DTR125 2T advice

[satelliteone]

hi all, blew my DTR up this week end, what i have found, seems like the power valve made contact with the piston rings,

the power valve is soo vey close to the rings, and the barrel was rebored 0.5mm over sized. its done a good few k miles before it went pop..

but i was 80+ for 15-ish miles 10k revs. backed off the throttle ,and pop. was thrashng the bollox off of it.

over sizing the bore makes the rings closer to the P.V. so my question is, would the massive heat in the piston at full chat. expand so far, that contact with the power valve caused this.

also, pic of the PV in the shut position , closesst to a piston ring.

why the dtr shuts the port down by roating the P.V backwards (clockwise) is beond me.

[2 Wheels]

Was your rebore done by a profesional?

you should not be able to see the piston or the rings through the PV chamber. (2nd pic.)

Looks like your bottom ring caught the lip of that excessive rebore on its way down and that is what done the damage.

Also with a bang like that you could now have a bent crank---- much checking required to see what damage has been done.

[Guest MaD.VeZ]

Looking at the pic it looks possible for the whole lot to heat up enough to cause contact between the two(I had no idea they were that close, im sure mine isn't ) , but if it caused the damage its hard to say. It does look that way though tbh, the marks on the piston seem to suggest most of the damage was done by the exhaust port/PV area on the upwards stroke by the way the piston ring is melted into the lower ring gap on the piston.

I would also be examining the barrel to make sure when it was bored out it was properly chamfered around the exhaust port area.

Depending on the year of your DTR depends on what direction the PV rotates. Yamaha changed the direction on the later models, this can be checked by examining the servo cables, if they are crossed over i believe it means that the PV rotates backwards(not 100% sure when it was done). Its not a simple case of swapping them either as the cdi controls the amount of rotation so it would mean swapping the servo and cdi etc to change it. Is it possible that the cables have been swapped and are wrong? just a thought.

Vez.

Edit: 2-Wheels, you been having speed typing lessons

[satelliteone]

  yup 2 wheels almost right, mad.vez ur spot on.. keen eye i see..lol..

to clear a few things up. the PV as fully operational, has been for the last 3K miles with this set up. the bike will go off the clock. It was rebored last year. new piston/rings. was orginally 56.00mm then after 20K miles on the original piston, rebored to the current 56.5mm. and done in a bike shop.

this one is a 2003 model and the cables are not crossed. deresticted front pipe and Dep end can.

Ive run the bike hard before, but saturday night, it was the amount of time it was running at 9-10K in 5th n 6th gear. about 15-20 miles at night. racing a car. at 80mph plus.

The bikes powerband was up all the time. till i was gettin to the end of the run and backed off the throttle, and within 30 secs.. power just went, barly running.blown.

When stripped, piston ring fragments were shaken out of the exhaust. the PV valve had horizontal dents. like where the edge of the rings were bedding into the alloy. No damage to the exhaust port., the bore only had 2 small light lines above the exhaust port. u can clearly see impact damage on the PV.

its deff PV valve that caused the ring strikes. got to be the heat and exspansion of the piston and the closeness of the rings to the PV.

and as mad said, when the piston was comming up and the PV rolling back/shutting down the port, that caused it.

Dremmel time me thinks on the PV!..lol

imagine if it was say 2.5mm over sized!. probally within limits on a standard bore.

[Guest MaD.VeZ]

If i were you i would buy a replacement barrel.

Standard OD for the later 3MB barrel is 55.95-55.955 mm

1st over size is 56.25 mm

2nd oversize is 56.50 mm

Total of 0.55 mm from standard to top oversize.

Where you get 2.5 mm from is just a scary.

Now imagine if that re-bore was done off center and towards the front.

Its always best to take it to someone you trust(quite literally with your life) to do re-bores or other serious work, there is no going back to complain when your dead or worse.

Vez

[Airhead]

Its always best to take it to someone you trust(quite literally with your life) to do re-bores or other serious work, there is no going back to complain when your dead or worse.

Vez

i'll second that, just had my 175 barrel done PJME

What a top quality job they made of it

38.000 miles on standard and 1st oversize, and no I dont try to do 80 for twenty minute stretches.

[2 Wheels]

Yea Vez, I think that re-bore was done off center.

There is no way it should be that near the PV.

