2001 CR125 A or B piston help unlock the mystery!

[500 dirtrider]

I'm working on a 2001 CR125. My cylinder is marked "B" and the piston isn't stock so the mark isn't there (B AR1T060). I looked on a few websites and I can't figure out the deal. Can you use an A piston with a B cylinder??? I was told that the diameter is different and it won't work BUT Wiseco only makes one size???? I want a stock Honda piston anyway but everyone has an A and noone has a B. What can/can't I use and what is the difference between the two? Thanks!

 

[MXP1MP]

your better off running a after market piston in this bike. The stock piston has to much clearance. Thats just the basic reason why.

 

[500 dirtrider]

The aftermarket piston that is in there now is a Wiseco, at least that was what I was told. It has some funny marks on it and I thought that stock would have been better as I have run in my other Hondas. You guys may be right about the aftermarket piston though...give me more details before I deciede on one. The Wiseco put a few light vertical lines in the bore and had some wear marks on the side of the piston near the wrist pin indentation. I assumed that this was from needing rings???? The marks were slight enough that a ball hone took them out. What about Pro X pistons?? I use them in my CR500 with good success. I would still like to know...what is the difference between the A and B piston.

 

[darringer]

I agree with MXP1MP, a friend of mine had an '01 CR 125 with the stock piston and the top end sounded like it was coming apart! After using a Wiseco piston it not only ran better but was a lot quieter. Tolerances between piston and cylinder wall quite a bit tighter.
Darringer

 

[MXP1MP]

To answer your question about the scuffing marks on the piston, They are on the skirt area? around the exhaust port bridge? on the cylinder area and the wrist pin. Sure Sounds like lack of lubrication during warm up, ya gotta make sure those forged pistons are warm before you scream the bike. Its like those fools you see who start thier bikes and from a dead cold wring the piss out of it. Or maybe just maybe it sucked alittle bit of dirt? How bad is the piston colored does it look like it had lots of blow by?

 

[500 dirtrider]

Looking at the piston, it looks brand new, the top of it has zero carbon on it. The wear marks are near the wrist pin on the side, not on the skirt at all. The exhaust port was loaded with 1/4 of carbon in the pipe flange when I tore it down, obviously from the last piston.

 

[MXP1MP]

What the hell did they do install a new piston before selling you the bike? The difference between A & B pistons is the cylinder size - piston size. Its like the YZ 125 which has an A, B, C, D pistons and cylinders. The difference in size is a few thousands of an inch its minor one is just a tiny bit larger than the other. I forget the exact measurements I don't have my CR anymore.

 

[500 dirtrider]

I don't know MX, the previous owner raced it 12 times and he told me that he ringed it often. He also told me that they put in a new piston last time and it "is probably due to be rebuilt". This thing still has the original front tire and the second rear tire on it. It seems like everything should be solid but the carbonless piston and "due to be rebuilt" has me in a quandry!! I have a digital pic if I could figure out how to post the darn thing!

 

[darringer]

The scoring you have on the piston sounds like a little dirt got through the filter. How's the crank? Maybe you have excessive play in the big end bearing and maybe that's why the previous owner said it needed to be rebuilt? The difference between the A and B cylinders is that the A is approx. .003 in. smaller than the the B. Too bad you have the B. A common practice is to run the factory Honda B piston in the slightly worn A cylinder to take up the excess clearance in the stock setup. I have used wiseco pistons in my 99 cr250 with good results.

 

[500 dirtrider]

I checked the rod for excessive play and all indications are that the lower bearing is good. I don't think that the previous owner was too swift, he may not even know what the deal was. The low speed jet was so clogged up, I'm surprised that this thing even ran. I had to run a piece of wire through it to clear it out. Here's my thoughts on the aftermarket piston at this point. If the aftermarket only makes 1 sized piston to fit an A or B, it would have to be small enough to fit the A size, thus making it loose for a B even when the tollerances are tight. Now that this cylinder has some wear on it, I'm thinking that a stock B size would more closely fit the bore size. Am I laying awake at night thinking about this stuff for nothing???? What do you think, about the size not my insomnia?

 

[oldguy]

My son races a 01 CR125 and when we ran up to Eric Gorr for some work in Sept. he was very clear to get an A piston kit. From what I recall he also said not to use a wiseco piston unless we wanted to hone the cylinder.
There was a slap noise on our bike before the work and when we pulled the head Eric showed us that the piston was topping out into the head. He said this was a common thing in Cr125s as 'the rod tends to grow'. You can see this on the top of the piston as a ring on the top surface edge as an about 1/8 in clear edge.
I posted in the mods forum for him to look at this thread next time he is online

 

[500 dirtrider]

Oldguy, are you saying to use an A piston even if it is a B cylinder? What about a B cylinder that has been run for a while, wouldn't the tollerances be better using a B? The manual from Honda says to use ONLY an A with an A/B with a B, but I guess they would??? You are probably right about the growing part, we saw some marks in the head and I couldn't figure out how they got there. We did ball hone the cylinder to freshen the cross hatch. I have also heard storied about slapping but I was told that it was with the stock piston and not the Wiseco. The parts man at the local Honda shop looked at me like I was crazy wgen I mentioned this. I have always been a fan of using the factory pistons, I have seen cracks in the skirt of my CR500 with the aftermarkets.

 

[oldguy]

I don't want to give bad info so I'll try to get Eric or Rich to respond. We did stay with the Honda A piston for his A cylinder.
If nothing else try to call Eric - his number is listed on his website and he is always happy to answer questions.

