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Cr250 piston wear

soap

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#1
The piston in my 02 CR250 has wear on the front (exhaust side) of the piston. There was enough wear to pinch the rings into their seats on the front of the piston so that there was too much blow by and not enough compression to start the bike. Then I found a topic in a forum here that said (at least for Wiseco) I should drill two holes in the skirt of the piston if I have an exhaust bridge.

From what I can see, I have what looks to be an exhaust bridge. I do have an OEM piston and was wondering if I can drill into it? The only reason for concern is that OEM is cast and the Wiseco is forged.
 
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#2
If the oem did not come with holes,I would shy away from drilling. More important is to take your new oem piston and a feeler gauge and measure your cylinder to skirt clearance in different areas,at right angles. Check it where the piston normally rides. Especially at the exhaust bridge. I would think between 3.5 and 4 thousandths,2 on the cylinder walls. When checking the bridge moving the piston up higher than normal is not safe if you have a ridge on the top. Do not force it. I have seen guys take pistons in and out the top,makes me nervous!
 

soap

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#3
I will check the gap with feeler gauges. But the same thing was happening to the piston that I pulled out of the bike when I bought it. And because of that I was thinking that it was either the cylinder is out of round, or the no lube spot that others are talking about. I was hoping that someone with a 02-07 cr250 had the same problem.

Heres the link for the wiseco piston

http://www.wiseco.com/PDFs/Manuals/MSeries.pdf
 

D36-108A

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#4
FYI you can get "gauge stock" made by Starret. Much longer than the cylinder height, just place it in the bore and use it like a giant feeler gauge. Measure at right angles. I found my TZ250 cylinders are oval, with .0025" front-back and .003" side-side.

Beats the heck out of crappy Chinese telescoping tools and micrometers.

Rick at Accu-Products.com has it.
 

soap

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#5
I did measure the cylinder and I got the same as D36 was talking about. My cylinder and piston were both oval, but kept the about the same gap all around. I didnt use a crappy chinese gauge to check it either. My decision was to drill the lube holes and fix the ring seats (since they were tight). I had to use very small files and 500 grit sandpaper to allow the rings to move more freely. I put the engine back together and it fired up.

Tomorrow . . . I guess in 6 hours, Im headed to the dunes to try it out. I'll let you all know how it worked out. And before anyone jumps on me for trying this, I took every precaution to get burs and small flakes of metal off the piston. I cleaned it with a solvent and used oil when assembling. So we'll see how it works.
 
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#6
I have 3 pistons showing the same wear, but I have not yet had the problem with the rings sticking. The pistons are 3 different brands, prox, namura and stock.

I don't think the wearing is a lubrication problem, rather I think it has to do with the exhaust bridge expanding when hot - but I am no expert.

My cylinder is out of round too, the piston shows 2 spots of wear indicating it is touching the cylinder unevenly.
 
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#7
Going to long on rebuilds,I believe is the prime source for distorting cylinders. They come that way occasionally,different plating does different things and the aluminum behind the plating moves. Us chrome has the thickest plating and very good at fixing bores. Checking and staying on the bridge clearance is important,but when a piston,ring wears it lays into the bridge and looks like a bridge seizure Whatever your bore clearance is the bridge relief should be double that! And the bridge is the best spot for flashing to build up.
 
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#8
If the piston is wearing a groove over the exhaust bridge, that's due to a bad right side crankcase seal. Your 02 CR has a similar exhaust port to my 87. Everybody told me it was the exhaust bridge clearance, but that spec'd out. Then, the bike started fouling plugs like crazy and replacing the right side seal fixed both problems.

Here is a shot of my wear. Is this the same thing you are seeing?

 
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#9
rerfurt said:
The pistons are 3 different brands, prox, namura and stock.
The funny thing is that all of those pistons, despite being different brands, are all the same. They're all made by ART.
 
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#10
Gm,where did you come up with this hypothesis about that seal? Do not get me wrong,I like it,but is there any facts to back this up?
 
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#11
It's all explained in Mr. Gorr's book. There's something about hot gasses moving past the bridge at high velocity creating an ionizing charge that pulls clutch debris out of the exhaust gasses and causes them to stick on the exhaust bridge. The debris is very abrasive and wears away at the piston. The clutch debris gets in through the crankcase seal.

Any time this type of wear shows up, most people are immediately suspicious of the exhaust bridge clearance. The deal is, once the exhaust bridge clearance is set, it tends to stay unless the cylinder is severely worn. I would only suspect an exhaust bridge clearance issue if the piston showed this kind of wear immediately after having a cylinder plated or sleeved. Otherwise, I would be replacing the right side seal.
 
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#12
76GMC1500 said:
The funny thing is that all of those pistons, despite being different brands, are all the same. They're all made by ART.
Well the prox and the stock one is marked ART, but are from different batches and different die cast equipment also has different markings and numbers cast into the aluminum. They are also tooled differently (stock appears coarser). The namura doesn't have the ART letters, doesn't have the cast numbers and there is a difference in the hatch pattern on the underside of the piston - also the dome is anodized and the skirt is coated.

The three of them doesn't measure identical either ( but within 0,05mm), and they don't use the same brand of rings. I can't measure it, but I can feel a difference when installing the small end pin, so the holes are cut with different tolerances too.

Also my stock (newest) piston has a definite piston slap at idle, which the namura piston did not.

Sorry if it appears overwhelming I just collected the data a few nights ago to diagnose my piston slap - driving me nuts. :bang:

I measured my gearbox oil recently after 3 hours use - it had gained 10 ml (???) . But then my exhaust bridge wear doesn't look as bad as yours either.
 

soap

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#15
My piston did not look like yours. It did have some wear in the same place but was not a severe. My piston had the dark carbon coloration past the rings, but the wear I could see didn't cut into the piston, it only wore off the coloration. I will probably be taking the engine apart once I get settled into school life again (less than a month) and I'll check the seal. Thanks 76gmc.

On another note the bike ran fine at the dunes today. We were riding for about 5-6 hours and I was running the engine at higher rpms. The bike right from the start seemed to have less compression than normal so I think the rings are still catching a little in the seats and allowing blow by. The compression did not get worse all day though. And the bike would always start within three kicks (normal). When I do pull the engine apart, I will check to see if there is more wear at the bridge.