CR125 big bored to 144cc has marks on the exhaust bridge (photo)

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3/17/2019 10:57 PM
Edited Date/Time: 3/17/2019 11:02 PM

Hi, i have a Honda CR125R 2007 With big bore that done by known workshop in US, on the first rebuild I’ve seen that the exhaust bridge is not polished good but i am not sure if it came that way
So, before i am paying for shipping to US (i live in Israel) and getting allmost one month of downtime
So what is those marks?
Can you guys tell me what cause it?
Can i fix this myself?
I ride mostly gnarly staff, my bike haven’t seen open power valve for more than 100 hours
The bike is basically running good butl t starts to loose compression after 40 hours of each piston that kind of bothers me (normally i can run 80-100 hours on each top end)
Plus, maybe the engine can run better with this problem fixed

Thanks guys










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3/17/2019 11:11 PM

It looks like the bridge has been ground down slightly. Some people to this to reduce the chance of the piston seizing here. If you put a straight edge on the bridge and it is lower than the cylinder, it has been ground down some. This will have zero affect on how it runs. Your piston looks like it might be close to seizing there the holes are drilled but it's hard to tell from the photo.

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3/18/2019 5:41 AM

Way too many hours on that piston.

Paw Paw

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3/18/2019 11:54 AM

Paw Paw 271 wrote:

Way too many hours on that piston.

Paw Paw

How many “too many hours”?
This piston was taken off after 68 hours as for preventative maintenance, not because of lack of performance.
The service manual says every 7.5 hours if i am not wrong but it refers to pro guy that race and taking a factor of safety/quality
I ride in tigh technical terrain, my bike never reach to the mid-top range
My KTM EXC 125 got piston every 80 hours for almost 600hrs in total and that is my reference

So consider the fact that this piston was taken off after 68 hours and according the piston condition, what you would do?

Thanks

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3/18/2019 11:58 AM

barnett468 wrote:

It looks like the bridge has been ground down slightly. Some people to this to reduce the chance of the piston seizing here. If you put a straight edge on the bridge and it is lower than the cylinder, it has been ground down some. This will have zero affect on how it runs. Your piston looks like it might be close to seizing there the holes are drilled but it's hard to tell from the photo.

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If so, why the surface of the piston that come in touch with the piston bridge seems to get more friction
It can’t reduce compression and piston “life”?

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3/18/2019 12:27 PM

You should be doing pistons every 25-30 hours in a big bore 144 bike. It’s not made to be big bore from the factory so you are going to wear out things faster with the extra force from the bigger piston in there. The exhaust bridge on there definitely has been relieved in order to keep the piston expansion from siezing it. That’s what the holes help with too.

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3/18/2019 1:16 PM

the single ring piston is a bizarre choice for longevity too, unless you replace them at half piston intervals or so.

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vomiting equals disqualification.

3/18/2019 1:32 PM

BR8ES wrote:

the single ring piston is a bizarre choice for longevity too, unless you replace them at half piston intervals or so.

Well this is Eric Gorr kit...
I wanted my CR to be a gnarly weapon and he did very good job with it
My bike fells very good and have lot of torque from idle, too bad he have no double ring set up

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3/18/2019 1:41 PM

BR8ES wrote:

the single ring piston is a bizarre choice for longevity too, unless you replace them at half piston intervals or so.

Yotam wrote:

Well this is Eric Gorr kit...
I wanted my CR to be a gnarly weapon and he did very good job with it
My bike fells very good and have lot of torque from idle, too bad he have no double ring set up

I hear you, they work of course. I would have no problems running the single ring, just not the same one for the life of the piston is all.

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vomiting equals disqualification.

3/18/2019 3:23 PM

barnett468 wrote:

It looks like the bridge has been ground down slightly. Some people to this to reduce the chance of the piston seizing here. If you put a straight edge on the bridge and it is lower than the cylinder, it has been ground down some. This will have zero affect on how it runs. Your piston looks like it might be close to seizing there the holes are drilled but it's hard to tell from the photo.

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Yotam wrote:

If so, why the surface of the piston that come in touch with the piston bridge seems to get more friction
It can’t reduce compression and piston “life”?

Nobody can tell you for certain why the front of your piston looks like that.

It has an insignificant affect on compression and piston life.



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3/18/2019 3:30 PM

barnett468 wrote:

Nobody can tell you for certain why the front of your piston looks like that.

It has an insignificant affect on compression and piston life.



It is make sense that it comes from the exhaust bridge becaus this point located just in front of it
This is exactly what i asked initially
Maybe should I sand it little with fine sand paper 1500+?
or maybe even diamond paper?

