RECENT POSTS

2002 CR250 Piston A or B

Joined
Jun 10, 2004
Messages
34
Likes
0
#1
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
 
Joined
Apr 8, 2007
Messages
292
Likes
0
#4
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

Member
Joined
Jun 9, 2004
Messages
1,409
Likes
0
#5
A caliper isn't a sensitive enough measuring instrument to show the differences in size between an "A" and a "B" piston.
 
Joined
Apr 26, 2007
Messages
147
Likes
0
#6
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 -
 
Joined
Feb 17, 2007
Messages
27
Likes
0
#7
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.
 
Joined
Jan 27, 2000
Messages
6,959
Likes
42
#8
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.
 
Joined
Apr 26, 2007
Messages
147
Likes
0
#9
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.
 
Joined
Jul 7, 2005
Messages
50
Likes
0
#10
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:
 
Joined
Oct 21, 2002
Messages
285
Likes
0
#12
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.
 
Joined
Apr 26, 2007
Messages
147
Likes
0
#13
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.
 
Joined
Jan 27, 2000
Messages
6,959
Likes
42
#14
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:
 
Joined
Oct 2, 2006
Messages
23
Likes
0
#15
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