YZ125 piston size confusion - PICS now added to assist!

Blue_SV650S · World Chat Champion Joined: 27 Nov 2008 Karma :

Decided to get a new ring, well probably a new piston as preventative maintenance on my Yamaha YZ125 MX bike. ~2001 model (not 100% sure).

Basically I have never had the engine apart, so thought I'd whip the head off and see what size piston is currently in there.

There is the number 95 clearly stamped on to ... so thought fine, a 0.95 oversize then!?!? ... seems they don't come in a 0.95 oversize ... and to be fair, the stamp does say 95, not .95 Confused

So I measured the bore ... it is 53.85 with my very-nears ... putting some force on them, I can get it to go to ~53.92 ...

Now, the piston is like a d1ck in a welly in there too, I'd say a fair bit of movement - perhaps not too much, but more than I'd expect if it were a virgin!!

ANYWAY, who knows about YZ125s of this era and can shed some light on the way they are sized ...

Oh and I am not sure if it is a nicosil bore or not ...

https://www.ride.net/Products/3785-pro-x-replacement-piston-rings.aspx

If you look at the above link, it seems there are several 'standard' sizes, I think mine might simply be a 53.95 standard size and not an oversize at all??? is that what the 95 is depicting!?!?!??

'elp!! Very Happy

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SillyMe · Trackday Trickster Joined: 11 Oct 2010 Karma :

Found the standard bore size on this model.
Bore x stroke: 54.0 x 54.5 mm (2.1 x 2.1 inches)
https://www.bikez.com/motorcycles/yamaha_yz_125_2002.php

https://www.georgiaoffroad.com/dirtbike.php?modelid=734

I believe it's a nikasil plate, if it is my best guess is that your gunna need to get it re-honed, the engineer will give an oversized piston kit. (This is if the barrel is not worn, they wear down the exaust port area but this is very difficult to detect by the untrained eye.) If not a rebore will be needed and oversized piston kit will be issued.

Apart from middle men there are actually only three engineers in the country that re-plate cyclinders, (Takes two weeks)I know this because one lives down the road from me and replated a KMX 125 cylinder for me. They will too give you the right piston kit.

I can give you names rough prices and places if you wish.
It sound as though this is the path you need to take.

Second thoughts looking at the sizes you have given you may well get away with new rings only. But then again just because there is no scaring it doeasn't mean the barrel isn't worn.
____________________
Once made equal to man, woman becomes his superior.
The man who says "I may be wrong, but -" does not believe there can be any such possibility
Owned and rode: Honda 100H, Honda MBX 125, Kawasaki AR 125, Suzuki RG 125 Pepsi, Honda NSR125 RK, Honda NRS 125 race bike, Suzuki GSXR 400 Bandit, Aprilia RS 125 O2, Honda CG125, Kawasaki KMX 125, Aprilia RS 125 O6, Suzuki XT225 and now Honda XR 400. ( A few scooters and other dead beats in between)

SillyMe · Trackday Trickster Joined: 11 Oct 2010 Karma :

Found this too looks good to me.

I have YZ125 1999 and I believe it's very similar to your engine size,
1. The piston to cylinder clearance should be 40~45 micron for standard and
Limits to 100 micron.
2. Cylinder B size is 54.004~54.006
3. Piston D size is 53.969~53.972
Subtract 3 from 2 and you will gate clearance of 32 micron to 37 micron,
So you can see that you will have less clearance (32~37) then the factory
Specification (40~45).
(By the way, using piston B in D cylinder it's O.K. you will have clearance of 48 micron ~ 53 micron and it's in spec.)
https://www.dirtrider.net/forums3/showthread.php?t=36861
____________________
Once made equal to man, woman becomes his superior.
The man who says "I may be wrong, but -" does not believe there can be any such possibility
Owned and rode: Honda 100H, Honda MBX 125, Kawasaki AR 125, Suzuki RG 125 Pepsi, Honda NSR125 RK, Honda NRS 125 race bike, Suzuki GSXR 400 Bandit, Aprilia RS 125 O2, Honda CG125, Kawasaki KMX 125, Aprilia RS 125 O6, Suzuki XT225 and now Honda XR 400. ( A few scooters and other dead beats in between)

Blue_SV650S · World Chat Champion Joined: 27 Nov 2008 Karma :

Thanks for help thus far ... but where does the 95 stamped on top on my piston come into all of this? Confused

That and I am not sure you can get a nicosil re-bored??
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SillyMe · Trackday Trickster Joined: 11 Oct 2010 Karma :

Blue_SV650S · World Chat Champion Joined: 27 Nov 2008 Karma :

SillyMe · Trackday Trickster Joined: 11 Oct 2010 Karma :

Oh yeah sorry my mistake.

