DT125R ARCHIVE

Calum

I always do the ring end gap according to Haynes manual Specs

Using castrol 2t Racing (its in a gold bottle about 12.99 a litre).

I now running a 260 main jet, any bigger i would expect a air leak some where, i have checked its all air tight.

i have checked the oil pump and it seams to be doing its job. i will put a clear oil line on to be sure.

it looks like on the failed piston, that the piston ring holding pin has either disintegrated or vanished allowing the piston ring to spin 180 degrees, allowing the ring end to get snagged on the exhaust port.

im thinking i may have had brought a defective piston in the first place. if you look at pictures you can see the ring ends are now at the front of the piston. and on the picture of the rear you can see the pin is missing.

Thanks calum for your help keep throwing ideas at me :)

Very bizare. Okay definitely get clear oil lines. People are always quick to put the blame on the piston when in actuality it's an underlying problem. I know it's expensive but actually spunds like a cheaper option when you discount the amount of times it fails. Just buy a forged wossner or prox piston. That way you can rule out defective pistons.

My next question is how have you go the oil pump setup. I did a video on how you should adjust the oil pump to prevent seizures when you shut the throttle off.

m.youtube.com/watch

You want the oil to be pumping with the throttle closed so put it just after the line.

Also you must remember to blip the throttle on down shifts and avoid high rpm usage with the throttle closed. If you're down hill coasting every now and then pull the clutch in a blip the throttle to inject two stroke into the barrel. I always blip the throttle on my bike when riding. Because that's how you ensure oil is getting delivered.

Personally if I were you I'd just consider going premix. That way you know for a fact that oil is being delivered correctly regardless of how often you blip. You still have to blip the throttle as the oil is then metered by the carb but you know even with no throttle it has got oil.

I'll keep thinking but it's likely that the locating pin has fallen out after seizure.

Another thing is that those rings have to be facing the right way. So it's inportant that they're installed correctly.

Also look at who bored your barrel for you. Are you sure they chamfered the ports correctly. It's important that after a rebore the ports are chamfed. You also have to install the piston with the arrow facing the exhaust port.

Worn/Used or poor grade small end bearings and gudgeon pins can also cause inappropriate behaviour. Again it'a worth your money investing in quality engine cmponents to rule out tjat as the point of failure

Very bizarre. So wouldn't surprise me if that barrel needs a rebore now mate. Looking at the damage to the piston, the rings won't seal right if you just buy a new piston.

Yeah Well I'm struggling to see whats going wrong. These engines are DEAD simple so there isn't much to go wrong.

Get some photos up of the spark plug. That will tell me how the fueling is.

If it's got oil, the barrel's clean and the fueling is right then that means it HAS to be the piston. I've used Mitaka and not had a problem, that said I would always pay the extra for quality pistons just for peace of mind.

You'll need that honed not sanded. You NEED crosshatching in the bore it can't be glazed/polished. The crosshatching allows for the oil to 'stick' to the walls.


You need to figure out why it's happened else it'll just go again. Perhaps the big end bearing has gone. Check the big end float make sure its within tolerances.

I don't know the ins and outs of Honing so I couldn't tell you. Just that you need to make sure the bore is within tolerance and is not going to cause piston slap.

Difficult to tell in the photos whether that's scoring down the bore. I'd definitely get a profession to hone that barrel for you though. That piston is an unusual colour underneath.

The jetting APPEARS to be fine. A nice golden brown colour in the bore does not indicate a lean mixture.

The only obvious thing there is that you're running a restricted 3BN cylinder head and that would be well worth source a derestricted head and fitting that. Noticeable gains to be head with that.

Very tough to gauge. I'd get that barrel honed properly. It's probable that the piston failed, but this could be due to poor installation or faulty parts. When honing and boring a barrel it's nice for them to the piston you're going to use to give to the engineer. No two pistons are the same so it's always a good idea to let the person honing the barrel to have this pistons you're going to use to measure against.

I'd try and save to get good quality parts. I'd increase the oil mixture as I show in that video. I'd use plenty of oil upon installation.

I'd check all the tolerances like big end float. Piston clearances. Piston ring end gap. Do everything, as slow and as perfect as humanly possible. All new gaskets and torque it all up properly.

Give it a PROPER run in period. There should be no reason for failure. Clear your fuel filter, new one if necessary, and clean the carb.

Make sure there are no metal filings in the bottom end. That's all I can really suggest. There is no obvious indication for failure.

Forgot to add my thanks and appreciation to take the time to help a fellow biker out. you time really is appreciated ill keep you updated

thank you

it could possibly be because i use the A1 (big motorway) everyday to get to work involving sitting at speeds excess 60+, although i do vary my throttle at them speeds and slow down every few mins. and blip the throttle alot the bike has had the gearing altered so i have a smaller rear sprocket so not redlining it or anything at them speeds,

maybe i should use the 4 thumper for daily commutes :) not as fun tho

or just get a bigger cc 2 stroke :) another yammy of course.

this is the second seizure, when you say float you mean carb float right, if so i have had the manual out and check float heights etc.. and cleaned.

no play in con rod what so ever....

in other word its probably my fault for the roads i use, which is a bit of a bummer. the DT wasn't built for speed lol

after a bit of research i found this on the Mitaka website. www.mitaka.co.uk/

Now finally, the most perplexing cause of failure. Ring pegs. On the face of it, if a ring peg moves either in or out, it would be natural to assume that is has not been a tight enough fit. Whilst we have seen batches of pistons where the peg could be pushed in with a finger nail, we have also seen peg failures where the peg has simply been worn away by the piston ring or worried until it has loosened. Under these circumstances, we need to look elsewhere for the cause of the ring peg failure. Obviously the peg is being loosened by the sideways forces exerted by the piston ring. These are brought about primarily by the ring moving in and out of the exhaust port (and to a lesser extent the transfer ports) causing it to change shape. If the ports have been enlarged, this will put extra load on the ring peg and may cause failure. If the peg is in the centre of the piston, the loads on the peg will be balanced and less likely to dislodge it. Another cause of ring peg loosening is damage to the exhaust port bridge. If the bridge has not been relieved after re-plating, serious wear and damage will occur and the plating will fail. This puts loads on the ring and peg, causing the peg to loosen and fall out. When the peg is seen to have failed, this is wrongly assumed to be the cause of the failure and not the result.

what do you think ?