Seized Engine after rebuild...

  • Hello!

    So I have a 1998 DT125 which needed an engine rebuild. I've messed it up and after a bit of advice.
    I sent the barrel away to get it rebored, and refitted everything with a new piston, new rings and new gaskets. After doing this, the engine wouldn’t start, it had a coolant leak and was left for about 2 months. I stupidly thought that I could just repair the coolant leak at a later date, give it a kick and it should fire up, but was busy so didn't get around to it.

    When I came back to it (a week ago), the engine wouldn’t kick and has seized. Upon opening up the engine, it appears coolant has got into the barrel, past the piston and into the lower part of the engine. With the barrel off and the piston still in place, the engine will still not kick, and it looks like there is now surface rust on the bottom end. So I think that coolant has got into the bottom end and sat there for a while, hence to bottom end is now seized.

    I’d like advice of what to do next really?
    I will happily rebuild the top end again, but need advice of how to free off the bottom end so the engine will turn.

    Thanks very much,

  • Administrator

    DOes the bore look okay?

    Personally, if you've coolant as reached the bottom end, then the mains could have rusted, which is just going to fall to bits once its under load.

    Ruining the bore, seals and everything else.

    My advice would be to split the cases and replace the mains. The gearbox bearings should be sound, and I don't see any reason why you can't reuse the seals. So it's just the mains that need replacing.

    But they must have seriously corroded to not crank the engine over.

    Something else sounds a miss.

    I would start by stripping the clutch cover off and having a look and seeing what moves and what doesn't.

    Are you sure the engine is in neutral.

    With the clutch off do the balance shaft move.

    Dunno something sounds a miss. Doubtful to be a simple case of rot.

  • The bore looked ok when we rebuilt it 2 moths ago, but it now has small rust pitting on the edges of the barrel. The piston won't rise with the barrel off.

    I'm sure it was in neutral, and had the bike on a paddock stand, so even if it wasn't the rear wheel should have spun around.

    By the mains do you mean the main crankshaft bearing? i.e. this:


  • Global Moderator

    I would advise not using coolant that allows rust to build up firstly... @Calum was talking about main crankshaft bearings yes.

  • Administrator

    Yeah mightyman that's why I'm puzzled. Coolant, surely any coolant that's not water, has rust inhibitors. So why you have rust in your bore wtf lol.

  • Wait coolant, or Water mixed 50% with Antifreeze like normal? Green or blue fluid? There has to be water to oxidise a hopefully pre oil coated bore and crank.

    But, a good shout, whip off the clutch cover and check the whole side, check the Impeller seal too.

  • Administrator

    @Darty I am not convinced a bit of water would have seized tge engine. Any good engine builder liberally coata a freshly built engine with lots of oil.

    Although now I think about it. If the leak was at the bottom gasket, and was severe. Then haven't you juat hydrolocked the engine?

    Clearly it neess investigating so until you split the cases you won't truly know the extent of the damage.


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