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Discussion Starter · #1 ·
I am working on the engine on a 1983 Honda CB550SC and have a question about cylinders.

In the service manual the cylinder measurements are listed as:
  • standard as 60.000-60.010 mm (2.3622-2.626 in)
  • service limit 60.10mm (2.3670 in)
My cylinders look clean & smooth, visually they look very good but... My measurements are outside the standard, but well within the service limit.
My cylinders measure 2.3637 - 2.3642 in

What action would you recommend?

Is it acceptable to consider those cylinder measurements good, leave as is, and reassemble?
or, should I replace the piston rings with the oversized rings and bore it out to oversized rings?
or is there something else you would recommend?

I'm just looking for suggestions, I don't have much experience with rebuilding engines and would love to hear what some experience engine guys recommend.

pics attached for fun :)
Thanks

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a few questions: what is your measuring tool? where did you measure the cylinders? what was the motivation for disassembling the engine? what's your budget for putting it back together.
a few notes: you can't replace the rings with oversize rings, you need to replace the pistons with oversize pistons and rings that go with those pistons.
 

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Discussion Starter · #4 ·
I took it apart because the cylinders had low compression. cyl 1&2 at 140psi, cyl 3&4 at 115psi.
Once apart, It was pretty obvious the compression issue was the head gasket and the valves that were in real bad condition. Since it was apart for cleaning the valves, I wanted to measure the cylinders to see what the situation was with that. I figured, before putting the head back on I should at least check them to see if there is any issue with the cylinders.

I measured the cylinders top, middle, & bottom (left-right and front-back) six measurements per cylinder. And all measurements were consistent, within 0.0005

Thanks
 

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Personally if i have the engine apart a top end refresh by boring out, oversized pistons and rings is my preference every time.

The cost is minimal in the grand scheme of things.
But thats just me, everyone is different.
If its within service tolerance then its within service tolerance and you could just hone and put in standard rings (obviously checking piston ring gap before assembly)
 

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Valves are usually harder then valve seats. What you do with the seats and guides is just as important as the valve, possibly more important unless the valve is obviously bent &or damaged.
 

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Discussion Starter · #9 ·
Valves are usually harder then valve seats. What you do with the seats and guides is just as important as the valve, possibly more important unless the valve is obviously bent &or damaged.
Oh, yea. I agree. I spent a lot of time cleaning up the valves and the valve seats. The valves and the seats cleaned up pretty well. I gently grinded the valves with the seats to be sure there was a very good seal. All measurements are within spec on the valves and seats. So I am hoping we're good there, but I have not done any tests to confirm that yet.

My biggest decision is what to do with the pistons. Roll with it as is (since within service tolerance) or replace. Of course, it seems to be nearly impossible to find oversized pistons & rings for the 1983 CB550SC/CB650SC. If anyone has a source for those, please share. :)
 

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if it's the .0005" gauge, I'd opt for a .0001" gauge. if they're all oversized by the same amount, and readings are consistent throughout, then they're not worn out, check your gauge against a standard. I will set up a grinding vise with gage blocks in it and set the bore gauge against the jaws, then use it as a comparator. as far as the valves go, I recommend against lapping, if you're going to do anything to the valves, just replace them and have the seats recut, it's important to have good surface area (flat faces) to transfer heat from the exhaust valve head into the cylinder head, lapped valves tend to run hotter than cut seats with new valves.
 

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My manual says to reface the valve seat to fit a replacement valve and that it requires grinders having 3 different angles, 32, 45 and 60 degrees.
Then you use "Prussian Blue" (which is a blue powder paint pigment) to determine if the contact patch between the valve and valve seat is correct.

ymmv but not by much, poppet valves are poppet valves.
 

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I'd hone the cylinders, remeasure to make sure you're still in the service limit, measure the pistons to make sure they're within the service limit then try and find a new set of rings to fit and bolt it all up.
 

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Discussion Starter · #13 ·
Thanks for all your replies.
Very helpful information and I appreciate the wisdom. Great resource & community here.

I never heard of the "Prussian Blue", I'll definitely take a look at that. And, knowing that it is acceptable to reassemble if they measurements are under the "Service Limit", makes me feel pretty good about putting this back together and getting on with the transformation into a café racer.

Thanks everyone!!
 
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