AndyDtr08 Posted July 4, 2012 Share Posted July 4, 2012 So I have accumalated a number of DT125R barrels all of which would need a re bore before use. I have used a local engineering shop called Boretech Engineering who do a stunning job. However they are expensive. They charge 54 for the bore, 66 for a mitaka piston kit and somewhere just short of a tenner for a small end bearing. All up it comes in at 130. I can get a mitake piston kit through ebay for 45 quid delivered and small end bearing is about 5 quid so I'm thinking about going elsewhere for a rebore. I have a friend who works at an engineering place and says he should be able to do the re bore so I have a few questions. If the piston size is 56.00mm then OS is +0.5 +0.1 +1.5 +2.0 what should the re bore be done to? Should be it bored to this exact size or is there a clearance that the rings take up? Or is this worked out and the piston is infact slighlty smaller and the "56.50" size is the size the barrel is bored to? Honing... has anyone honed their own DT125 barrel? I have a draper flexi hone that i used on my 50cc barrel but the blocks appear too small and would get stuck in the ports. Any help would be much appreciated! Andy Link to comment Share on other sites More sharing options...
Moderator mervin Posted July 4, 2012 Moderator Share Posted July 4, 2012 1 does your mate know what he is doing re clearances etc 2 PJME rebore for £25 +vat supply mitaka pistoms at £39 +vat ot wossner £70 _vat, Vertex £56+vat and they know what they are doing http://www.pjme.co.uk/acatalog/Yamaha_DT125R_Engine_Parts_Top_End.html Link to comment Share on other sites More sharing options...
Moderator drewpy Posted July 4, 2012 Moderator Share Posted July 4, 2012 I did my own honing and you can get different lengths of hone stone. any engineering shop would be glad of your business. I use western rebores on Oldham road and always take my parts with me and he's happy just to the the machining. Link to comment Share on other sites More sharing options...
AndyDtr08 Posted July 4, 2012 Author Share Posted July 4, 2012 Cheers guys. Yeah he does know what he's doing and would probably be embarrassed by me asking advice on here but I thought if try and get sizes etc in case it was any use to him. I'll look into those places though. Those prices are what I worked out including vat. They are 45 + vat for the bore and 55 + vat for the piston kit. If my guy can do it then I'll get him to do all the ones I have as he'll have it all set up and then I can just buy the piston kits as and when I need them. He just txt me saying his work has a boring bar big enough and he can hone them aswell. I'm helping him renovate an import tzr 4dl so he will do me a good price I'm just settling down to a night shift so will probably end up scrolling eBay for Dt parts all night. When you say clearances, that's what I'm after. I'm sure he will probably "bore" me with the details at some point but if any one could enlighten me it would be useful to make me look clever. The bore pun wasn't homo erotic before anyone makes that connection! Link to comment Share on other sites More sharing options...
NEV Posted July 5, 2012 Share Posted July 5, 2012 Don't forget Andy, if you're re-boring the barrel, you'll also need to ream out the head gasket, skim the powervalve (or the piston will rub against it), head will need doing too for cylnder volume change, but most importantly, allow for the piston expansion, Cornish Motorcycles did a decent job of mine, but didn't quite take enough off the barrel, as it was semi-seizing due to the piston expanding, and aparently, different makes expand differently. Link to comment Share on other sites More sharing options...
AndyDtr08 Posted July 5, 2012 Author Share Posted July 5, 2012 Hmm some interesting things to think about there. My current build has had the barrel re bored and an oversized piston gone in. I didnt skim the powervalve, head or anything like that and never did on the 2002 DT i re built ages ago. I can't believe that Yamaha would design it so that the powervalve would hit the piston after a re bore it just seems mad. Have you read this up anywhere or is it common knowledge or what? Just wondering where it's come from as I've never heard anything about that before. Link to comment Share on other sites More sharing options...
Moderator drewpy Posted July 5, 2012 Moderator Share Posted July 5, 2012 always bore to the piston ie get the piston first and give to the borer. Each piston has slightly different sizes and need to be matched. I don't know the clearances and they will be different with each manufacturer's pistons as it depends how much silicate they used in the alloy. you used to be able get an overbore gasket kit, but I haven't seen these for donkeys years! Link to comment Share on other sites More sharing options...
