate



  • @bobbym94 just go get your bore measured and take the guess work out of it find the original bore specs and go from there


  • Administrator

    @declan Or read the top of the piston as this will have a number ontop of it if it has been rebored! I.E. 0.5 or 1.0 or 1.5

    I think you are jumping to conclusions if this is the case.

    It's possible that something broke off and hit the piston and valve, hence the scoring marks.

    Usually when you hit the powervalve the damage will be catastrophic. As you are going to hit the valve when it's open, so 6'000 RPM or higher, at which point it's going to be self destructive.

    I think maybe the valve is a red herring, it can't be inserted the wrong way easily so should be obvious if it is.

    Also the fact it's been running for 7 months prior tells me that it's unlikely to have been that. It wouldn't be thermal expansion since the piston would have locked up in the bore long before the valve hit the piston, unless the barrel has been rebored. Then it will.

    So there is a lot of factors at play here.



  • @Calum nah bud the piston could be too small and could be causing piston slap always measure the bore


  • Administrator

    @declan You can't measure the bore with conventional micrometers.

    You need to measure it with a laser from 6 points as a bore is not spherical.

    If the piston is 0.5mm too small it should be obvious just from a visual inspection, let alone IF you got it running it would run like a bag of nails.



  • @Calum I get mine measured at my local shop



  • @declan and no there was a guy on here that had a piston too small and it looked normal but still slapped the barrel when cold but ran good when warm



  • Verniers should be ok to measure and see the difference. 0.5mm is really big difference even for ruller or measuring tape.



  • What year bike is it mate as the early dtr ran a bigger piston as standard 56.40
    The newer ones use 56.00 ,had this problem myself some years ago,the piston slapped around causing it to eventually shatter and scrape the valve on its way outta the exhaust port 😣 (I was young then tho haha)
    Definitely get the bore measured by a local engine shop,they won't charge u to tell u the size if ur nice to em😎
    Then u can buy a piston that fits £35-40 or have them bore it to the next size and supply a piston £75 WILL BE LIKE A NEW BARREL HAHA


  • Administrator

    @castrolsniffer The barrel 3BN run larger bore, shorter stroke.


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