DT125R ARCHIVE

MDK wrote:

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MDK wrote:

maybe take exhaust of if possible have a look at piston and rings

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first post   get this feeling I was talking to a wall  ?

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 Unfortunately seems to be the way at the moment that people seeking advice are not liking the answers and ignoring them. Although im sure im not a shade on you MDK I've learnt alot and always like to offer up the easiest way to check things like this. I also mentioned exhuast a few posts down. I just wish that if people had a problem they could take the advice and then post up some detailed photos to give everyone wanting to help, the best chance to diagnose the issue.

Atleast we've found the problem and Mad will now know that you can ride5 miles with damaged rings :P

Mad if you could measure the bore size and let us know it would be useful. Some say its only over 1.5-2m oversize that you need to modify the powervalve whereas others think it should be done eveytime. If you do find it to be too big to rebore again or want a replacement barrel / powervalve then let me know. got some for sale in the appropriate section.

Almost certainly will be the piston/rings rubbing against the powervalve where it has been rebored to an oversize so that the valve protrudes into the bore. It will have just worn down the piston ring until it got to the point that the mixture was getting passed the piston at that point and the explosion didnt have enough force to push the piston all the way round 1 cycle.

Those prices sound relativly expensive. PJME or Grampian motors will charge you 25 quid for a rebore then around the same for the piston kit. Around £12 return postage but only costs you £5 or less to send via my hermes/ collect plus. All depends on your finances and personal preference. You'll need to get the powervalve modified. Theres plenty of good info on here as to what you need to do and where you need to do it. Or you could pay your local engineering shop to measure and do the work but obviously it will be more cost. I would ask them if they can include that in their price of the rebore and tell them its £25 most places. That way you get it all done locally and all proffessionally.

Also remember to account for a top end gasket kit and small end bearing ( Athena or Genuine for Head gasket is an absolute MUST) the others can be cheaper pattern but you might as well buy an athena top end gasket kit.

If you havnt done a top end rebuild before ( or even if you have) read up on here thouroughly and avoid the mistakes that others have made. They are easy to do and its cheaper to learn from someone elses mistakes than to make them yourself first.

A few that spring to mind are

-Keep the crank blocked with CLEAN rag whilst removing/refitting piston circlips ( best to keep there at all times)
-Fit circlips so the gap is up or down. NOT SIDEWAYS
-Ensure all gasket remains are removed and the the mating surfaces CLEAN, FLAT AND OIL FREE.
-Use genuine or Athena head gaskets
-Ensure the entire barrel is clean and free from metal filings ( wash it) before you fit.
-Use 2 stroke oil to librally lubriate the Piston, rings, small end bearing, bore surface and crank during assembly.

Whilst you've got it all apart its also worth de cokeing the exhaust port to remove all the carbon deposits( and powervalve), checking the reeds. Cleaning the combusion area ( i personally polish the combustion area, piston top and powervalve surfaces using autosol or some similar metal polish but the benefits of that are debatable. Cleaning the exhaust would also be beneficial. Look up Caustic soda for that but be very careful!

Oh and is a vernier gauge accurate enough to measure bore size and do you know the turnaround time with pjme

-- Edited by madrevs on Tuesday 11th of February 2014 10:37:25 AM

What would you recommend using

In my opinion it is. I think a vernier gauge is adequete but Cal may be able to convince me otherwise. I'm only offering my personal advice but I've never had an issue with using one on steel bore cylinders. If youre checking clearances on nikasil plated bores then its not....but..... on steel bores where we will be reboring and going up a piston size in 0.25m incriments its fine. Plated bores have tighter clearances and usually use 4 different oversize piston sizes as the plating wears very slowly.

e.g for a nikasil plated bore on a Dt we would see these piston sizes as oversized pistons.

A 56.00
B 56.01
C 56.02
D 56.03

As you can see they go up in 0.01mm incriments and so a proffesional bore measuring devis would be needed.

Whereas on our steel bores we get

1st oversize 56.25
2nd oversize 56.50
3rd oversize 56.75
4th oversize 57.00
5th oversize 57.25
6th oversize 57.50
7th oversize 57.75

The old gauge i use measures relativly accuracte and will defintly tell me if it is close to one of these sizes. Then i can make my own decision on the next steps.....as below...

When working on a used barrel all we need to know is if the ring will seal against the bore surface all the way around. You can put the piston ring inside the bore, ensure it is the same length down the bore (from the top) all the way around. Holding it up to the light, if you can see light around the ring between it and the bore surface then the bore/ ring is worn and will need a rebore at worst or a new set of rings at best.

*Obviously it depends on your finances but to me it seems a false economy to have the engine apart like this and not do the best job. Its different with twins or triples but single cylinders are relativly cheap. I see it like this. You HAVE to buy a new gasket set (£20). You almost have to buy a new small end bearing (£8) If you are going to replace the rings (standard maintenance on 2 strokes) (£20) then you may as well buy a piston kit which includes them (£40). If you are buying a new piston then a rebore is the only other option to consider and this depends on the condition of the bore but for only an extra £25 it seems like a bargain and when you sell the bike and can say its had a fresh bore piston etc at xxxx mileage the £25 will reek its benefits.

