Dtr 125 pressurizing problem!

  • Had this problem for a long time, really at my wits end with this bike now.

    Cooling systems keeps pressurizing therefore dumping coolant into expansion tank then out overflow, after a few days the head gasket then blows in the same place at the from towards exhaust port,

    I have tried most things that I have been advised from various mechanics,

    Thermostat tested and working ✅
    Cylinder head skimmed ✅
    Crank seals replaced ✅
    Genuine head gasket✅
    Head torqued down to spec✅
    Compression test is 55 psi ✅
    Inspected for cracks in barrel but nothing visible ✅
    Replaced base gasket ✅
    Cylinder bored 1mm oversize✅
    New piston and rings ✅
    Water pump works✅
    tempurature normal✅

    I have noticed that the cylinder barrel and head are not the same code, could this have an impact? Head it 3bn Y-2 and barrel is not readable but definetely not 3bn.

    The only thing I have not tried is replace barrel, possibly has a hairline crack you can’t see when cold but at the moment I’m unsure which one I need.

    Read on another forum that 3bn is the earlier models and the bike year is 03,

    Inside clutch casing is 3bn y-2

    The rebore I had actually made it worse.

    Any help please thanks guys.

  • Did you open up the bore in the head gasket? Believe when you go 1mm bigger it's advised. Also is the filler neck where the rad cap seals against free from damage?

  • @DTR12503 you say that the only thing you haven't checked is replace the barrel but you haven't mentioned the radiator pressure cap. Don't overlook the importance of this item as it regulates the pressure in the cooling system and at a certain pressure it opens enough to allow expansion into the expansion bottle but if it opens too soon it would cause the problem that you are having. Too low pressure in the cooling system would cause the water to boil at a lot lower temperature hence the head gasket failing. the idea is water under pressure boils at a much higher temperature and the pressure is regulated by the radiator 'pressure' cap and they can fail.

  • @markus-w forgot to mention the rad cap, it looks ok an the spring on it has good free play, I was thinking the overflow pipe on rad is higher than the seal on rad cap and when you unscrew cap theres pressure, so surly if the cap was failing the last of the pressure would go through overflow into expansion bottle.

    But there cheap so I may as well order one just in case

  • @DTR-NSR I did think this so I checked and it wasn’t hanging over the bore plus I had this problem before I had it bored

  • @DTR12503 yes it is meant to cover the outlet for the overflow and it lifts at a certain preset pressure which should be stated on the top of the cap and it could be worn and lifting slightly too soon. Different caps are set for different pressures so make sure you order the correct one.

  • @DTR12503 obviously it's not failed completely as you still have some pressure but it's supposed to open at I think about 20 psi but if it's worn and opening at say 10-15:psi then obviously you'd still have pressure in there but not enough and could easily cause the problem that you're having.

  • @markus-w yeah I see what you mean, I never really worried to much about cap because the gasket blows after a week so I’m thinking there’s definelty to much pressure

  • Pressure Cap!

    I wouldn't believe it but I have seen it a few times.

    Always start off with something cheap and look at the more expensive items later.

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