2002 CR250 Piston A or B

soap · May 29, 2007

I was putting in a new piston for my 2002 CR250R and I went with an OEM piston. There are two kinds of pistons offered stock (A and B). My cylinder says A on the side so I ordered it or so I thought. I was hoping to ride the bike as soon as I got it back together so I was in a rush (setting you up for the stupid part). When I got the piston, it was marked B. Not wanting to wait 2 weeks to get it all sorted out and pay a restocking fee, I measured the original piston and the new piston with a caliper and all the important measurements were the same. So I just used the B piston but I haven't fired up the engine yet.

Now I found out from my doctor that the bone is not healed as I hoped and have to wait 2 more weeks to ride (Please be only 2 more weeks). So now that I have time, should I pull the B piston, pay the 20% restocking fee, and be safe? Or is there any difference between pistons? And remember that ALL aftermarket companies only offer one type of piston. Thanks

SQUIRLEYMOFO · May 29, 2007

I'm following this closely because I've wondered the samething.............

rerfurt · May 29, 2007

I am wondering too...

84cr125 · May 29, 2007

i think the 20% restocking fee is not right. you ordered the A piston, but they gave you the B piston. take them to school and get a full refund.

oh and ya i'd switch it to the A piston just to be safe......
my .02 cents.

mtk · May 29, 2007

A caliper isn't a sensitive enough measuring instrument to show the differences in size between an "A" and a "B" piston.

Bunya · May 29, 2007

I don't know what the differances are between the A & B, but if they went thru the expense of marking things, it must be important to have the correct one. If you ordered and A and got the B, the company shouldn't charge the restocking fee. Even if you made the error, I'd still be surprised if they charged you a restocking fee assuming you were purchasing the correct one from them.

Marc -

matego · May 30, 2007

Directly from the service manual.... piston A 2.6114-2.6117 and piston B 2.6111-2.6114. so roughly it is .0003 difference between piston A and B. I will let you make the call on keeping it in. They wouldn't stress it in the manual if it was not important though.

Ol'89r · May 30, 2007

mtk said:
A caliper isn't a sensitive enough measuring instrument to show the differences in size between an "A" and a "B" piston.

As mtk pointed out, you can't accurately measure the difference with a caliper. You have to use an inside micrometer to measure your cylinder and an outside micrometer to measure the piston and determine the clearance. The bigger piston can be used in a worn cylinder to take up the extra clearance but, it has to be measured. You can't guess.

For some models you can get aftermarket pistons like Wisco and Pro-X that come in A, B and C sizes.

Bunya · May 30, 2007

When you get down to measuring tenths, everything changes. It takes a lot of skill to make accurate, repeatable measurements down to a tenth. At those ranges, forget the snap (telescoping) gages as well as your inside mic (too clumsy to use in the center of the bore). You need a good quality bore gage. Temperature also becomes a factor and proper techniques have to adopted. Because you are using 2 different instruments for your measurements (bore gage, outside mic), calibration is essential. All this makes accurate measurements down to a tenth outside the capability of most of us.

rerfurt · May 30, 2007

Ok, this is what I don't get - sorry for using metrics by the way,

A brand new wiseco/prox piston is 66.34 mm and first oversize prox would be 66.35. All wear limits is in hundreds of a millimeter in my manual - ie. piston outside diameter service limit is 66.28mm.

The bore is 66.40 mm and standard piston to cylinder clearance is 0.06 mm and service limit is 0.09 mm

Knowing this, why do anybody care if it is an A or B piston when the difference is only 2 thousands (0.002) of a millimeter - I really have a hard time understanding how that comes into play ?

Often I find that dealers only offer the B piston, which is the smaller one, that makes even less sense to me ??? :coocoo:

griffbones · May 30, 2007

Wiseco will probably only list one piston too!

steve.emma · May 31, 2007

Ok, this is what I don't get - sorry for using metrics by the way,

dont be sorry, metric is the most accurate way to make measurements....

as for the issue of clearance, this being an 02' bike there is probably some wear in the cylinder anyway (unless its brand spanking new..). so the tiny amount in size difference between an A or B piston will not make the slightest bit of difference anyway. when you are talking bout sizes this small the bikes jetting will have more affect on clearance due to the heat expansion of the piston than the A or B selection.

