[Richard_Pitman]

My CB175k4 engine has never been apart, only has ( so far as I can tell ) some 8k miles on it in the last 48 years.

At some point in the future I won't be able to resist having a peak inside, and if the cylinder bores are still OK it probably wouldn't hurt to fit a new set of piston rings. ( Along with cam chain and tensioner wheels etc )

Obviously, standard size would be best, but if I could only source say .25mm oversize, would it be Ok to fit these, adjusting ring gap as required ?

 

[TOOLS1]

Yes, it would be OK to use the oversized rings.
TOOLS

 

[spirograph]

Others will know better, but I’ve heard before that it’s OK to get oversized rings, you just have to carefully file the ends until the ring gap is within spec in the bore.

Edit: TOOLS beat me to it see, others usually know better!

 

[ancientdad]

I may be wrong on this Richard, and the size difference is very small in this case as well and thus may not matter, but the curvature of larger rings would be slightly different and filing the end gaps to proper size, though it would meet one set of needs, might still not be completely correct for the best fit. Again, not too sure it would make much difference in a step up of only .25mm (.010")

 

[tbpmusic]

....... the curvature of larger rings would be slightly different and filing the end gaps to proper size, though it would meet one set of needs, might still not be completely correct for the best fit. .... (.010")

That's true of course, but doesn't matter much in this application.
I've done it myself more than a couple of times.
For some reason CB175 and CB200 rings are really hard to find.

 

[ancientdad]

That's true of course, but doesn't matter much in this application.
I've done it myself more than a couple of times.
For some reason CB175 and CB200 rings are really hard to find.

Yeah, I said so myself but thought, in case he hadn't thought about it, that the reasoning behind it might be good to point out. He's a smart guy, he probably knew already...

 

[longdistancerider]

If the bore is at the high side of the specification sometimes this is the only recourse to get the ring end gap into spec. Rings are typically manufactured to the smallest bore sizing for end gap. I've done it a few times and know of many others that have. In this case it's parts availability rather than bore size but there won't be any problem doing it. Just file slowly, doesn't take much

 

[Richard_Pitman]

Thanks for the replies, I'm pleased to hear that this has been done before without problems.

Just mulling it over, a sort of thought experiment, hope this makes sense.

A perfectly round rubber O ring of a given diameter can be reduced in size by removing a segment then rejoining the ends. It will still form a perfect circle, with a smaller diameter, because it is made of elastic material.

Piston rings are perfectly round, at the bore size they are specified for. Made out of cast iron, which is not elastic in the same way as a rubber O ring is. Without really thinking it through, I imagined that increasing the ring gap would simply result in a ring with a smaller overall diameter, still perfectly round, and I guess that for .25mm on a 52mm bore size this is still pretty much true.

However, I suppose that if you took this to extremes with a much larger gap, the compressed ring would then become oval, rather than perfectly round, and obviously wouldn't seal properly ?

 

[ancientdad]

I think there's something to that Richard, which is why I shared my thoughts about something I learned from an experienced tech when I was a lot younger. At the size difference you mentioned, it's negligible, but certainly a larger difference between oversized rings and existing cylinder size might make the curvature near the ring end gaps less than the perfect circle that makes the best seal - and increase the potential for blow-by

 

[Boomer343]

Way too much thinking going on here ..... the oversize rings I have fitted have always resulted in excellent compression numbers. I choose used cylinders very carefully though and check them in several places for wear.

I have seen far more damage done on cylinders from people power honing with a drill and either ball or stone hones. No bore is perfect and proper honing will bring it closer. At home honing is usually over done and incorrect procedures followed. This usually results in tapered and out of round bores. Far more of a concern in my opinion than the curvature difference between a standard ring and a .25 over ring fitted.

Best filing method I have found is by clamping a thin diamond disc made for a dremel tool into a vice and by carefully holding the ring working both of the ring ends on the disc at the same time. This works very quickly and keeps the ends square. Also be sure to very lightly chamfer the edges of the fresh filing before checking in the cylinder.

 

[Richard_Pitman]

Interesting stuff.

Off topic a bit, but when I was an impecunious teenager I had a Yamaha YG1 73cc 2 stroke. Pre Posilube, ran on premix.

After seizing it up, I decide to fit cylinder that someone had left lying about outdoors in the rain. The cylinder bore was rusty, so I cleaned it using wet and dry paper and 3 in 1 oil. Fitted back on the bike with a used piston and rings, ran better than it did before.

 

[Boomer343]

Interesting stuff.

Off topic a bit, but when I was an impecunious teenager I had a Yamaha YG1 73cc 2 stroke. Pre Posilube, ran on premix.

After seizing it up, I decide to fit cylinder that someone had left lying about outdoors in the rain. The cylinder bore was rusty, so I cleaned it using wet and dry paper and 3 in 1 oil. Fitted back on the bike with a used piston and rings, ran better than it did before.

You weren't doing anything that many limited budget or stranded biker hasn't done. BTW for the record I have used muratic acid to clean up rusted cylinders, double cut piston ring lands on a lathe to accommodate 2 rings to fix damage from broken rings, used solder on loose piston pins and put the pins back in with a large hammer .... fun times with no money but places to go ....

 

[longdistancerider]

You weren't doing anything that many limited budget or stranded biker hasn't done. BTW for the record I have used muratic acid to clean up rusted cylinders, double cut piston ring lands on a lathe to accommodate 2 rings to fix damage from broken rings, used solder on loose piston pins and put the pins back in with a large hammer .... fun times with no money but places to go ....

There's been lots of "cannibal fixes" done over the years, some are really amazing. Necessity is the mother of invention

 

[mannydantyla]

I had a hard time sourcing .25mm+ oversized rings and pistons for my 200. I found some on ebay, but could not find all, and I had to order the rest from cmsnl.com which was much more expensive. Good luck!

 

[ethano]

At least in the US market, in my recent search for rings and pistons for my CL 175, I have found piston rings of all four sizes for the 175. There seem to be plenty. My question for you, Richard, is are you looking for OEM rings in particular? Do you stay away from "generic" rings?

 

[Richard_Pitman]

Actually, I originally posted this question because I'd seen some reasonably priced .25 oversize rings on Ebay, and thought I'd better check before purchasing them.

I've got them now, in the spares box in the loft, ready for the day that engine gets a rebuild.