Gearbox failure

[SpookDog]

@dtr-nsr
I don’t know, I’m not an expert. But it looks like an ‘aggressive’ heat seizure, whether mixture or lube, to me...
Then again I’ve only ever seen other people’s work. I had a near seizure once but caught it in time when the engine ‘note’ changed. So it never smeared the piston.

[DTR+NSR]

Pictures of a dt piston that heat seized due to lean mixture, from the base gasket blowing. You can see the classic 4 point seize and a little detonation damage to the piston crown nearest the exhaust port. Believe it or not, pistons aren't perfectly round, And are normally narrower along the line of the gudgeon pin, so it would be very unlikely to get a heat seizure at that point on the piston.

[Calum]

@DTR-NSR See that death ash on the top as well.

[DTR+NSR]

@calum yeah it certainly wasn't a happy piston, also was a dark heat spot on the underside of the crown.

[theportingmaster]

@lllDennis Two-stroke piston seizures are either thermal-related or lubrication-failure-related. That one appears to me to be lubrication in nature. Could be overly rich jetting that allowed liquid fuel to wash oil off of the piston skirt as the fuel comes through the transfer ports, causing a lubrication failure.
If you need a gearbox have a couple of complete low mileage boxes can do you cheap.

[lllDennis]

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[lllDennis]

Hey guys thanks for the replies. It's probably the harder break in tha caused the damage. I only had 300km's on it and probably forced it with the top speed run.

I also opened the gearbox and seem to have stripped a gear. Lots of heat was Involved.
I included some pictures below. I've never seen damage like this before. Is this normal?

https://imgur.com/a/BH66681

[lllDennis]

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[lllDennis]

@theportingmaster The jetting was extremely rich. Apparently there are 2 sorts of mikuni jet 'sizes'. In my carb there was a 250 mikuni jet. So i ordered aftermarket 250-300 for the Athena kit to test with. But the aftermarket 180 size is the same as original 250.

It was overlay rich while tuning the carb until i figured this out.

If you got a excellent low milage gearbox for me send me a message with some details. Really appreciate it
I looked at new original parts but it's a little expensive for my student wallet

[Calum]

@llldennis dayum, that's not a stripped gear, that's completely melted

[theportingmaster]

@lllDennis Best jetting for Athena 170 is 300 main jet on the
Mikuni tm28 but with the powerjet blocked off. Just cut the
rubber tube in half from the float chamber to the powerjet then block the 2 halves of the rubber tube with a couple of 5mm threaded bolts or similar.

[lllDennis]

@calum Any recommendations on a crankshaft? While everything is open I want to replace it anyway (Has some small grooves where the bearing is). Best with original or aftermarket?

[Calum]

@llldennis Didn't know they do aftermarket cranks?

[Nottsbiker]

@calum

There's some on ebay for less than £200

[jimmydc98]

@calum pretty sure they sell the aftermarket cranks on PJME as well

[jimmydc98]

@nottsbiker https://www.pjme.co.uk/acatalog/Yamaha-DT125R-Complete-Crankshaft-Top-Racing--4596.html#SID=103

[Calum]

@jimmydc98 Cheap

That's good to know at least. Connecting Rod for my RS is that price alone