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3/18/2019 3:35 PM
Edited Date/Time: 3/18/2019 3:36 PM

The piston skirt shows a lot of wear and indicates that the piston in rocking in the bore due to too much wear.
I would suggest that between 25 and 30 hours is a very long life for this set up and you have gone way beyond that.
I would also suggest a ring change at 10-15 hours max.

Paw Paw

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3/18/2019 3:39 PM
Edited Date/Time: 3/18/2019 3:39 PM

Paw Paw 271 wrote:

Way too many hours on that piston.

Paw Paw

Yotam wrote:

How many “too many hours”?
This piston was taken off after 68 hours as for preventative maintenance, not because of lack of performance.
The service manual says every 7.5 hours if i am not wrong but it refers to pro guy that race and taking a factor of safety/quality
I ride in tigh technical terrain, my bike never reach to the mid-top range
My KTM EXC 125 got piston every 80 hours for almost 600hrs in total and that is my reference

So consider the fact that this piston was taken off after 68 hours and according the piston condition, what you would do?

Thanks

68hrs on a 125 piston is way too many. On a 125 I'd do 20-25 hrs at the most. For a fast guy, probably 10-15.

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2018 KTM 150SX

3/18/2019 3:49 PM

Ok, 20-30 hours will be from now on
What about the exhaust bridge?
Should I do something with it?

In the photos stock vs big bore, you can see the difference



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3/18/2019 3:59 PM

Yotam wrote:

Ok, 20-30 hours will be from now on
What about the exhaust bridge?
Should I do something with it?

In the photos stock vs big bore, you can see the difference



What are those scratch marks in the cylinder?

Is this a nickasil cylinder?
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3/18/2019 4:41 PM

Yotam wrote:

Ok, 20-30 hours will be from now on
What about the exhaust bridge?
Should I do something with it?

In the photos stock vs big bore, you can see the difference



barnett468 wrote:

What are those scratch marks in the cylinder?

Is this a nickasil cylinder?
.

The bottom pic is his stock bore, the one with the scoring issues. He's just showing to give a reference of the exhaust bridge.

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2018 KX450F
2005 KX250
2003 KDX 220
1984 KX 80
1982 RM 80
1980 RM 80
1977 XR 75
1969 Honda Mini 50

3/18/2019 5:13 PM

Exhaust bridge looks like it may be cracked to me?

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3/18/2019 5:27 PM

Paul_Pitzonka wrote:

Exhaust bridge looks like it may be cracked to me?

I don’t think so
But here’s some more photos, all these where taken long time ago, the bike is running new piston since then for about 40 hours.
It is just starting to feel like little slower and lacking compression that was one of the triggers to this thread (hard to start it cold).











uv

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3/18/2019 6:12 PM

I’ll give you a factory secret.

Heat the cylinder as hot as you possibly can (I’m talkin hot, torch right in the bore hot) then drop it in a bucket of the coldest water you can get!!

Works like a charm every time

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3/18/2019 6:30 PM

40 hours out of a 144 big bore piston is really good.

Get a new one in there and go ride... I am not seeing anything wrong with that cylinder... if there was a problem, it wouldn't have gone 40 hours.

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3/18/2019 9:46 PM

Looks like the bridge was repaired at one point and then ground back just a little. This keeps it from getting too hot and cracking.

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3/19/2019 2:57 PM

Spudnut wrote:

I’ll give you a factory secret.

Heat the cylinder as hot as you possibly can (I’m talkin hot, torch right in the bore hot) then drop it in a bucket of the coldest water you can get!!

Works like a charm every time

What does this accomplish ? Thanks

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3/19/2019 6:33 PM
Edited Date/Time: 3/19/2019 6:37 PM

You have to have relief of the exhaust because it will expand into the cylinder and stick the piston. It looks like it was relieved and holes drilled in the piston to help with this. On my vintage Elsinores they have a .003-.004" relief on the bridge. It looks like a bit of a rough job on the relief on that cylinder.

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Take it to the limit, one more time!

3/19/2019 6:36 PM


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Take it to the limit, one more time!

3/20/2019 5:34 AM
Edited Date/Time: 3/20/2019 5:37 AM

Exhaust bridge is fine it looks to have been relieved a little bit when it was replated which is normal it helps with cracking and catching of the piston. Use a little scotch-brite green and wd40 to knock off the glaze on the inside of the cylinder. It has some scoring but it looks pretty superficial scotch-brite should help that. Wash with warm dawn soap and water. As previously stated 68hr is to much on a 144 you need to be at least putting a new ring on the piston every 25-30 hr. Hope this helps.

