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YZ 125 barrell - Exhaust bridge - Cylinder repair info.

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I thought I would share this with some of you as you might have expereinced this or be interested in this. It is a note I wrote for my future reference for our Small engine rebuilding shop with regards to the exhaust bridge cracking on the YZ125 - 2001 model. 

 

Here is a link to our Facebook page if you want to follow. https://www.facebook.com/CleanRebuilds

 

Information from the Cylinder replaters and from Clean Rebuilds experience to date. 

 

The YZ125 exhaust bridges are thin and are prone to cracking. This is mainly due to heat. This heat can happen for a number of reasons, Jetting, Poor cooling, running lean due to, oil mix ratio, magneto side crank seal leak or general leaks in the intake system, Air fuel screw mixture and other heat related reasons. There are options for cooling the exhaust bridge so as to try to eliminate the potential for a crack in this area. 

This cylinder had a Woosner piston installed and these pistons are forged. A forged billet piston will expand faster than the cylinder will as the cylinder is cast and has pourous holes in the casting. The original OEM piston is cast and so cast on cast has the same thermal expansion rate. I would assume that the clearance is 0.050mm or 2 thou. I don't know if the OEM pistons come with the oil lubricating exhaust bridge holes in them.  

 

We have a completly cracked exhaust bridge in the upper portion and lower portion of the exhaust and it has seperated but still in place. There is good news for this as it can be repaired.

I am assuming that the following few things have occoured here to create these two cracks but it has all stemmed from one thing and that is heat.

  1. A forged piston has been installed and this had lead to a dissimilar thermal expansion rate for the two alloys.
  2. There are no oil lubricating holes in the piston thus the exhaust bridge can't be cooled and has lead to the bridge expanding out into the bore. 
  3. The piston to cylinder clearances for a cast on cast piston and cylinder can be a 2 thou with hat appers to be little o no damage but the forged piston would have been better with 2.5 - 3 thou clearance. 
  4. I can't be sure but I believe there is no Exhaust bridge relief built into the the cylinder from stock thus the bore size is the same for the entire cylinder including the exhaust bridge. 
  5. I have also been informed but can't be sure that the plating is thinner from stock than what is available from our replater.

I have checked the jetting and is is stock factory. 

Another thing that will effect the engine with regards to heat is holding the throttle wide open for extended periods. This is not good for it adit experienced a bit of this before it lost compression.

The fuel mix is 32.1.

 

The total cost for a new OEM cylinder is $ 308.00 USD so landed here in NZ at the current exchange rate of 0.81 is about $ 450.00.

The total cost to replacte this cylinder is $ 450.00 plus GST so this equates to a figure of $ 517.00 plus a bit of freight. 

 

So here's what we have decided to settle on..................A replated and repaired cylinder from here in NZ.  The reasons are as follows, 

  1. We can bore the cylinder to 2.5 - 3 thou to allow for extra piston to cylinder clearances for the forged single ring piston. 
  2. We can add 3 thou into the exhaust bridge before plating. This is something we can't do with the stock OEM cylinder.
  3. Add two 1 mm oil holes into the piston and chamfer the outlet side for better oil dispersement.
  4. We will be getting a thicker layer of plating than that of stock.
  5. A tig welded repair to the bridge. 

All of the above mentioned custom mods and extra clearances will allow for better cooling in this area thus providing us with a bit more confidence to operate this machine on the track. 

I we stayed with the stock order cylinder we would have,

  1. Thinner plating.
  2. Stock clearances meaning that a stock OEM piston should only be operated.
  3. No Exhaust bridge relief.
  4. Possibly no oil lubricating holes.
  5. The potential for the new cylinder bridge to crack and be in the same situation all over again. 

 

   

So in short a customised cylinder set up for better heat transfer and limited risk of cracking again compared the the stock cylinder. 

 

Y1.jpg

 

Y2.jpg

 

Y3.jpg

 

Y4.jpg

 

Y5.jpg

 

Y6.jpg

 

Y10.jpg

 

Y11.jpg

 

Y12.jpg

 

Y13.jpg

Edited by GEOF

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I was under the impression that oiling/cooling holes were required in every application where a bridged exhaust port was used.  Whoever put this thing together without drilling the piston made a big mistake.

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Correct, the piston must have holes over the exhaust bridge. If they're not there already when purchased, they need to be drilled.

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So when I look at the Piston Pictures I am beginning to wonder about the failure?

 

My thought was that the intake side of the piston appears to show signs that the Piston Ring retaining pin may have come loose.  When the pin was coming loose it may have jammed in the intake ports and forced the top of the piston forward into the exhaust bridge on the opposite side when the piston became too big for the bore size as a result of the pin hanging out of the piston and contacting the intake ports.

