Cylinder bridge crack cr 125

[robert robkzns]

Hello,I have a 2002 cr 125 i hase around 25 hour since new piston ring and reeds.winter is coming and i decided take it apart and clean the bike up i took the head off and i found a hairline crack in the cylinder also the head doesnt look very good.Is the cylinder fixeable?Also bike was running good with this cylinder before i tore it apart i will include pic of the spark plug.i ran the bike at 50:1 on motul 800 

Edited November 30, 2024 by robert robkzns

[BV's]

platers in the USA would be able to repair that, you should be able to find one in your neck of the world as well.  

[Eric Nyysti]

I haven’t run 50:1 in a 2 smoker since my bel ray days 😂 I have been in the business a  long time and I have learned that the key to longevity of your engine is lubrication! Lubrication is your friend! We run everything out of our performance shop at 32:1 that’s 4 ozs of oil to a gallon of gas. The cracked exhaust bridge is common with too much heat! Like someone said earlier a good replating company will take care of that no problem.

[mlatour]

4 hours ago, robert robkzns said:

around 25 hour since new piston ring

Do you remember if the piston had two oil holes drilled in line with the exhaust bridge?

It's a common 'must-do' mod, many aftermarket pistons already have them.

 

Many European riders have unfortunately been brainwashed that 50:1 is fine in all applications for many years now.

+1 with Eric above, no matter how good the premix oil (and Motul 800 certainly is)

and while it could be acceptable in larger displacement engines (250-500cc)

50:1 remains not much of a safety margin in high revving 65-125 motocross race engines.

Consider running closer to 30:1 (as the manufacturer also recommended)

if excess dripping/smoke occurs changing the premix oil to something better suited to your usage.

 

 

[robert robkzns]

My mechanic told me to run 30:1 after the rebuild to break the bike in and after to run it at 50:1 he is well known in slovakia and a very good mechanic

[robert robkzns]

Many peopel run 50:1 here in ktm 125 kx rm.havent heard anything about the bridge cracking on there bikes 

[Eric Nyysti]

7 hours ago, robert robkzns said:

Many peopel run 50:1 here in ktm 125 kx rm.havent heard anything about the bridge cracking on there bikes 

Comparing a KTM to a Honda doesn’t really compare the KTM cylinder has a better bridge design and are you comparing it to a 02 KTM or the modern version? Here at our shop where we build around 8-900 engines a year every seizure we’ve had both crank or piston has been a 85 or 125 running 50:1. I forgot to mention they were all KTM’s or Husqvarna engines. This has been a debate for as long as I can remember back in the 70’s! It really comes down to how well you maintain your bike? Our full build race engines we change pistons at 10 hours where for a normal engine we’ve gone as long as 40, but that’s with Vertex pistons and not Japanese cast pistons! That bridge probably cracked from running a worn out piston slapping around in the cylinder!

You can run your premix at whatever you want I’m just stating that some of our trail riders are getting 200hours out of top ends at 32:1 it’s tried and true! 

[robert robkzns]

That piston was new i think its wieco 

[mlatour]

Since you're halfway there, for the price of a new base gasket I'd further disassemble the cylinder

and have a look at the piston and ring in line of where it contacts the bridge.

 

Running 50:1 may not be the only cause of the crack but, it's certainly not 'better' in any way.

 

Edited December 1, 2024 by mlatour

[chase chance]

The last gen CR's are prone to exhaust bridge cracking due to a weak narrow bridge and improper warm-ups. you must drill the lube holes in the piston, this helps cool the bridge. You must take extra time on warm-ups as well.......... I learned this the hard way. Never had another one crack.

[CR-Dude4]

Almost looks as if the power valves were striking against the bridge...almost. Looks like it could be repaired. Check Ebay for another cylinder just in case.

[chase chance]

I had a 02 cr250 cracked cylinder bridge repaired. It held up. You really need to take extra time on warm up to prevent cracking. I would even put around few min after a hard ride, to cool it down a bit before killing the engine.

[CR-Dude4]

On 12/9/2024 at 6:26 AM, chase chance said:

I had a 02 cr250 cracked cylinder bridge repaired. It held up. You really need to take extra time on warm up to prevent cracking. I would even put around few min after a hard ride, to cool it down a bit before killing the engine.

Spooky. In aviation maitnence school, we magna fluxed LYCOMING engines for cracks, and boy if we found one, its off to the bin. I'm no fan of cracks or scratches on any engine, except for honing non Nicky cylinders.