Bunya · May 31, 2007

steve.emma said:
....dont be sorry, metric is the most accurate way to make measurements....

Huh???? What does the unit of measure have to do with accuracy? Accuracy is determined principally by the measuring instrument and the skill of the user. It makes absolutely no difference if you're using imperial or metric units. Neither system is inhearently any more accurate than the other. It's certainly easier to measure something in whatever system was used in it's manufacture to eliminate having to convert back and forth, but unless you're splitting tenths, it's just not an issue.

Ol'89r · May 31, 2007

steve.emma said:
so the tiny amount in size difference between an A or B piston will not make the slightest bit of difference anyway. QUOTE]

If you have a cylinder that is slightly worn, lets say .002 oversized or .002 out of spec. The difference between the A and B piston will put you back in spec. In a race engine, 1/10's are important.

Bunya.

Thanks for asking that. I was wondering the same thing. :coocoo:

NWDirt · Jun 2, 2007

The gap between the piston and cylinder wall is an engineered distance that needs to be maintained. A Japanese manufactured A cylinder with a D piston will seize. Ask my riding partner.

If you have ever honed a press fit hole on a mill, or bored an accurate ID on a decent lathe, than you realize how little it takes to open up an ID. An extra half a second on the up stroke of a hone can open up an ID more than you wanted. The machines used for mass machining have tool wear etc to deal with so being perfect is pretty cost prohibitive.

If you only had one tolerance range for the ID of the cylinder, you would be scraping a lot of cylinders. The answer is to have 3 or four tolerance ranges and 3 or 4 different diameter pistons manufactured for each range.

They machine the bores and nikosil the cylinder wall. They measure the bore accurately and depending on the bore diameter, they mark the cylinder A (tightest bore diameter) through C or D (largest bore diameter). That way they can match it to a piston that will allow the designed gap between bore and cylinder.

You can measure the piston with a dial caliper (digital is better) just above the intake opening (largest diameter) and if that falls within the piston dia tolerance given in your manual for an A cylinder, its good. chances are it was not marked B for no reason though.

Eric Gorrs book does a nice job of describing all of this.

Working in Metric is also easier as a system. It was designed way later than empirical system and thought through so going from volume to weight as an example, is less painful. It doesn't matter what system you use, as long as anyone else wanting to use a different system (machining, measuring, designing around) understands how to convert tolerances correctly. Many talented machinists will get a metric print that says 10.5 +/- .02 and the first thing they do is convert 10.5 to .4134" and then accidentally us .02 (20 thou) as the tolerance. Oops. Converting measurments can cost a lot of scrap parts.

2 cents.

Dave

Bunya · Jun 2, 2007

NWDirt said:
You can measure the piston with a dial caliper....

No, you can't! Sorry Dirt but there's no way a caliper is accurate enough for these measurements. In the hands of a skilled machinist, they're good for maybe .001". In the hands of a novice, you can't expect any better than .005". A machinst NEVER uses calipers for measurements that are more critical than several thousanths. According to Matego, the mean difference between the A & B is .0003", more than 10x smaller than what can be measured by the average guy with a caliper. These measurements have to be taken with a micrometer. Again, attempting to take accurate measurements down to tenths is not a trivial task and beyond the capabilities of most folks.

....(digital {calipers} is better)....

No way! Assuming equivalent quality, a vernier or dial would be a better choice. Digital calipers only read to the nearest .0005 and due to display errors that's +/-.0005 on top of the basic accuracy. The average differance between the A & B is .0003 so a digital can't resolve the difference. With a vernier or dial, you can interpolate the reading down to a few tenths. But again, this is beyond reasonable accuracy for a caliper!