 

Is there a scratch or ding in the cylinder on the intake side?

 

It does not appear that this is a Heat related failure but more of a mechanical malfunction.

 

Please provide your comments.

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I rarely see a bridge failure on a stock bore cylinder.  In fact - I don't think I've seen any if the engine didn't have a different problem (such as ring failure snagging port)

It's not too hard to relieve  the stock bridge if you wish to do so - and you should on a tight bore with aftermarket piston.

 

Once a bridge has cracked  - very rarely can it be fixed.  Even after the plater welds it and replates - it usually cracks again.

I have seen engines with cracked bridges continue to run with no issue for many hours - depends on the location and the motor.  In your pictures the bridge looks shifted over a little - suggusting possibly the ring pin and ring issue may have been the ultimate culprit. 

For the cost of repair after parts on that - owner should consider a newer/different bike possibly

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I rarely see a bridge failure on a stock bore cylinder.  In fact - I don't think I've seen any if the engine didn't have a different problem (such as ring failure snagging port)

It's not too hard to relieve  the stock bridge if you wish to do so - and you should on a tight bore with aftermarket piston.

 

Once a bridge has cracked  - very rarely can it be fixed.  Even after the plater welds it and replates - it usually cracks again.

I have seen engines with cracked bridges continue to run with no issue for many hours - depends on the location and the motor.  In your pictures the bridge looks shifted over a little - suggusting possibly the ring pin and ring issue may have been the ultimate culprit. 

For the cost of repair after parts on that - owner should consider a newer/different bike possibly

 

Thanks for the comments chaps and I will reply in depth a little later.  

Harrperf.....I am interested to know how you can relieve an exhaust bridge on a plated cylinder? I know this can be achieved on a sleeved cylinder with ease but plated is a little harder to remove stock. How do you do it at your end? 

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Lot's of patience and time with the porting tools...

Plater does the same, proper prep if I send a cylinder in that I've bored is for me to relieve the bare cylinder such that once the plater relieves it after plating and final hone - there is still plenty of thickness of plating (plating thickness not decreased due to relief.)  then they stick in a porting tool and grind away...

In a perfect world we would cnc machine the relief to be most at the middle of the bridge tapering to normal size as it meets up with the bore. 

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Lot's of patience and time with the porting tools...

Plater does the same, proper prep if I send a cylinder in that I've bored is for me to relieve the bare cylinder such that once the plater relieves it after plating and final hone - there is still plenty of thickness of plating (plating thickness not decreased due to relief.)  then they stick in a porting tool and grind away...

In a perfect world we would cnc machine the relief to be most at the middle of the bridge tapering to normal size as it meets up with the bore. 

So if I am reading this correctly you are saying that the plater does a bridge relief after plating aswell and then hones the cylinder? You do the initial relief in your shop during the boring process and this would expose a bare aluminium cylinder wall with no plating on it that you could then put relief into it and then send it to the platers. If so then this would mean that special carbide or diamond type porting tools would be required to relieve the plating at the plating end of the process? I would have thought this would have been very difficult to achieve but I think I understand what you are saying.

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So when I look at the Piston Pictures I am beginning to wonder about the failure?

 

My thought was that the intake side of the piston appears to show signs that the Piston Ring retaining pin may have come loose.  When the pin was coming loose it may have jammed in the intake ports and forced the top of the piston forward into the exhaust bridge on the opposite side when the piston became too big for the bore size as a result of the pin hanging out of the piston and contacting the intake ports.

 

Is there a scratch or ding in the cylinder on the intake side?

 

It does not appear that this is a Heat related failure but more of a mechanical malfunction.

 

Please provide your comments.

 

Hey Cannon. Thanks for your reply. I am starting to see what you are saying and I am leaning more towards your comments and Harrisperf. It does look like more of a mechanical failure now that I have had a better look at it after your comments. I will post some pics later but your are correct about the piston pin snagging on the inlet side. There is a bit of a gouge as you have suggested and it is directly in line with the pin. Half of the piston ring pin is still there. So this would suggest that the pin has let go, gouged the back of the cylinder, pushed the piston forward towards the exhaust bridge and cracked the bridge at the top and the bottom. All the scoring, gouges and cracks would suggest this as you and harris have mentioned. Thanks for your input. Now what do I do with this......Fix it and replate it or buy OEM and stay with stock 2 thou clearances and no bridge relief but drill two oil lubricating holes to minimise the risk of cracking on the new cylinder. What do you guys think? 

