Finally got the "expert" to confirm my piston and bore measurements, of course I left all the cylinders behind so I'm doing this from memory...

So I got a pair of "C" cylinders, both measured out at 54.008 which is OK for a C cylinder, correct?
Pistons came out at 53.928 and 53.927, ring end gap 0.45-0.50 on all 4 rings, I assume those pistons are junk and rings were recently replaced? Could this possibly be the rattle I was hearing?

Other set I have is an A and B cylinders, both have scratches on the bore as if the bike ran without an air filter and sucked in some dirt/sand, those pistons measured out both at 53.947-.948, can they be used with "C" cylinders? that will make is 0.06mm gap.

Appreciate any advice
Hope it's OK to discuss piston clearance in the open forum...

You did say,... you were doing this off memory.
I would like to suggest, in the nicest possible way,.. NOT doing this off memory.

Measuring the diameter of the bore down to 0.001 of a millimeter seems over-accurate to me.
At that level I would expect to see some variation in the bore from top to bottom, at least.
Did you measure the bores in a few places? If so, was there any variation?

So lets just say you have bores of 54.01 and used 'C' pistons of 53.95.
Assuming that these are standard Honda pistons and that you made the measurements 15mm up from the bottom of the piston skirt,..
Then this gives you a piston to bore clearance of 0.06mm.
Honda recommend a piston to bore clearance of 0.035 to 0.044mm with a service limit of 0.08mm

I guess this means that your 'C' pistons (if measured as assumed) are out of spec, but not outside the service limit.
So you could potentially use them if you wanted, but,..
the arguments for and against doing so will vary depending on your specific situation and thinking.

Me,.. personally,.. I would buy some new pistons.
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[color=#808080][size=9]Yes,.. I too know how to waste Time and Money,...

I'm sure it is totally unintentional, but the original post can be taken as a kind of troll.
http://en.wikipedia.org/wiki/Troll_(Internet)

VERY basically the question being asked is:
I have a part that is half worn (=half new?), should I use it.

Argument is almost guaranteed to follow. See what I mean?

I am not trying to insult Dmitry in any way.
I am sure he just wants to know what to do.
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[color=#808080][size=9]Yes,.. I too know how to waste Time and Money,...

Measured the bores with a "inch" micrometer that reads 0.0001 (0.00254mm) wrote the numbers down and converted them to metric
bores were nearly perfect with variation of .0001" from top to bottom and another .0001 in the last few centimeters where the sides open up. I remember writing down 54.008 on both cylinders which would be the measurement in the middle so they vary from 54.005 at the top to 54.01 at the very bottom
Manual says C is 54.000-54.005 with service limit 54.075
B is 54.005-54.010 so my cylinder worn out to B standard size?

Now to the piston

C bore needs D piston and the spec is 53.961-53.965 with limit 53.030
I clearly recall measuring and writing this numbers over and over because I had a machinist double check, 53.927 and 53.928, I have very good visual memory, those numbers are quite clear in my mind, and yes 15mm from the bottom, and just a hair under the service limit.

The other pair of cylinders came out of A and B bores that I did not measure but pistons both read 53.947( this one I also recall because I kept checking and re-checking, crossing 7 for 8 etc. these pistons are above the D piston limit of .930 so can they be used in a C bore?

TuckerBag wrote:
VERY basically the question being asked is:
I have a part that is half worn (=half new?), should I use it.

No worries mate, not easily offended, but the question is from someone who does a fair bit of 2 strokes but never had to deal with this tight of a clearance, for example on a Vespa ring gap is 0.5mm with service limit of 2mm! I ran them in the past at 3mm with good power (for a 50 year old Vespa) and always matched the pistons with much less precision, you know 150cc, 4-6 hp @ 6000 rpm... they run forever. Seizing a hole is followed by a light hone and scuffing a piston, then re-assembly and fixing an air leak or checking the timing. Unless it's a performance top end the damage is always minimal.

So now I have this .005 mm increment in the bore size (Vespas use .2mm oversize step) and I don't know how important (or unimportant) it may be to get it right.

Also my question is can the used pistons be re-used from another cylinder, not weather my original piston is any good. I also want an answer to the noise I was hearing, what does the worn out piston sound like? On a Vespa it gets louder when hot (air cooled) and what I have does sound like piston slap but I don't want to shovel money in to this bike replacing everything

OK, I just want to throw a bit of perspective on this:

Cylinders:
The difference in diameter of A -> B -> C cylinders is 0.015mm (54.015 -> 54.000)
The amount of wear on the cylinders required to hit the service limit is 0.07mm (54.085 -> 54.075)
(This probably has to do with the thickness of the nikosil coating)
Note that the wear limit of 0.07mm is much larger than the 0.015mm difference in cylinder bore size from A to C.