 

[bclapham]

Originally posted by 500 dirtrider
The parts man at the local Honda shop looked at me like I was crazy wgen I mentioned this.

i wouldnt worry about that, every parts person i have ever deal with has looked at me in that way when i wanted to buy something he didnt stock but didnt want to buy something he did stock.

we have a couple of people on here (steve and marcus) that have done quite some things with the honda 125, from what i remmeber running an aftermarket piston was one of the most imprtant things to improve the"CR death rattle". just be patient and i am sure you will get some very good advice in the next few days.

 

[500 dirtrider]

I like that "death rattle" LOL...any info is appreciated. This is the kind of stuff that I'm lookin for, you guys have been there done that. I'm from the old school, this 2001 is the closest thing to current that I ever purchased, and it's my son's. For cryin' out loud I'm riding a 1990 CR500R! Yehhhh, some day when I get OLD I'll buy a 4 stroke and forget about this maintenance stuff!

 

[Spammer125]

I just rebuilt my 2001 Cr125 due to a dirt sucking incident...

I used an A piston with an A cylinder. The A piston I got from servicehonda seemed to be better quality than the old stock one. I have no idea why.

Also, the piston is not topping out as oldguy described. Hopefully my rod is not going to grow.

Why do they make B cylinders and A cylinders? why cant there just be one type of cylinder?

 

[Jman271]

The only piston I found that worked in my 02 cr125 was a Wiseco pro-lite, no rattle, the stock piston is noisy as hell. The reason they make diff cylinders, A,B. I believe has to do w/ mfg tolerance discrepancies. They run off a 100 jugs at a time, but they don't all come out the same. They measure them, if the are like .0123-.0126, they get marked an "A", if they are like.0125-.0128, they get marked "B". Don't quote me on the #'s, I'm just using them for an ex. I hope I'm saying that right-

 

[oldguy]

Jman that is correct. from what Eric told me the quality control was lacking and instead of tossing the slightly oversized they made them B cylinders.
If you have an A I believe you are stuck with an A piston. A B cylinder will acomodate a wiseco piston.

I hope Eric or Rich sees this soon

 

[500 dirtrider]

Me too Oldguy, I posted a question on Erics board and it got rejected and thrown off for being "off subject". My son lays awake at night wondering when dad will FINALLY order his parts LOL. I sure as heck don't want to do this twice! I sure appreciate all of the good info that you guys are providing...keep it coming! :worship:

 

[Rich Rohrich]

Originally posted by 500 dirtrider
Me too Oldguy, I posted a question on Erics board and it got rejected and thrown off for being "off subject".

It wasn't off-topic it was a repeat of the same question. We try to avoid that when a thread is already running.

Eric hasn't had much opportunity to get on the boards lately so I'll field this one.

If you have a B cylinder you can run an A piston but you'll have additional piston to wall clearance and a noisy, but safe engine. The Wiseco is a better choice for the B cylinders.

The A cylinder is too small to accomodate the expansion requirements of the Wiseco unless the A cylinder is honed to a larger size. The A pistons are the simplest choice for the A cylinders.

Hope this helps.

 

[marcusgunby]

I think a good answer to all these questions on pistons is to measure what we have-and then purchase the right piston-i found out this lesson at a friends expense when we fitted a new wiseco to his 133 kit-i didnt measure it and it ran great for 1 hour-no rattle as you would expect-it was too tight and caught the exhaust port roof.It took out the centre bridge and all the ports-the bottom end needed to be rebuilt as all the particles went in the mains.

On the honda i believe the A cylinder is the bigger unlike the YZ.In reality you could fit almost any aftermarket piston in any std CR cylinder and get away with it but its a big gamble.The std pistons come in A and B-wiseco come in one size and are unlikly to cause any problems if you warm them up correct.Vetex make nice affordable pistons that come in a range of sizes.SRS pistons have a longer skirt for less noise and they come in the largest sizes but cost more than all the others.The SRS race pistons say they last 7 hours for good reason-under hard use they really do last less than 10 hours.The wiseco can do 50 hours of easy riding.The vertex i would say is good for 25hours and the std ones i would run for only 12hours.

 

[Rich Rohrich]

Originally posted by marcusgunby
I think a good answer to all these questions on pistons is to measure what we have-and then purchase the right piston-

ABSOLUTELY :thumb:
Marcus - Thanks for bringing up a very important point that I shouldn't have glossed over.

I always hate answering these types of questions for the exact reasons Marcus outlined above. Unless you are measuring the new parts you are taking it on faith that the manufacturer didn't screw up. That leap of faith can be costly even when buying parts from the best companies. Real mechanics and tuners would never just fit parts together without measuring first, but I understand that not everyone has precision tools in the garage.

This brings up a useful topic. You can purchase a good quality Fowler bore guage from any number of sources for under $100, and I've seen them on sale for as little as $79 at J&L Industrial. A similar investment can get you a workable micrometer in the approriate size to measure a piston. For those of you who are serious about racing and your maintenance this $200 investment will pay for itself quickly, and given the proper care the tools will last a lifetime.

Cheap insurance by my estimation.

 

[500 dirtrider]

Thanks Rich....I must not have read close enough to see my question already posted. I think that between info from you guys and some local help, I'm ready to buy some parts. We mic'ed the cylinder and compared it to the specs of the available pistons. I friend's son races/rebuilds CR125's and he is somewhat of a parts supplier. We're going with the ProX piston, a new ring/wrist pin and bearing, and a gasket set. He claims that he has never had a knocker with this setup.....I hope that he's right :scream:

Thanks!

 

[Dragmeistr]

Does this hold true for 250's as well. I just ordered a Wiseco for an almost new CR250 with an "A" bore. Should I hone the cylinder for clearance prior to installation?

 

[marcusgunby]

It does hold true for any bike but i belive a 250 has more clearance than a 125 so its less of a problem.