Oh just to clarify I m not a professional engine rebuilder. It is a hoobie of mine and I am very much in the early stages and so is the name. So we are still learning so apologies if I have come across as a know it all as that was not my intention. Thanks for your comments and remarks as they help me learn more. I am currently studying at Penn Foster doing their motorcycle and ATV technician course to try to advance my knowledge. 

Once again thanks for you help chaps. 

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Hey Team. 

Here are a few more pics I took today after reading what you guys have written. 

Please let me know what you think and your input. 

 

1.jpg

 

2.jpg

 

3.jpg

The above three pics show the gouging from what looks like what you are saying Connor is the piston ring retaining pin.

4.jpg

This one above shows the general overall location on the inlet side. 

6.jpg

In this pic you can just see a little burning on the top of the piston on the side above the ring. 

7.jpg

In the above pic and below pics you can see there is only half the pin left. Although it may be hard to see. 

8.jpg

 

9.jpg

 

10.jpg

Edited by GEOF

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This touched on one of the things I worry about forged pistons.

We know they grow more. And the perfect situation would be a completely uniform growth. Well considering shape and the uneven heating that occurs. Can one really expect a piston that's perfectly round under operating conditions?

Even if heated uniformly I wouldn't expect it to maintain concentricity based on it's complex distribution of metal.

Could try heating one in an oven then measuring it. Use gloves. Lol

Point being you'd seem to have better luck starting with a more thermally stable piston that will maintain its dimensions under use.

Thus maintaining closer tolerances and better performance, key in a 2t.

My preference has always to use cast except in situations of extremely high output engines when cast just aren't strong enough.

Those types of engines are usually only seen on gp style road race engines and some karting applications.

Which leads to another possible solution if desiring bridge relief. One of the best piston I've ever seen is the rs125 piston. It requires extensive break in. After getting tired of that procedure I helped it along on some of my builds by reliving the piston along the area that runs on the bridge. Sorta a reverse relief you could say.

Since they can get away with narrower power bands they can be tuned to higher peak hp which isn't common on a wide band style mx set up.

Edited by Supertuner_tb
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This touched on one of the things I worry about forged pistons.

We know they grow more. And the perfect situation would be a completely uniform growth. Well considering shape and the uneven heating that occurs. Can one really expect a piston that's perfectly round under operating conditions?

Even if heated uniformly I wouldn't expect it to maintain concentricity based on it's complex distribution of metal.

Could try heating one in an oven then measuring it. Use gloves. Lol

Point being you'd seem to have better luck starting with a more thermally stable piston that will maintain its dimensions under use.

Thus maintaining closer tolerances and better performance, key in a 2t.

My preference has always to use cast except in situations of extremely high output engines when cast just aren't strong enough.

Those types of engines are usually only seen on gp style road race engines and some karting applications.

Which leads to another possible solution if desiring bridge relief. One of the best piston I've ever seen is the rs125 piston. It requires extensive break in. After getting tired of that procedure I helped it along on some of my builds by reliving the piston along the area that runs on the bridge. Sorta a reverse relief you could say.

Since they can get away with narrower power bands they can be tuned to higher peak hp which isn't common on a wide band style mx set up.

 

Hey Supertuner. I am hearing you on all accounts there and I total understand what you are saying. Thats well writtten and I thankyou for your knowledge and input. Cheers.  

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This touched on one of the things I worry about forged pistons.

We know they grow more. And the perfect situation would be a completely uniform growth. Well considering shape and the uneven heating that occurs. Can one really expect a piston that's perfectly round under operating conditions?

Even if heated uniformly I wouldn't expect it to maintain concentricity based on it's complex distribution of metal.

Could try heating one in an oven then measuring it. Use gloves. Lol

Point being you'd seem to have better luck starting with a more thermally stable piston that will maintain its dimensions under use.

Thus maintaining closer tolerances and better performance, key in a 2t.

My preference has always to use cast except in situations of extremely high output engines when cast just aren't strong enough.

Those types of engines are usually only seen on gp style road race engines and some karting applications.

Which leads to another possible solution if desiring bridge relief. One of the best piston I've ever seen is the rs125 piston. It requires extensive break in. After getting tired of that procedure I helped it along on some of my builds by reliving the piston along the area that runs on the bridge. Sorta a reverse relief you could say.

Since they can get away with narrower power bands they can be tuned to higher peak hp which isn't common on a wide band style mx set up.

 

 

Pistons aren't*** round to start off with - modern machining process is a specialized CNC that makes them whatever shape the designer wants - all pistons have a huge taper from top to bottom, top being the smallest (for heat expansion).  Pistons also aren't round - and with modern machining it's possible to machine in the additional relief at bridges into the piston as well instead of having to do it by hand.

