Confused with this cylinder



  • So I'm currently building up another DTR 125 at the moment it's been a learning curve which has actually been going really well in the cosmetic department but engine wise I'm confused! Basically the plan was to fully rebuild the engine bottom end and top end and so far the bottom end has been completely rebuilt and reassembled so I'm now planning on rebuilding the top end next month and getting the cylinder rebored but now I'm really looking at the cylinder it's different from my other dtr125's as on the left and right side of this one there's just covers over where I presume the power valve is? On my other bike in the left side there's pulley cables going in to it but on the left side on this cylinder there's just a cover with a screw going through the centre of it so I'am wondering if this is normal for some dt's as I'm totally unsure ??was told the bottom end of the engine was from a 2001 bike and the top end was off a 1995 are these parts even compatible? They do seem to fit perfectly mind you lol but I'm confused 😬.



  • The YPVS assembly might have been installed and removed by a previous owner. 94' onward bikes all had Servos fitted by then I think. What's the Barrel stamped as out of curiosity? 3BNs/3MBs all work together.



  • @Darty Thanks maybe that's what's happened then. The barrel is stamped " 3BN00 " and the head is stamped " 3MB Y1 " . Will the bike still run ok without the servo being fitted?



  • @Biker_123 yep. Sounds like the powervalve, if there is one, has been pinned open.

    That's not uncommon. From what I remember the 3BN has a ever so slightly bigger bore than the 3MB mate. Be careful when rebuilding the top end. You will need the right size piston.



  • Thanks @Calum . Well I'm hoping the local bike shop will hit me up with the right size piston and rings after the rebore so fingers crossed it all goes to plan! wasn't sure weither to even get the cylinder rebored as its the first time I've seen the inside cylinder of a 2 stroke so thought I better play it safe as there is a few light scores inside that I can just feel ever so slightly if I run my finger of them .. Is there a downfall of running this engine with the power valve pinned open without buying a servo set up then? I was just going to use this bike on a day time mot over the summer anyway and get some off roading done on it while keeping my other DT in prestige condition 😃



  • @Biker_123 It will probably get away with just a hone and a new set of rings. Depending on the condition of the piston mate.

    A servo will help ten fold on the offroading section.



  • @Calum Good to know thanks 🙂 ! The funny thing is though the piston was a mess when I removed it and when I say a mess I don't mean broken it was quite burned out looking at the top and around the edges at the top it was slightly chipped in 2 places just above the top piston ring but the confusing part is it looked like it had been filed down as the chipped parts are really smooth and no damage from what I can tell to the cylinder apart from what I described above and also when the bottom end of the engine was rebuilt when disassembled there was no sign of any broken parts etc 🤔 also the piston rings looked in good shape I don't know if the last owner was a cheap skate and thought reusing a damaged piston with new rings was a good idea lol. Maybe I'm better trying to put some pics up of inside the cylinder when I get time to see what you think as I'm serverly lacking in knowledge with these engine jumped right in at the deep end although I must admit the bottom end seemed not to bad although I was merely a spectator whilst my cousin who's a mechanic took the lead lol



  • @Biker_123 Yeah it's the best way to learn mate. Get Stuck in.

    These engines are so simple mate. Few cogs and nuts and that's it.

    Yeah I know people who will do something similar. You'd be surprised what you can get away with to be honest. Sure it'll be down on power but that isn't a concern to some people.


Log in to reply