DTR 125 pressurizing problem!!

[Dtr12503]

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 front 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.

[Mr Hammers]

Compression seems awfully low at 55. IIRC it should be at least twice that.
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[Nobby the Bastard]

I'd be looking for a crack in the head or cylinder to the water jacket.
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[jeffyjeff]

Be aware that it is possible your cylinder head or block has a crack that is not detectable to the naked eye. A good machine shop can deploy one of several crack detection methods depending on the metal these components are made of.

Be aware that a faulty pressure cap will cause coolant to pass through to the surge tank. Your case, however, sounds way too severe for that, since you say the surge tank passes coolant out the overflow.
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[Dtr12503]

Thanks for advice, compression it definitely way to low I think the recommended is around 90 to 110 psi yeah rad cap seems fine and theres no visable crack in bore or water jacket like you said.
i won’t bother getting it tested if I can get a cheap enough cylinder

3bn cylinders seem to be quite rare but there’s more 3mb ones about, can a 3mb cylinder go with a 3bn head I already have?

[Robby]

I assuming you're running an oversize piston to go with the rebored cylinder?

Crack detection isn't expensive, you can buy the spray on amazon, although it is better done by someone who has used it before - ask whoever did your rebore.

Replace the rad cap anyway, seeing as its a cheap part and you're throwing money at the bike.

Are the gasketing faces on the head and barrel spotlessly clean?

You're missing a lot of compression. Compression test should be done on wide open throttle, closed throttle will give a lower reading. Even then, I would expect a cocking massive crack in a head or cylinder to give a reading that low. Running a too-small piston and rings would do it though.

[Dtr12503]

Yes running an oversize piston ,I don’t have an oversize gasket on because the standard gasket is not overlapping bore, I had this problem before I had it re-bored aswell, it has just made it worse

coolant use to take a few weeks to start going through the overflow then gasket would fail, but after rebore it’s a daily problem

And yeah I may as well replace rad cap anyway

[jeffyjeff]

Don't overlook the cylinder head. In all likelihood, it is water cooled as well, and a potential source for combustion pressure to charge the cooling system.
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History shows again and again how nature points out the folly of men - BOC