If we've got the bore back after a rebore there is no need to measure anything aside from ring gap. This is the gap between the ends of the piston ring. If this is wrong then as things expand the ring ends can hit against each other and the locating peg, knocking the peg out which will then obviously make a nice mess of your engine. I personally have never actually checked this before and never had a problem but will always from now on ( as reccomended by the mitaka website). It is a good read so go check it out.

As for pistons, I've used mitakas always in the Dt125r's ive worked on and they have always been great. An example being my full bike rebuild following by 2052 mile trip ( Sussex > lands end>john o groats > sussex) which including the engine run in!

I dont rag the hell out of my engines and dont go passed redline. I use wossner pistons in my YZ125 which is obviously a race engine (and also single piston ring) and DOES get treated like the dirty beast it is and wants to be. If you plan to ride the bike like youre being chased by tribe of cannibals on R1's then invest in a wossner. If you plan to ride the bike as it is designed ( hard but not exteme) and dont plan on modifying the engine/ bike for more speed etc. And most importantly......MAINTAIN THE BIKE! then go for a mitaka and save 50 quid.

You must always check ring gap using Wossner. They send them with one within range and the other too small. I've ALWAYS found this with Wossner rings. For the sake of five minutes just check the ring gap.

The reason why I'd use the lasers is cause my micrometre is solid to get accurate readings from. Because you don't just want to measure at TDC but other places down and using conventional verniers this is impossible.

You also need to measure the bore in 6 different places at 60 degrees apart so you get a good indication of how the bore is.

I've just had a barrel back and the technician was baffled at how different the shape was nearer the bottom than it was near the top.

Personally I can't see how you'd get an accurate reading using verniers. Interested in finding out.

Rebuilt my dt top end this weekend. The 4dl cylinder i put on there back in August has been shipped off to have some porting work done to it.

But yeah. Sent away my original dt barrel that's covered 15'000 miles on original bore and 350 miles was stolen with little/no oil. And he commented saying that it took some honing.

Yeah the 3mb00P.

But no I got my original bore back that came with the bike. And the guy who honed it said it was a really odd shape and said it needed quite a bit of work done to it. It's fine now but was odd when he measured it.

It's this precision of measuring that I'm talking about. The cylinder didn't need a rebore. There was some slight scoring but that came out with the hone.

But he noticed as he measured it that it was out of shape. And other areas of the bore needed more honing. The guy is a two stroke nut and after I picked the barrel up he phoned me up and were talkingg for about 30 mins about bikes and stuff. Top lads really helpful. Yamaha dealership. I paid £35 incl vat. They're fairly competitive not what I expected from a Yamaha dealership. And even gave me discount on some two stroke oil i bought today.

Okay yes i've spent £500 in the last couple of months there but still most of that was parts. Always very quick to help and best of all they understand that the bike is a rolling project and don't pressure me into taking out servicing on it or any other sales tactics. Nah truly rate them.

wish I could say the same about the yam dealer down here.theyre useless however never used the workshop side

They do sound very helpfull and quite cheap for a main dealer I think the wossener on pjme is the same price as that.the only answer I ever get out of the yam dealer in norwich is point to which bit u want or all I can show you is the picture.not very helpfull at all

madrevs wrote:

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Can see what your both saying but I think as im getting it rebored anyway they will do all the measuring.all I need is a rough size so I know whether the cylinder is in limits.today I measured it with a digital vernier and it measured 57mm.also measured in a few different places and it ranged from 56.96 to 57.22 so I think I meed to go for a 57.50 to be on the safe side.will make sure I check the end gap as well.also one more question have noticed on the its stamped 3bn and on the cylinder its 3mb are they compatible together.am going to replace all the bits I can while the top ends in bits.as if you done over 2000 miles trip on a 125

-- Edited by madrevs on Wednesday 12th of February 2014 07:32:29 PM

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 I can assure you i did lol. Me and a friend on a CBR125. Heres a few photos!

 

 Day 1 at lands end

As you can see we intended to camp. That lasted 1 night, the rest was 15 quid a night hostels. The bottom photo is at loch ness, the one above is a random old petrol station just north of inverness. The only real problem we had was when the cbr's chain snapped ( half the link snapped) just north of watford gap service. iphones out and we had to buy and fit a new one in the car park. The photo is him adjusting it on the way up around birmingham i think. He refused to lubricate it at all! It was an adventure and id recceommend it to anyone who wants to push themselves and their arse to the limits haha!

Andy that looks/sounds amazing! Espeically on 125's wtf! Didn't know that was even possible!

Oh and the initial plan was to do the entire coastline. We made it to lands end the started coming up the << side of cornwall and followed it all the way round to somewhere near you cal, before we decided to head straigh from john o groats. We only had 9 days off work so realised we wouldnt make it. The whole trip took 7 in the end.

 

Thistledown Farm, Tinkley Lane, Nympsfield, Gloucestershire was the campsite. Well reccommended!

-- Edited by AndyYam on Thursday 13th of February 2014 01:38:38 PM

Yeah make sure you mod the pv then. If its pushing one side of the piston it makes sense that the other side would get damage too. Now you've found the problem its just a case of getting the work done and rebuilding. Look forward to a video of it running again. keep asking questions if you need anything or are even unsure. Piston looks like it was sealing well up until the final moments! Any idea what piston or those rings are? the rings look cheap to me but not 100%.