DT125R FORUM

@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

@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

@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

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.