I finally go around to inspecting the top end on my MC21SP and found that I have no mark pistons in the C cylinders. I assume the pistons are standard Honda, they have the markings "IN H KV3" and "HF2" on the underside of the skirt.

I have a caliper for measuring, but it is not likely accurate enough to verify the wear of the cylinder and piston. Both cylinders appeared to be 53.924mm while the pistons were ~53.900. I guess I need some more accurate tools, because I do not expect the bore to be less than 54. Or could it? If it were re-plated it could have a smaller bore than 54.

At least I was able to measure the ring gap and found that 0.45 would pass, so new rings are in order.

It has been running fine like this for the past several thousand km when I purchased the bike used. Any thoughts on the no mark pistons in the c cylinder?


Last edited by doug3 on Sat Apr 11, 2009 9:11 am; edited 1 time in total

You're right, all cylinders start over 54mm.
As there is only 0.011mm across the production tollerance for all three cylinders you're going to need a very very accurate vernier gauge indeed to measure them like that.
I guess that's why the cost of veniers runs from about a fiver up to several hundred pounds.

What really matters though is not the overall size or the letter on the cylinder / piston but the clearance which as I mentioned somewhere recently should (according to the book) never be under 0.039mm and has a service limit of 0.1mm.
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lol You're wrong.

Most stock NSR cylinders will measure under 54mm as you have found.

Most B cylinders measure between 53.880mm and 53.920mm. It is very rare to find any NSR cylinder over 54mm dia.
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If I have to take the carbs off once more...

I'm not saying you're wrong, but if you're right, the Honda service book is wrong;

A cylinder; 54.008-54.011
B cylinder; 54.004-54.008
C cylinder; 54.000-54.004

Service limit (which does imply you can measure it) is 54.02 for all cylinders.
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o o


Last edited by DifferentStrokes on Mon Sep 28, 2009 6:05 pm; edited 1 time in total

fontyyy wrote:I'm not saying you're wrong, but if you're right, the Honda service book is wrong;

A cylinder; 54.008-54.011
B cylinder; 54.004-54.008
C cylinder; 54.000-54.004

Service limit (which does imply you can measure it) is 54.02 for all cylinders.

Well, this is why I was asking the question. It sure seems odd to me that my cylinders would have a smaller diameter than the spec in the manual. I guess I did not accurately measure it, or is it like StephenRC45 says? Was the lol meaning that I am measuring wrong, or do you really find stock cylinders less than 54mm?

Remeasuring, I think I came up with 53.924 for the piston, and can't get a decent measurement of the cylinder with my caliper. I think the service limit for the no mark piston is 53.91mm (new, 53.962-53.965).

Anyway, I guess I will not replace the piston at this point, and only put in the new rings. When I do a top-end rebuild next time, I will put in the smaller D pistons to match the cylinders.

I went though all my barrels last night with a bore guage and not one of them was over 54.00mm

What Honda quote them/want them to be is one thing. What they are is another.

They say their crank are true as stock.... far from it!
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If I have to take the carbs off once more...

o o


Last edited by DifferentStrokes on Mon Sep 28, 2009 6:05 pm; edited 1 time in total

You are confused. HRC know what they are doing most of the time. It's production Honda that let us down. Not that it matters, who uses the proddie stuff anyway?
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If I have to take the carbs off once more...

o o


Last edited by DifferentStrokes on Mon Sep 28, 2009 6:04 pm; edited 1 time in total

StephenRC45 wrote:I went though all my barrels last night with a bore guage and not one of them was over 54.00mm

Thanks for the confirmation. I guess I am not crazy?!?

doug3 wrote:
Thanks for the confirmation. I guess I am not crazy?!?

You're in the wrong place then, cos we're all mmm... muh... muh... maaaaaaad sir!
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