Speed typing----- im getting a tad faster

[satelliteone]

I see what you are saying, off center. perhaps, but the guy that done it,has been doing it 40 years, i can check too, as the cylinder sleave has a width, i can check with the calipers offset.

as to 80mph..lol.. 10500 rpm is 90 on current gearing. and ive hit 11000 a few times. my chainsaw see`s that all day long.!

"38.000 miles on standard and 1st oversize, and no I dont try to do 80 for twenty minute stretches"

Grandad! hahah nice to know tho, i dont do 80 all the time.. it was just that saturday night.. i was thrashing the bollox off it.but then again..i always can just flick the clutch in an a emergency..which i did at supersonic speed on a B road. anyway would be nice to know that i can!

If it was realiable then it would`nt of done that. and the last piston in the bike lasted 20K and Yam says to rebuild the top end every 10K miles.

2.5mm is the largest piston ive seen FOR the dtr.. scary piston here on ebay! look>> http://cgi.ebay.co.uk/Piston-Kit-Yamaha-2-50-TZR-DT125R-TDR-TZR250-58-90mm_W0QQitemZ150420615638QQcmdZViewItemQQptZUK_Motorcycle_Parts?hash=item2305c471d6

well my final thoughts.The design of the standard engine is ok. basically when it wears out..buy a new one. we all know we will repair the one we got. and to resleave the barrel to standard size is not cost effective at 180 quid! PS. that included the piston/rings/gaskets.

not only that, design flaw in the bottom end too. the left hand(magneto) crank bearing wears faster than the right.why? i know. not enuf thought.

Yam got it wrong!, no tollerence in effective rebores. simple. why the PV valve rotates backwards? if it rotated forwards, i would`nt be here now.

[Guest MaD.VeZ]

2.5mm is the largest piston ive seen FOR the dtr.. scary piston here on ebay! look>> http://cgi.ebay.co.u...=item2305c471d6

That is a piston for a DT250 . If you can get one to fit a DTR125 i will be impressed. Also a four stroke 250 i might add.

why the PV valve rotates backwards? if it rotated forwards, i would`nt be here now.

As i said earlier, the PV no longer rotates backwards, so effectively Yamaha learned from their mistake.

And Maybe your PV is fitted wrong. Is it original for the bike ?

[satelliteone]

hi mad.. u sure u see the same page as me..lol.. its says

Piston Kit Yamaha 2.50 TZR,DT125R,TDR,TZR250(58.90mm)

tzr125 DT125r tdr125 and the 2 cylinder TZR250 (uses the same pistons). the pic is just a pic they have, not the correct for the item.

http://www.pjme.co.uk/acatalog/copy_of_Yamaha_Engine_Parts_Top_End_.html

if ya notice its a 58.9mm thats 2.9mm over sized than standard 56mm DTR piston. scary! for the PV. so in theory, that 1.45mm closer to the PV.

i have seen crossed cables on the older dtr`s..mine is a 2003 model and it came like that from new.

even in the manual it shows them straight.

any way.. my new piston turned up..

old PV! 20 thou clearence 0.5mm.

new modded pv..

now 40 thou.. about 1mm

hate to think what a 58.9mm oversized piston would look like with a standard PV..lol

PS. offset is spot on!

[Guest MaD.VeZ]

hi mad.. u sure u see the same page as me..lol.. its says

Piston Kit Yamaha 2.50 TZR,DT125R,TDR,TZR250(58.90mm)

tzr125 DT125r tdr125 and the 2 cylinder TZR250 (uses the same pistons). the pic is just a pic they have, not the correct for the item.

http://www.pjme.co.u...s_Top_End_.html

if ya notice its a 58.9mm thats 2.9mm over sized than standard 56mm DTR piston. scary! for the PV. so in theory, that 1.45mm closer to the PV.

i have seen crossed cables on the older dtr`s..mine is a 2003 model and it came like that from new.

even in the manual it shows them straight.

any way.. my new piston turned up..

old PV! 20 thou clearence 0.5mm.

new modded pv..

now 40 thou.. about 1mm

hate to think what a 58.9mm oversized piston would look like with a standard PV..lol

PS. offset is spot on!

Nice bit of modding, shouldn't need to be done if it was all correct to start with. I've had more than 1 DTR barrel to 56.5 mm and never had a problem like yours.

