Overpressure in cooling system



  • Hi guys,

    Well, it seems everytime I solve an issue with my DT, along comes another. I just replaced the piston because I struggled with piston slap - turned out to be the piston fitted when the barrel was rebored was too small - while I was at it, I fitted another 3MB head a had laying around in mint condition, moved the thermostat and housing over etc. Fired the bike up, sounds great, no weird noises anymore - however, when filling the cooling system I discovered that somehow the pressure in the system is way too high - with the cap off the radiator at squirts a large amount of coolant right out, everytime I give it some throttle, this happens from cold start and continues when the engine is hot, if I fit the radiator cap it just comes out from the coolant resorvoir instead.

    Any ideas why this happens? Only thing I can think off is a bad bottom or head gasket, but this seems unlikely as it would have to be very leaky to make this kind of issue, and I was very thorough as always.

    https://www.youtube.com/watch?v=iwrBbsG8YN4


  • Global Moderator

    Man that engine does sound sweet! 😁 You shouldn't really remove that cap when the bike is warmed up as there is always going to be some sort of pressure in the system. I don't actually know if your video shows TOO much pressure though. Anyone else?



  • As the coolant heats it expands hence pressure.

    The radiator cap is supposed to handle some sort of pressure, replacing that can sometimes fix the problem.

    If the pressure is massively undue then it is likely to be a headgasket.

    The head and barrel needs to be skimmed. They are both ally and both warp.



  • @Mightyman said in Overpressure in cooling system:

    Man that engine does sound sweet! 😁 You shouldn't really remove that cap when the bike is warmed up as there is always going to be some sort of pressure in the system. I don't actually know if your video shows TOO much pressure though. Anyone else?

    Yeah, I know there are supposed to be some pressure, but this is definetly too much, i've filled the system a number of times by now, and i've never noticed anything like this before 🙂

    @Calum said in Overpressure in cooling system:

    As the coolant heats it expands hence pressure.

    The radiator cap is supposed to handle some sort of pressure, replacing that can sometimes fix the problem.

    If the pressure is massively undue then it is likely to be a headgasket.

    The head and barrel needs to be skimmed. They are both ally and both warp.

    Might be the cap yes, but this wasn't an issue a month ago, before I changed the piston, so I doubt it helps changing the cap - it's more likely the head that maybe is warped yes, I didn't really check it since it seemed to be in a very good condition.



  • Looks like too much pressure to me and like calum mentions I'd say the mating surfaces need skimming, guessing you used a decent gasket?



  • @andrewj1680 said in Overpressure in cooling system:

    Looks like too much pressure to me and like calum mentions I'd say the mating surfaces need skimming, guessing you used a decent gasket?

    Yes, used a Athena gasket kit when refitting the top end.

    Well, I'll order another gasket kit and get to it as soon as possible, hope this solves the issue, if you guys have any other thoughts, please let me know.



  • @MDN91 never used the Athenna gasket but am told they are next best thing to a genuine, I use genuine Only, I know it sounds silly but you have put the head gasket the right way up haven't you as I've seen that before too, I'm sure I've seen a tread on the site about a mild skim using a sheet of glass and cutting compound, by calum if I remember rightly



  • Sooo, time for an update.

    Turns out it WASN'T Athena gaskets I fitted, rather some chinese noname ones - I had the top end gasket kit laying around for ages (can't remember where I bought it from) and didn't look much at the package when I fitted the new parts, but now I have done the light skim of the head and barrel, fitted Athena gaskets and guess what - no more overpressure in the radiator 😃 And it feels like the compression has increased quite a lot too (duh).



  • Head gasket has gone mate 100% ha he that and all will be sorted 👍🏻



  • Yeah that's the usual suspect on these bikes. I dismissed it because you said you had used an Athena gasket. I would love to build an engine with those cheap gaskets and NOT have it fail.

    They are notoriously bad!



  • @Ads123 said in Overpressure in cooling system:

    Head gasket has gone mate 100% ha he that and all will be sorted 👍🏻

    Yep, it sure was the head gasket

    @Calum said in Overpressure in cooling system:

    Yeah that's the usual suspect on these bikes. I dismissed it because you said you had used an Athena gasket. I would love to build an engine with those cheap gaskets and NOT have it fail.

    They are notoriously bad!

    I really believed I did use Athena gaskets, my mistake - won't do that again, that's for sure. 😄



  • @Calum wow 😳 you've got some good knowledge!!!