CB125K4 piston / ring question

HondaJR

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May 17, 2024
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Location
Belgium
Hi all,

While taking apart my top end on a cb125k4 ( twin ) i measured the end gap of the piston rings. They were 1.3 mm which is almost double of the max service limit. When inserting new piston rings the end gap was still 0.9 mm which baffled me . So i measured both cilinder bores and it was bored out to the first oversize ( +0.25 ) pistons and rings that were used were standard size.

I am currently looking for new +0.25 pistons and rings which are really expensive and not easy to find anymore.

My question : what happens if i would use +0.25 rings on the standard size pistons in this engine? The end gap would at least be in spec. I guess there would be (more) piston slap ? As the engine did "run" with standard size rings and pistons and did not blow up. Except for faster wear would this be somewhat "safe / ok" to run it that way? How bad / dangerous would this be?
 
The cilinder is bored already to +0.25mm.
I know the only good way is to have the correct pistons and segments (+0.25).

Those cilinder blocks are impossible to find ( on ebay 400 euro +)

I have my eye on a second hand set ( cilinder block + rings + pistons ( +0.5) which i probably will onder if they ship to my location. As well i found a set of pistons +0.25mm in france. Which i will order to get this cilinder as a spare set.

I was just wondering if it would be possible to run it that way without damaging the piston skirts. It ran ( ok not good as the compression was low and probably a fair amount of blow by) with the standard size piston and worn standard rings without really damaging the pistons. I was really surprised to see the previous owner installed standard size pistons and rings in a bored to first oversize cilinder (+0.25mm)
 
I wouldn't run it that way because you're starting with an additional .010" (0.25mm) piston to cylinder clearance beyond factory spec, which means far too much piston movement in the bore. No matter how well you gap the rings that rebuild won't last long.
 
Richard Pitman said:
Radical suggestion - slip a complete 175 engine in there. :geek:
Click to expand...
Always an option 😅 but i want to keep this bike as stock as possible.
The 175 does not add that much more power compared to the 125 ( The better torque is there though)

A newly bored second hand cilinder with correct pistons and rings is on its way.
 
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