Help....Acceptable Clearances for Standard Piston & Rings??


Been Around the Block

I'm currently working through my first engine rebuild, a 1975 CB550K, and am trying to ascertain what piston and ring sizes will be correct to use for rebuild.
The motor has done 16000 miles. The cylinders look pretty good, the usual build ups at the top and bottoms. Heres a pic. Any texture you can see is just a light coating of oil & WD40.


The cylinders are all smooth to the touch, and in the centre of the stroke, I can see the original cross hatching.
I have measured the cylinders top and bottom, in X and Y axis with digital verniers. They all come in between 58.50mm - 58.52mm.
I also made a device to measure deeper in the cylinders, and am confident that the cylinders are not tapered or out of round.
I gapped the original piston rings in the cylinders, and their ends gaps were all within spec, but I plan to replace with new rings.
The pistons diameters are within the specs of above 58.35mm, and I plan to use the original pistons.
The Clymer manual says that a rebore of the cylinders is needed if cylinders measure 58.60 mm or over.

My plan was then to hone the cylinders with a 320 Grit Silicon Carbide Flex Hone to remove the carbon build up and make a good cross hatch. Then re-measure the cylinders once they look clean. As this is my first build, I'm not sure how much metal I will need to take out during honing.

SO, my question is, am I right in assuming that standard pistons and rings are ok to use in cylinders up to 58.59mm? If not, what is the acceptable clearances on ring sizes?
Obviously I would imagine the closer to 58.50mm the better, as I'd assume more clearance would lead to quicker wear in the rings/cylinders?

Ideally I would rebore the cylinders, but given my location, that isn't really practical. That said, I want to do it right, so if thats the best approach, then I will do that.
Just hoping someone out there with more experience would be able to confirm whether or not these clearances and my assumptions are correct.



I make things.
a real bore gauge is really what you need to measure cylinders, when tolerances are measured in the .0001"ths, you need something that'll read to that. given your statement that reboring isn't practical based on location, i have to ask, where are you located? there are good shops that bore and hone for a very reasonable rate and work through UPS. if you're dead set on using the stock pistons and new rings though, then you should break the glaze with the flex hone before reinstalling, I wouldn't bother measuring if you're not boring over


Been Around the Block
Thanks for the reply. I'm located in Australia, not near a major city, so unfortunately there isn't anyone really nearby to get to re-bore them. Definitely understand that a bore gauge would be the best, but I don't have one at this stage.
Yeah, well the pistons and cylinders look OK, so I figure I may as well use them, and if a re-bore is needed sometime down the track, then I will do it next time.

So with 58.50mm being the stock size, and 58.60mm being the dimension at which a re-bore is required, can I take it that standard size pistons and rings are OK with up to + 0.090mm clearance? I wouldn't think that I will need to take out that much material honing, but I'd be interested to know at what point other people decide what is too much clearance. This is for a street use bike, not a race bike.


I make things.
well, you're not really honing with those flex hones, they're glaze breakers, a hone straightens and rounds a hole, thus correcting to a true cylinder, those flex hones have no guidance and will just keep rubbing material off basically at random, so you should only use them to scuff up a cylinder so the rings can wear in a bit. I've experimented with using them on a fresh bore that was perfect, and for every .001" average they remove, they go out of round at least .0005" and taper the same... so use them only briefly to remove glazing or you'll make matters worse


Coast to Coast
Doing ANY cylinder work without bore gauges and mics is a bad need to be able measure within .0005 down the entire length of the bore. Tops and bottoms wont help you. Id love to see this device you made but I can guarantee its probably not accurate enough. You can get bore gauges for a decent price online.


Been Around the Block
Thanks for the info guys, appreciate it.
I made some basic extensions for the verniers I have, definitely not accurate to .0005", the calipers themselves are only accurate to 0.02mm at best. Well I might give the cylinders a quick run with the flex hone, and see how they come up, and see if I can get my hands on a bore gauge like suggested and go from there.
Thanks again.


Been Around the Block
Great, will do, thanks. :)
Not having used one, do the T Handles one get measured with a micrometer after being removed from the bore? As apposed to being connected to a dial indicator?


Over 1,000 Posts
yes you use them to transfer the measurement to a mike or caliper and it is very tricky to do, not something you will be able to do without being shown how
looking at the pics of your bores they are toast,bands of rust clearly showing
the only way you could possibly save them is with a precision sunnen hone and then check them for size with again tricky precision measurements, the machine shop can do that for you
but really you need to get new pistons and bore


Been Around the Block
Thanks for the inputs guys, much appreciated!
Sourced and ordered a set of CB500 +3m pistons, and I'll have the cylinders bored and honed to suit at 59mm.

I was wondering if you guys might be able to give me a little help on another matter. Just split my cases, and I'm looking to work through with plastigauge to see if my crank bearings and rod big ends are OK.
I was wondering if any of you guys might be able to help me understand how to figure out the clearances I need to be looking for, I've had a read, and I think I kinda understand, but the tables still have me a bit lost...

My cases are stamped BBAAA. The rods are all stamped 2D.



These are the tables for the CB550 from Clymer.



If anyone would be kind enough to show me the light, that would be rad. :)

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