NEV Posted July 5, 2012 Share Posted July 5, 2012 Think about it Andy, normally the powervalve is just missed by the standard size piston/rings, so the bigger the bore, the bigger the piston, but the powervalve is still in the same place, so unless you shave some off the powervalve, the piston/rings will hit it. and yes I had to have it all done for mine, although to be fair, I'm on maximum re-bore. I literally took the whole engine to Cornish Motorcycles for them to do the top end properly, thats the difference between using a good engineering company and a good motorcycle engineer, the engineering company will re-bore/hone to your specifications you ask them to do, whereas a good motorcycle engineer, will have knowledge about the head gasket needing reaming out, and the powervalve needing shaving to suit the oversize piston. Link to comment Share on other sites More sharing options...
AndyDtr08 Posted July 6, 2012 Author Share Posted July 6, 2012 Think about it Andy, normally the powervalve is just missed by the standard size piston/rings, so the bigger the bore, the bigger the piston, but the powervalve is still in the same place, so unless you shave some off the powervalve, the piston/rings will hit it. and yes I had to have it all done for mine, although to be fair, I'm on maximum re-bore. I literally took the whole engine to Cornish Motorcycles for them to do the top end properly, thats the difference between using a good engineering company and a good motorcycle engineer, the engineering company will re-bore/hone to your specifications you ask them to do, whereas a good motorcycle engineer, will have knowledge about the head gasket needing reaming out, and the powervalve needing shaving to suit the oversize piston. Hmm. Yeah I see your point but surely the valve won't pertrude past the barrel lining? Yamaha would have thought of that when they designed it. Otherwise they wouldnt have had oversized pistons. The shop I use is Boretech engineering who are a motorcycle engineers based at a motocross track. They arnt just an engineering shop. Well the one that's built at the moment will stay as is I think but I'll consider that with the next one. Cheers Nev. Link to comment Share on other sites More sharing options...
dt502001 Posted July 9, 2012 Share Posted July 9, 2012 Sorry to say but yamaha didn't alow for valve clearence,the shop you took it to to have bored cleaned it up but prob didn't tell you. Another thing all ports should be chamfered after boring to stop a ring from cliping the sharp edge. General rule is 2thou per inch of bore for clearence, but as note above different pistons expand differently. Any time i bought pistons they came with bore/clearence specs. Sometimes just a sticker on the box but normaly a note inside The ball type hone work nice and don't get caught up in the ports Link to comment Share on other sites More sharing options...
Ryan09 Posted July 9, 2012 Share Posted July 9, 2012 When i had mine honed and bored i just took the cylinder to an engineering firm along with the new piston, they asked how much clearance is recomended which pjme said about 2 thou iirc and they done a brilliant job of it. Was only the first re bore and it was miles away from the pv. I would have thought Yamaha would have stated a max overbore before you would start having problems with hitting the pv? Sounds a bit mad to me. @Andy, cant you supply your own piston at the engineering place you go to? Link to comment Share on other sites More sharing options...
AndyDtr08 Posted July 9, 2012 Author Share Posted July 9, 2012 Cheers guys. I'll keep all that in mind. Ryan, I'm pretty sure they said no when I asked last time but thinking back they said it was because they couldn't guarantee the quality using an inferior piston. I'll speak to my mate and see what he thinks about it and if he can do all that. If not I'll go back to Boretech and ask if I can supply a mitaka piston. Thanks for all the help guys it's much appreciated. Link to comment Share on other sites More sharing options...
Moderator mervin Posted July 10, 2012 Moderator Share Posted July 10, 2012 Andy It would be better to get the pistons first and then he can measure them to get the bore clearances correct i reckon, I dont think pistons measurements will be as accurate as you think , ok if it is bored a bit tight it can be honed out a little but if it is bored to big you either end up with a sloppy rattly engine that will not last long, or having to bore again to the next size up Link to comment Share on other sites More sharing options...
Recommended Posts