Getting this profile correct is where good piston design seperates itself from poor design.  OEM's have this aspect of a piston much more thoroughly designed than an aftermarket piston.  On current four strokes, we've found stock pistons to perform best in almost all applications - even at a lower compression ratio etc.  The design from the OEM holds round under temp and load better for better ring seal etc...

The aftermarket isn't stupid however - and over time their shapes evolve to fit any problems they encounter.  For the applications we have - the majority of aftermarket pistons we use outlast, and perform as well as oem...

Edited by harrperf

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Pistons are round to start off with - modern machining process is a specialized CNC that makes them whatever shape the designer wants - all pistons have a huge taper from top to bottom, top being the smallest (for heat expansion). Pistons also aren't round - and with modern machining it's possible to machine in the additional relief at bridges into the piston as well instead of having to do it by hand.

Getting this profile correct is where good piston design seperates itself from poor design. OEM's have this aspect of a piston much more thoroughly designed than an aftermarket piston. On current four strokes, we've found stock pistons to perform best in almost all applications - even at a lower compression ratio etc. The design from the OEM holds round under temp and load better for better ring seal etc...

The aftermarket isn't stupid however - and over time their shapes evolve to fit any problems they encounter. For the applications we have - the majority of aftermarket pistons we use outlast, and perform as well as oem...

Oh yeah you can have non round pistons. But when you think about the type of tolerances involved I'm skeptical.

For example. Say you have 3 thou clearance, at the top.That being the difference in bore size and piston diameter. That's 1.5 all around. You get expansion in both the cylinder and piston. Now to create a non round piston for expansion compensation. Which first you have to be able to predict, possible with better finite element analysis software available these days. Still tough with the complex and variable heat loading in an engine. You're talking about differences in roundness from the RADIUS out around a piston off half a thou or less.

possible of course. However, in mass produces pieces that's really tough.

Also when you refer to piston life oft forged piston s that also include cylinder life?

Edited by Supertuner_tb

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I rarely see a bridge failure on a stock bore cylinder. In fact - I don't think I've seen any if the engine didn't have a different problem (such as ring failure snagging port)

It's not too hard to relieve the stock bridge if you wish to do so - and you should on a tight bore with aftermarket piston.

Once a bridge has cracked - very rarely can it be fixed. Even after the plater welds it and replates - it usually cracks again.

I have seen engines with cracked bridges continue to run with no issue for many hours - depends on the location and the motor. In your pictures the bridge looks shifted over a little - suggusting possibly the ring pin and ring issue may have been the ultimate culprit.

For the cost of repair after parts on that - owner should consider a newer/different bike possibly

When you say you've only seen bridge cracks on pistons with ring failure how did you determine which came first?

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Hey Cannon. Thanks for your reply. I am starting to see what you are saying and I am leaning more towards your comments and Harrisperf. It does look like more of a mechanical failure now that I have had a better look at it after your comments. I will post some pics later but your are correct about the piston pin snagging on the inlet side. There is a bit of a gouge as you have suggested and it is directly in line with the pin. Half of the piston ring pin is still there. So this would suggest that the pin has let go, gouged the back of the cylinder, pushed the piston forward towards the exhaust bridge and cracked the bridge at the top and the bottom. All the scoring, gouges and cracks would suggest this as you and harris have mentioned. Thanks for your input. Now what do I do with this......Fix it and replate it or buy OEM and stay with stock 2 thou clearances and no bridge relief but drill two oil lubricating holes to minimise the risk of cracking on the new cylinder. What do you guys think? 

Oh just to clarify I m not a professional engine rebuilder. It is a hoobie of mine and I am very much in the early stages and so is the name. So we are still learning so apologies if I have come across as a know it all as that was not my intention. Thanks for your comments and remarks as they help me learn more. I am currently studying at Penn Foster doing their motorcycle and ATV technician course to try to advance my knowledge. 

Once again thanks for you help chaps. 

 

Geof:

 

My recommendation along with Derrick's input, I would not attempt repair of you cylinder with Damaged exhaust bridge. I would do one of a few things depending on the price in your area.

 

1 find good used cylinder measure and inspect and decide to use it as is or have it replated.

2 find new or used cylinder and bore and replate to 144CC depending on your needs and racing rules consider porting during this process. (this is what I will do for my son's 125)

3 buy Athena 144 kit if price permitting Consider porting on new cylinder if budget permits

4 buy new OEM cylinder with corresponding letter piston.