Pistons:
I don't have exact figures for new piston sizes but, going off the info you posted above and using common sense, lets just say that,
The difference in diameter of B -> Blank -> D pistons is 0.015mm (53.965 -> 53.950) This should be pretty close I hope?
The amount of wear on the pistons required to hit the service limit is approx 0.02mm (53.945 -> 53.930) Again, this should be pretty close I hope?
Note that the wear limit of 0.02mm is comparable to the 0.015mm difference in piston size from B -> Blank -> D.

So for a man who is used to working on Vespas, it would probably seem that Honda are being pretty 'finniky' about the piston wear, given the allowable cylinder wear.
Perhaps there is a good reason for this? I do not know how to explain this one.

But, what is important here is the piston to bore clearance.
It seems that a minimum piston to bore clearance of 0.05 is required.
On the large side: 54.015 - 53.965 = 0.05
On the small side: 54.000 - 53.950 = 0.05
Note: this is true for when the piston measurement is taken 15mm up from the bottom of the skirt.

Now I know that I have already quoted Honda as recommending a piston to bore clearance of 0.035mm to 0.044mm with a service limit of 0.08mm
So, assuming that we are on a budget here, trying to re-use old pistons, I am going to hack that statement and just say that we want,
A piston to bore clearance of 0.05mm with a service limit of 0.08mm

Your C barrels have worn a bit so as to effectively now be B barrels at 54.01mm (close enough) The nikosil will still be thick enough given the 0.07mm wear limit I assume.
Your D pistons measure up at 53.93 giving a piston to bore clerance of 0.08 (right on the service limit)
Your other pistons (from A and B barrels) measure up at 53.95 giving a piston to bore clearance of 0.06 which is not too bad if on a budget.

So basically, I will just say the same thing again:
You could potentially use them (the pistons from the A and B barrels) if you wanted, but,..
the arguments for and against doing so will vary depending on your specific situation and thinking.

Me,.. personally,.. I would buy some new (blank) pistons for what are now effectively B barrels.


Regarding the piston slap.
I have found that the noise is loudest when cold and reduces as the engine warms up.
It sounds like a semi-muted kind of clacking? That's the best I can come up with anyway.
Perhaps replace gudgeon pins and small end bearings just to eliminate that one as well. They are fairly cheap.

I hope there aren't any errors in this post as I have lost the ability to edit and even though I do preview my posts I don't always get it right.
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[color=#808080][size=9]Yes,.. I too know how to waste Time and Money,...

I got the numbers from the manual posted in the members section, piston clearance is .035-.044 with service limit .08
with 54.008 bore and 53.927 piston I'm at the service limit

I would normally want to run a piston to bore clearance of 0.035mm on a road bike. On a track bike I would run 0.045-0.050mm depending on what type of track its manly going to live on. I wouldn't ever want to run mine with a clearance of 0.08+mm or anything near it.
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If I have to take the carbs off once more...

Yes i agree with Stephen ,the 0.08mm clearance is too much even for 2stroke bikes with iron cylinder sleeve.
The piston rings,piston skirt and cylinder's wall will suffer at this clearance.
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Owner of : Honda NSR250 MC28 95sp colours / Honda NSR250 MC18 R6K SP / Honda RS250 NF5 89 in boxes / Honda C50 12V cub / Honda C50 12V super cub Japanese version /Yamaha XT600 89 benetton

stefanos-MC28 wrote:Yes i agree with Stephen ,the 0.8mm clearance is too much even for 2stroke bikes with iron cylinder sleeve.
The piston rings,piston skirt and cylinder's wall will suffer at this clearance.

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Owner of : Honda NSR250 MC28 95sp colours / Honda NSR250 MC18 R6K SP / Honda RS250 NF5 89 in boxes / Honda C50 12V cub / Honda C50 12V super cub Japanese version /Yamaha XT600 89 benetton

StephenRC45 wrote:I would normally want to run a piston to bore clearance of 0.035mm on a road bike. On a track bike I would run 0.045-0.050mm depending on what type of track its manly going to live on. I wouldn't ever want to run mine with a clearance of 0.08+mm or anything near it.

Well than, that decides it, time for new pistons!