2005 dtr piston and barrel

  • So I have bought a cheap DTR 125 for £650. Spent a bit treating it to new bits and getting it back on the road as well as ordering a new CDi to fix the powervalve, Haynes manual, exhaust packing - all the general stuff that hasn't been done to it for years. The guy that sold it to me said he had it rebuilt by a friend and thinks he ordered a piston that was too small as it was immediately down on power. The compression does seem low but its riding okish considering it has no power valve set up at the moment. The problem is I've taken the top end apart as there is a bit of a rattle and the piston seems quite loose in the barrel, also one of the fins at the bottom a has a fracture that means i NEED a new piston! The vottom end has no play and the top end bearing has no play, sound like piston slap. Both barrel and piston say 3bn00 but the piston has 50 stamped on the top - does than mean it's a 56mm piston plus .50 oversized? The barrel measures about 57-57.50 (I've ordered a proper measuring device - that's with a ruler). How do I know if the barrel has had a rebore and what size piston to get!?!? I was tempted to send the barrel away for a rebore and piston deal for £61 but the barrel is on very good shape and I really dont want to unnecessarily bore it after reading all the issues with contacting the power valve. The current piston measure 56.00-56.50 I can't really tell with a ruler!!!!

    Is there figures somewhere that says what piston ia needed for the certain width of the barrel bore?

    Cheers for any help. Want to order something ASAP

  • Global Moderator

    Contact that same guy on eBay offering the £61 deal and ask for a quote on just supplying the piston based on your cylinder size. I would imagine he might knock off a tenner or £15 as it's not a lot of work to rebore it.

  • Administrator

    The 3BN00 barrel has a bigger bore than the 3MB00 barrel.

    If that isn't the standard barrel, which foe a 2005 it isn't, then that also means the stroke is not the correct length.

    This throws out all the port timings and what not. As such it would take a lot more work to get the power back up. My advice is to find a 3MB00 and put it back to how it is supposed to be.

    The alternative is that that botton end is built to hoise the 3BN barrel. But if that's the original DTRE Engine then unless they have swapped the rods then that isn't the case.

  • @MIGHTYMAN thanks, i did think of that and that's my backup option. But a bit of extra work to post if there is a way of just ordering the right piston from home. Surely there must be a way

  • @Calum good spot and my mistake! It is a 3MB00 not 3BN00 , didn't realise there were two types! So it is the right barrel and piston but the barrel definitely seems bigger than standard. Checking in the Haynes manual it says the barrel bore width should be 56.00-56.02. I'm guessing the mate that did the rebuild for him just ordered a standard piston and rebuilt and that's why its rattling around in there like a bull in a China shop. Ordered some propper digital caliper measurers and then I should be able to work out what must on is needed. Cheers for your help!

  • Global Moderator

    Even with a set of cheap vernier callipers I find it difficult to measure the bore exactly. I didn't measure properly once and ended up with a piston too small. Won't do that again. For me, it's worth the extra couple of quid to get someone who knows what they're doing to measure it accurately.

  • Administrator

    As Mightyman says. You don't measure the bore with verniers. Just let a specialist sort it out

  • @MIGHTYMAN cheers, yes I got it too small. I took the barrel and piston to some one near me today who sells the pistons on eBay. He immediately sorted me out with the right piston which was a .50 oversized and a new gasket set. The one thing I need now is the little orange gasket in the power valve set up. It came with the o-rings but not that gasket and mine is ruined.


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