 

If you send your cylinder in on an exchange basis for a repaired/replated cylinder please let us know where you sent it as none of us want to to receive that cylinder in return after repairs have been made.

 

Your decision will be influenced by what your local market has to offer as I have no idea about your local market or availability. 

 

I would not spend a lot of time and effort worrying about Cast vs Forged as both work well and have pro's and Con's

 

We could speak at length about Bore clearance and its effect on performance but this decision cannot be made in a vacuum and motor reliability and are also very important factors.  You need to finish the race before you win it.  There are lots of performance mods that will give you better gains than a piston to bore clearance of 1/2 a thousands of an inch less clearance . (just my opinion)

 

Good luck with your study program your doing well so far.

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When you say you've only seen bridge cracks on pistons with ring failure how did you determine which came first?

 

I usually don't see cracked bridges on stock bore engines...and typically chalk it up to the mechanical failure - not the bridge failure.

It's very common for 144's to crack at the bridge - it's why the kit we offer now is welded up thicker at that spot (where powervalve machine slots are cut) and powervalve is machined narrow on our kits (adding lots of cost too)  But in those cases the bridge is only 1mm thick if not welded - and cracking would be expected.

Even when they crack - I've seen them go a long long time with no issue...so I wouldn't say the crack (even if it happened first) caused your failure.

Regarding pistons - this day and age - it's pretty well sorted out.  Cast - or forged. I run our 144's looser than spec on bore clearance in order to handle the increased thermal load our engine produces (when making 44 horses vs 35).  This has helped our end reliability go up with users who may not warm it up - or maybe they lost coolant - or are jetted too lean - to keep from having issues as well.

I haven't seen a repeatable measurable difference in power going looser.  For power - most important variable is ring conditition.  After 3 hours there is a noticeable drop on any modern race engine....and after 10 the ring is basically totally worn as far as pure racing efforts are concerned.  It's a weak link in high performance two strokes if they every wished to find their way into endurance or production vehicle engines...(very possible) if ring life could be sorted.  Just a challenge when you have huge holes (ports) in the cylinder.

Edited by harrperf

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I usually don't see cracked bridges on stock bore engines...and typically chalk it up to the mechanical failure - not the bridge failure.

It's very common for 144's to crack at the bridge - it's why the kit we offer now is welded up thicker at that spot (where powervalve machine slots are cut) and powervalve is machined narrow on our kits (adding lots of cost too) But in those cases the bridge is only 1mm thick if not welded - and cracking would be expected.

Even when they crack - I've seen them go a long long time with no issue...so I wouldn't say the crack (even if it happened first) caused your failure.

Regarding pistons - this day and age - it's pretty well sorted out. Cast - or forged. I run our 144's looser than spec on bore clearance in order to handle the increased thermal load our engine produces (when making 44 horses vs 35). This has helped our end reliability go up with users who may not warm it up - or maybe they lost coolant - or are jetted too lean - to keep from having issues as well.

I haven't seen a repeatable measurable difference in power going looser. For power - most important variable is ring conditition. After 3 hours there is a noticeable drop on any modern race engine....and after 10 the ring is basically totally worn as far as pure racing efforts are concerned. It's a weak link in high performance two strokes if they every wished to find their way into endurance or production vehicle engines...(very possible) if ring life could be sorted. Just a challenge when you have huge holes (ports) in the cylinder.

Right on.

I agree with ring seal being the critical situation.

And quick wear is part of the trade off for performance. I'm sure if longevity were priority there would be more than one and not so thin. But when you want low friction less is more right?lol

I've also seen cracks in the plating on bridges that really are no concern.

I imagine it's a combination of a crack that would under proper maintenance would be no issue. But in conjunction with a piston and ring left in too long it eventually gets sideways enough to snag.

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Right on.

I agree with ring seal being the critical situation.

And quick wear is part of the trade off for performance. I'm sure if longevity were priority there would be more than one and not so thin. But when you want low friction less is more right?lol

I've also seen cracks in the plating on bridges that really are no concern.

I imagine it's a combination of a crack that would under proper maintenance would be no issue. But in conjunction with a piston and ring left in too long it eventually gets sideways enough to snag.

 

Sounds like a good diagnosis for this engine.

If the ring wore too much - it may have hopped the ring pin - and created a host of issues.

I had a motor we built years ago pass hands through a few owners.  It started running funky on a guy - he opened it and ring had spun around hopping pin - so he replaced top end only to have it happen again in 2 hours.

 

In that case - plating had gotten so worn it was just way too big - and allowed the ring to hop....I think it's possible this happened - or ring wore to that point.

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