No Compression '99 Honda 125

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2/24/2019 6:17 PM
Edited Date/Time: 2/24/2019 6:20 PM

I bought a '99 Honda CR125 with no compression. I took off the cylinder head and cylinder and found out that the piston took some beating. The connecting rod is a Hot Cam and the piston looks aftermarket. Could the damage be from an incorrect sized piston or slop in the connecting rod? There is not any play up/down in the connecting rod but there is some side to side. I want to make sure that installing a new piston and cylinder is going to resolve the issue.
Any help is appreciated.


2/24/2019 7:17 PM

Looks like a lean condition condition to me... the exhaust side of the piston looks like it’s melted from the picture of the dome; so you have excessively high combustion temperatures, and you have a siezure by the transfer port... I would lean towards you have an air leak or incorrect jetting...... Connecting rods have side to side play; there’s a spec, if you have a manual. There should be no vertical play...


2/24/2019 7:20 PM
Edited Date/Time: 2/24/2019 11:00 PM


2/25/2019 6:26 AM

Good stuff. Thank you Paul for the feedback. I'll start digging into the carb, reeds and air boot and find out the jetting and hopefully pin point possible air leak areas.


2/25/2019 1:44 PM

Older bike I’d definitely hang crank seals in it... more than likely you’re pulling air from the seal on the stator side...


2/25/2019 3:26 PM
Edited Date/Time: 2/25/2019 3:27 PM

The piston shows signs of a "cold seize". The ring is froze into the piston ring groove. This is when the engine is nor allowed to warm up prior to putting it under load.
Check cylinder size and the piston to cylinder clearance.
Crank sounds ok.
Like said, change the crank seals.
FYI: Hot Cams do not make cranks, so I have to think it is a "hot rods crank.
Paw Paw


2/25/2019 8:15 PM

Looks like you have some detonation going on also, I'd cut your fuel with some race gas to try to get 96 to 98 octane minimum.


2/25/2019 9:00 PM

I'm with Paul, even though I can't see the entire exhaust side of the piston I do see piston going into meltdown on the exhaust side (looking at the dome). I would bet the exhaust side is really scored. As for the scoring on the sides, that is a byproduct of the lean condition, excessive heat, and the oil film being burnt away.

I would follow Pauls lead and replace the crank seals, along with a new top-end (new Nikasil/piston/ring). Seeing how it's a new bike to you I would also check the jetting against stock numbers and dump any old gas out of the tank and start fresh.


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