I tend to agree with Ol'89r, that if the difference between A & B is a few tenths, I wouldn't be too concerned about it unless you really push the engine hard all the time.

rerfurt · Jun 2, 2007

NWDirt in your case it does make sense, and I also read the same thing in Eric Gorrs book.

The thing I don't get is that honda CR's only have 2 sizes, A & B, where B is smaller than A. The difference between A & B diameter is "only" .002 mm. Does it really matter if the piston to cylinder clearance is 0.060 or 0.062. Honda specifies that piston to cylinder service limit is 0.09, thus it would seem it doesn't matter - but why does honda then use resources on having two different pistons.

Bunya · Jun 2, 2007

Calipers and accuracy

For those of you who are interested in valid information on the subject, here's a link to a discussion about calipers on the Home Shop Machinist board. A lot of the guys that contributed are or were machinists so they know what they are talking about. There's a lot more in depth information concerning the accuracy and pros & cons of digital vs dial than I presented here. They also touch on the subject of how temperature starts affecting things when you get down to measuring tenths.

Calipers and Accuracy

NWDirt · Jun 2, 2007

Bunya,

Mellow out. You need to try some decaf. With so many responses in the last few weeks, you must be tired.

Rerfurt,

I dont know. Start a new thread maybe with that question in the title? Of course, youll just be going back and forth with Bunya.

Moving on.....

Dave

Bunya · Jun 2, 2007

NWDirt said:
Bunya,
Mellow out. You need to try some decaf. With so many responses in the last few weeks, you must be tired.

Nah, don't drink coffee - that's for grown ups. I'm just trying to help some folks out while I'm killin' time waiting for my cylinder to get back from US Chrome. And I hate to see people get inaccurate info. I've done a lot of stupid things in my days and learned a lot as a result. I guess part of getting older is wanting to pass some of the hard earned experience onto others and maybe save them some of the cash and aggrivation it cost me.

steve.emma · Jun 3, 2007

Quote:
Originally Posted by steve.emma
....dont be sorry, metric is the most accurate way to make measurements....
Huh???? What does the unit of measure have to do with accuracy?

yeah fair point guys, after reading this again i should have said metric is easier rather than more accurate. :bang:
and having said that theres bound to be some machinists out there that have been using the imperial system for years and would disagree with that too. i guess it comes down to what you have been taught, and in my corner of the world its metric. personally i think metric is miles better! :laugh:

(thanks D.T.)

Pete Payne · Jun 3, 2007

Use the B piston and ride on !

soap · Jun 4, 2007

Alright, I see the piston a and b question has been turned into accuracy with calipers and units. haha First off thanks to Matego for the piston specs. Once I saw the difference was on average of thirty thousands I didnt care about the size difference. The wiseco piston is said to be 2.114 in (the exact measurement the a and b piston share) but they dont give tolerances. So Im assuming they dont care too much either.

The caliper I used did read out to 0.0001 in. But as many people have said, it's not likely to get an accurate number out to that range. The small difference i did read was less than 0.001 and so I dismissed it as my mistake or slight carbon build up. You guys can fight over the importance of that one.

I did go against doctor's wishes and went riding today, and everything ran great. The kickstart felt like it had the same compression (like I could tell the difference after 2 months anyway), and although I didnt use more than 3/4 throttle all day (Im trying to break it in) it did feel to have the same amount of pull.

So final verdict is dont worry about a and b unless you have an A cylinder that has been badly worn. Then Im sure the POSSIBLE extra 0.0006 inch would come in handy.

2002 CR250 Piston A or B

soap

SQUIRLEYMOFO

rerfurt

84cr125

mtk

Bunya

matego

Ol'89r

Bunya

rerfurt

griffbones

steve.emma

Bunya

Ol'89r

NWDirt

Bunya

rerfurt

Bunya

NWDirt

Bunya

steve.emma

Pete Payne

MX-Tech Suspension Agent

soap

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