From what I have detected it's just related to manufacturing.
The numbers stack up so I wouldn't worry about it too much.

SillyMe · Trackday Trickster Joined: 11 Oct 2010 Karma :

scroll down for nice picks of mine.

https://www.bikechatforums.com/viewtopic.php?t=205873
____________________
Once made equal to man, woman becomes his superior.
The man who says "I may be wrong, but -" does not believe there can be any such possibility
Owned and rode: Honda 100H, Honda MBX 125, Kawasaki AR 125, Suzuki RG 125 Pepsi, Honda NSR125 RK, Honda NRS 125 race bike, Suzuki GSXR 400 Bandit, Aprilia RS 125 O2, Honda CG125, Kawasaki KMX 125, Aprilia RS 125 O6, Suzuki XT225 and now Honda XR 400. ( A few scooters and other dead beats in between)

Blue_SV650S · World Chat Champion Joined: 27 Nov 2008 Karma :

Right, took the top end off and had a proper look-see ...

Before I can think about replacement piston, I want to correctly identify what I have in there now!!

Oh and I am pleased that I have removed the piston as the ring locating pin is loose (not sure it truly came out in pics)!! Shocked

SillyMe · Trackday Trickster Joined: 11 Oct 2010 Karma :

Nice pics, as previously mentioned I can't comment on the 95 stamped on the piston other than to say it is related to manufacturing, Specs say stock is 53.94.
"judging by the number crunching earlier in this thread, this has to be standard piston size"

Barrel to me looks like a replate job unfortunately, I say this because you previously mentioned the piston slap and you can see where it has worn down the exhaust port side. Only an engineer can verify this with a good feel before commiting to a replate. Did a KMX replate not too long ago and looked exactly the same.

As for the 95 on top unless you get a Yam mechanic on here I think your gunna need call your local Yam dealer and speak to parts.
Manufactures process of the pistons and cylinder is not very accurate, Yamaha sort the cylinder and pistons to 4 different size (A,B,C,D) .
They want to keep the piston cylinder clearness in spec, so the cylinder A has a larger diameter then B (about 16 micron) and the piston A also has a larger diameter then B (again about 16 micron), the different between A and B size is the same as B to C and the same for C to D.
So if you go from B to D you will have at least 32 micron of over clearness and as I remember it larger then the proper specifications.
____________________
Once made equal to man, woman becomes his superior.
The man who says "I may be wrong, but -" does not believe there can be any such possibility
Owned and rode: Honda 100H, Honda MBX 125, Kawasaki AR 125, Suzuki RG 125 Pepsi, Honda NSR125 RK, Honda NRS 125 race bike, Suzuki GSXR 400 Bandit, Aprilia RS 125 O2, Honda CG125, Kawasaki KMX 125, Aprilia RS 125 O6, Suzuki XT225 and now Honda XR 400. ( A few scooters and other dead beats in between)

Blue_SV650S · World Chat Champion Joined: 27 Nov 2008 Karma :

I don't think Mr yamaha will recognise the piston as I am almost certain that it is NOT a yamaha one ... look at the symbol on the inside ...

My spidie senses coupled with some google-fu make me think it is a Mitaka piston ... CAN ANYONE CONFIRM?? ...

My spidie senses also make me think it is a piston for an YZ125 '02-04 ... NOT an YZ125 '97-01, which is what the "5HD00" markings on the side of the barrel indicate ...

The 02-04 is a 53.95, the 97-01 is a 53.94 ...

Is it just a coincidence it has 95 on top?? Thinking

https://www.proaccessories.com.au/products/mitaka/piston.htm
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Carvel · World Chat Champion Joined: 01 Jun 2010 Karma :

Blue_SV650S · World Chat Champion Joined: 27 Nov 2008 Karma :

SillyMe · Trackday Trickster Joined: 11 Oct 2010 Karma :

I may have solved the problem regarding the 95 stamp, it may just be a cut down an abrevation of the following relating to the year it was made but not 100% sure.
It could be an old piston in a newer bike.

Mitaka pistons
PT.90YZ125 note the PT.90 made YZ125 '90-'93
https://www.mitaka.co.uk/YAMAHA%20PISTONS/PT.90YZ125.htm

PT.94YZ125 note the PT.94 YZ125 '94-'96
https://www.mitaka.co.uk/YAMAHA%20PISTONS/PT.94YZ125.htm

PT.97YZ125 note the PT.97b YZ125 '97-'01
https://www.mitaka.co.uk/YAMAHA%20PISTONS/PT.97YZ125.htm

And so on.