I specifically looked for the " Standard picture, not the product you will receive warning" on the ebay link. but meh ,its not easy on a phone screen

Never seen a piston that big in a DTR, do the newer DTR's(post 98) have a bigger barrel lining? Any one got a manual or proof either way? I'm intrigued now.

Vez.

[Cynic]

That piston is for a TDR250, and the barrel specs for the 125 and the tdr250 seem to tally, my TDR motor is Prefixed 1KT. BUT and its a biggy. The 250's PV is TOTALLY different to the one in the pictures, its a lot thinner and is more of a blade, its also made from several parts instead of one piece and it is chamferd both sides presumably to give piston clearance. Dont forget the 1KT TDR has iron liners not the nikasil and goes up a couple more sizes.

Maybe its a TDR/TZR250 barrel with the 125 valve?

[Guest MaD.VeZ]

The pics look like a DT barrel, what bothers me is the possibility of going over size by 2.5mm on a DTR. What kind of oversize can your TDR go to Cynic? And does the TDR share the same piston as the DTR, ie starts at 55.95mm upto 56.5mm?

[Cynic]

 The pics look like a DT barrel, what bothers me is the possibility of going over size by 2.5mm on a DTR. What kind of oversize can your TDR go to Cynic? And does the TDR share the same piston as the DTR, ie starts at 55.95mm upto 56.5mm? 

I'll have a look and see what there is.

[Cynic]

 The pics look like a DT barrel, what bothers me is the possibility of going over size by 2.5mm on a DTR. What kind of oversize can your TDR go to Cynic? And does the TDR share the same piston as the DTR, ie starts at 55.95mm upto 56.5mm? 

Yep same, the barrel in the pics must be a DT one though, the tdr has a different PV and there are no seals in the PV openings, they are o rings in machined groves on the PV is self.

This has got me thinking though, what is the difference between a TZR/TDR250 barrel and the 125 version. With the 250 good for 50 hp, surely you would get over 20 with the right pipe and jetting. Got to be worth a look at the ports and wheather they will actually swap.

Surely the 250 must have bigger transfers and such to get the extra go, but still at heart a 125 barrel.

What do you think?

[Guest MaD.VeZ]

Yep same, the barrel in the pics must be a DT one though, the tdr has a different PV and there are no seals in the PV openings, they are o rings in machined groves on the PV is self.

This has got me thinking though, what is the difference between a TZR/TDR250 barrel and the 125 version. With the 250 good for 50 hp, surely you would get over 20 with the right pipe and jetting. Got to be worth a look at the ports and wheather they will actually swap.

Surely the 250 must have bigger transfers and such to get the extra go, but still at heart a 125 barrel.

What do you think?

Would make sense that the ports be a major factor.

The DTR PV has the same type of o-ring seals in grooves on each side of the PV as well as one on each end cap.

Could there also be a difference in the crank diameter and a longer con-rod setup on the TDR giving a higher compression?. 

Also i wonder if the barrel lining may be different in the TDR to give a bigger compression chamber and a larger piston?

Been onto the wiseco website regarding piston sizes and the biggest you can get for a DTR is a 58mm giving a good 2.05 mm oversize.

What size piston does the TDR take at standard and 2nd oversize?

[Cynic]

Would make sense that the ports be a major factor.

The DTR PV has the same type of o-ring seals in grooves on each side of the PV as well as one on each end cap.

Could there also be a difference in the crank diameter and a longer con-rod setup on the TDR giving a higher compression?. 

Also i wonder if the barrel lining may be different in the TDR to give a bigger compression chamber and a larger piston?

Been onto the wiseco website regarding piston sizes and the biggest you can get for a DTR is a 58mm giving a good 2.05 mm oversize.

What size piston does the TDR take at standard and 2nd oversize?

Same as the 125 at 55.95mm and 56.5mm respectively from Yamaha, the pattern boys go up in .25 incriments. Thats with the earlier 1KT engine with iron liners. The later generation motor (although near identical) had the, one nip an its knackerd nikasil coating.

The bore is the same the stroke is the same, supposedly the pistons are interchangeable so in theory the deck height and crown are the same. The head IS different as links both cylinders together with cooling and such and i've never had the top off the TDR so i can't say what its like first hand. I can imagine the squish is probably different.

There are a couple of proper engine heads on the TDR forum i'll see what gives on there.