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3/20/2019 9:03 AM

So the reason the exhaust bridge is ground down a bit is mainly because the piston most likely used was a forged piston. Forged pistons expand more than cast. With that little piece of metal, the combination of grinding it down AND drilling the small holes in the exhaust side of the piston, allow the bridge to cool better, which substantially lowers the risk of a seizure.
Looking at the piston and the wear on it, I'd grind it down a small amount more. You could call Eric Gorr and send him these same pictures, I'm sure he'd tell you if you need to do a little more grinding.

I'd have to agree with the rest of the comments that 50-60 hours on a 144 is a lot.

Before reassembly, I'd hone the cylinder, then make sure the new piston clearance is good, just to be sure.

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3/20/2019 9:08 AM

danman wrote:

So the reason the exhaust bridge is ground down a bit is mainly because the piston most likely used was a forged piston. Forged pistons expand more than cast. With that little piece of metal, the combination of grinding it down AND drilling the small holes in the exhaust side of the piston, allow the bridge to cool better, which substantially lowers the risk of a seizure.
Looking at the piston and the wear on it, I'd grind it down a small amount more. You could call Eric Gorr and send him these same pictures, I'm sure he'd tell you if you need to do a little more grinding.

I'd have to agree with the rest of the comments that 50-60 hours on a 144 is a lot.

Before reassembly, I'd hone the cylinder, then make sure the new piston clearance is good, just to be sure.

Thanks for the reply mate but i think you’re wrong with the honing stuff
It worth checking but As much as i know 2 smokers shouldn’t be honed no matter what, especially those with nikasil plating

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3/20/2019 10:27 AM
Edited Date/Time: 3/20/2019 10:39 AM

danman wrote:

So the reason the exhaust bridge is ground down a bit is mainly because the piston most likely used was a forged piston. Forged pistons expand more than cast. With that little piece of metal, the combination of grinding it down AND drilling the small holes in the exhaust side of the piston, allow the bridge to cool better, which substantially lowers the risk of a seizure.
Looking at the piston and the wear on it, I'd grind it down a small amount more. You could call Eric Gorr and send him these same pictures, I'm sure he'd tell you if you need to do a little more grinding.

I'd have to agree with the rest of the comments that 50-60 hours on a 144 is a lot.

Before reassembly, I'd hone the cylinder, then make sure the new piston clearance is good, just to be sure.

Yotam wrote:

Thanks for the reply mate but i think you’re wrong with the honing stuff
It worth checking but As much as i know 2 smokers shouldn’t be honed no matter what, especially those with nikasil plating

I can't say with 100% certainty about honing, but, that is what I've been told by several people that do motor work.
The honing process doesn't take a bunch of surface off any plating. Even after a fresh plating, those cross hatches have to get into that surface somehow, and I've been told that a new honed surface will allow a better mating of the ring and cylinder wall to seal better. Like I said, I've been told that by a bunch of people, but that certainly doesn't make it correct.
If you call Gorr, ask him. I'd be interested in what he says.


EDIT - you got me curious so I did a quick search. Of course there are always conflicting viewpoints, but here is a Thumpertalk thread that talks about honing, but points out that with a plated cylinder, you are actually just deglazing it.
https://thumpertalk.com/forums/topic/294800-honing-nikasil/

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3/20/2019 10:33 AM

Good lord, I've had more 144s than I can remember and none looked as good as this or went as long on a piston.

Clean the cylinder, fit a new piston and go ride. It shouldn't be this complicated.

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3/20/2019 11:02 AM

danman wrote:

So the reason the exhaust bridge is ground down a bit is mainly because the piston most likely used was a forged piston. Forged pistons expand more than cast. With that little piece of metal, the combination of grinding it down AND drilling the small holes in the exhaust side of the piston, allow the bridge to cool better, which substantially lowers the risk of a seizure.
Looking at the piston and the wear on it, I'd grind it down a small amount more. You could call Eric Gorr and send him these same pictures, I'm sure he'd tell you if you need to do a little more grinding.

I'd have to agree with the rest of the comments that 50-60 hours on a 144 is a lot.

Before reassembly, I'd hone the cylinder, then make sure the new piston clearance is good, just to be sure.

Yotam wrote:

Thanks for the reply mate but i think you’re wrong with the honing stuff
It worth checking but As much as i know 2 smokers shouldn’t be honed no matter what, especially those with nikasil plating

The nikasil is so hard you could scrub at the cylinder with green scotchbrite pads until your arm fell off, and it still wouldn't hurt the coating. Heck, I've used muriatic acid with a stainless steel scrub pad to remove aluminum piston deposits from a coated cylinder with no ill effects. The scotchbrite pad is just getting rid of the glaze created by oil that's burned during the combustion process.

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