Scroll to the bottom.
https://www.mitaka.co.uk/pistons.htm
____________________
Once made equal to man, woman becomes his superior.
The man who says "I may be wrong, but -" does not believe there can be any such possibility
Owned and rode: Honda 100H, Honda MBX 125, Kawasaki AR 125, Suzuki RG 125 Pepsi, Honda NSR125 RK, Honda NRS 125 race bike, Suzuki GSXR 400 Bandit, Aprilia RS 125 O2, Honda CG125, Kawasaki KMX 125, Aprilia RS 125 O6, Suzuki XT225 and now Honda XR 400. ( A few scooters and other dead beats in between)

Old Git Racing · World Chat Champion Joined: 08 Aug 2009 Karma :

As said earlier the race pistons come in a, b, c, d. You choose which one to suit your barrel. If you have it replated it should come out to suit one of those piston sizes. measure it accurately with an internal micrometer, ring a good Yam dealer such as Fowlers and get an a b c or d piston to suit. If you have it replated get it done by someone reputable, Wrathall did mine. They can then recommend which piston is needed.
This is how it works on my 250TZ v twin, basically 2 x 125 cylinders similar to yours. I have to run an A in one and a C in the other with the barrels on at the minute.
I'd use genuine pistons, you get what you pay for in the end.

OGR
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Blue_SV650S · World Chat Champion Joined: 27 Nov 2008 Karma :

SillyMe · Trackday Trickster Joined: 11 Oct 2010 Karma :

You could be right, I could be wrong about the number on the mitika.
Old Git Racing is spot on (Glad he came to the rescue), as long as the barrel doen't need replating.

There's two engineers listed in this thread that do piston replating.
Good luck.
____________________
Once made equal to man, woman becomes his superior.
The man who says "I may be wrong, but -" does not believe there can be any such possibility
Owned and rode: Honda 100H, Honda MBX 125, Kawasaki AR 125, Suzuki RG 125 Pepsi, Honda NSR125 RK, Honda NRS 125 race bike, Suzuki GSXR 400 Bandit, Aprilia RS 125 O2, Honda CG125, Kawasaki KMX 125, Aprilia RS 125 O6, Suzuki XT225 and now Honda XR 400. ( A few scooters and other dead beats in between)

Blue_SV650S · World Chat Champion Joined: 27 Nov 2008 Karma :

But I don't want to send it off ... all I want to do is decode what piston I have now and then get a new one!! .. it shouldn't be this hard! Laughing

So I repose the original question ... WTF does the 95 on the crown mean?? Wink

I have measured the bore and there is no way it is 54mm. To be honest, it isn't even 53.95mm ... it is smaller than that ... as is the piston ...
____________________
The purpose of life is to fight maturity.

SillyMe · Trackday Trickster Joined: 11 Oct 2010 Karma :

Well sorry can't help you on that count. If no one else has any idea or answers give this chap a bell in the morn, Ask to speak to the engineer about it, he's realy good. He helped me a lot.
Just say you need to speak to the engineer about a technical issues before you send some work his way, this way you will get through.
Nod Nod Wink Wink.
I found he's the helpful sort anyway.
https://www.pjme.co.uk/

01902 307457
____________________
Once made equal to man, woman becomes his superior.
The man who says "I may be wrong, but -" does not believe there can be any such possibility
Owned and rode: Honda 100H, Honda MBX 125, Kawasaki AR 125, Suzuki RG 125 Pepsi, Honda NSR125 RK, Honda NRS 125 race bike, Suzuki GSXR 400 Bandit, Aprilia RS 125 O2, Honda CG125, Kawasaki KMX 125, Aprilia RS 125 O6, Suzuki XT225 and now Honda XR 400. ( A few scooters and other dead beats in between)

Blue_SV650S · World Chat Champion Joined: 27 Nov 2008 Karma :

Right, just so everyone knows, I ended up buying a genuine Yamaha piston kit.

Kit part number 4XM-11630-30. The piston was a 'B' pistons and has the part number 4XM-11631-30-B0

The piston fitted perfectly, so it is safe to say that my original piston was for a standard bore.

Just so anyone in future google search knows if they have a piston like in my OP it IS the same as a standard piston Thumbs Up

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The purpose of life is to fight maturity.

Syris the Indomitable · Crazy Courier Joined: 22 Oct 2010 Karma :

K. Cheers!
Did you ever get tothe bottom of what that number was stamped on the top of the piston?
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Blue_SV650S · World Chat Champion Joined: 27 Nov 2008 Karma :

Syris the Indomitable · Crazy Courier Joined: 22 Oct 2010 Karma :

Oh well!
Is it running good now?
____________________
In Greek mythology Ichor is what immortals have in the place of blood
Those that mind, don't matter...Those that matter, don't mind. Do you agree with this?

Blue_SV650S · World Chat Champion Joined: 27 Nov 2008 Karma :

coggie · Two Stroke Sniffer Joined: 20 Sep 2010 Karma :