Piston A, B or C?

[wence]

G'day,
I have just pulled the head and piston out of the bike and will be doing a big end up rebuild now I have more time.
Just wondering whether it states anywhere on the head or piston what size I need ( A,Bor C),as I can't see it anywhere?
Thanks in advance,
Mark

 

[gasser]

Back of the cylinder, at the bottom an A,B, or C will be stamped in the middle.

 

[wence]

Beautiful...
Thanks Gasser, I had a quick look but couldn't see anything.I will check again.
Cheers Mark

 

[GMP]

You can safely go up a size as well for a first top end, so if your an "A" cyl like most are, shop for a "B" piston.

 

[wence]

Thanks Glenn,
I am about to measure it tonight as a second check.Does anyone know what the factory clearances are piston to bore or ring to bore?
Cheers Mark.

 

[GMP]

Unless you have a very good bore mic and/or a shop that has one is doing it, don't bother. The margin of error with lesser instruments is greater than what you are trying to determine. I have measured this years ago on a '00 XC250 with a good 3 point mic and verified its fine, but don't have the numbers handy. The "micro" size difference between A.B,C.D is only 0.01mm. Clearance spec is pretty much industry standard for a liquid cooled 250 2-stroke, I just don't remember off hand. Its related to the piston type (forged vs cast) for the same size. You will be fine with a "B" in an "A" cylinder, many of us here have done it as standard practice. I've even gone to "C" in an older "A" bore on a friends bike that was in good shape after deglazing.

 

[wence]

No worries Jeff,
I had a bloke measure it tonight( does it for a living), and he measured 72.01 basically everywhere in there .
It is a B cylinder by the way.
Piston had about 5 thou difference(tolrance), is that about right?
I cannot get a workshop manual from the gasmoto link(only the handbook).
Cheers Mark

 

[jostby]

tolerance depends on size per GG workshop manual:
200cc STD: 0.3mm, limit 0.6mm
250cc STD: 0.4mm, limit 0.7mm
300cc STD: 0.5mm, limit 0.8mm

But differant pistons require differant clearances based on whether they are forged or cast. My Vetex and Athena pistons both came with a small slip of paper stating what clearances were required for each. Go with whatever your piston calls for.
If you can find a supplier that knows what A, B, C etc sizes are then you can just ask them for a B or whatever you need without worrying about it.

I should add my '00 was a stock "A" piston and it still sits on my desk at work. It is marked 66.33mm.

 

[jostby]

I will also add that based on your cyclinder size you have a 300cc.
cylinder diameters:
200: std, 62.50mm limit 62.60
250: std, 66.38mm limit 66.50
300: std, 71.99mm limit 72.10

 

[wence]

Thanks Jotsby,
I was talking to Scotty of Scotties dirtbikes today and he said the same.
By the way where is the workshop manual found these days?
I can find the user manual but not the workshop manual.
Cheers Mark

 

[eff]

Thanks Jotsby,

I can find the user manual but not the workshop manual.
Cheers Mark

It used to be on the GasGas USA site, but now for some reason the site took a step backwards and is almost useless.

 

[jostby]

I found it at GGUSA also and printed it off at that time.

 

[GMP]

Thanks for posting that clearance spec jostby, it demonstrates my point.

Mark,

As you can see, the spec provided by GG has a tolerance an order of magnatude greater than the difference in micro sizes of the new pistons. Even if you were to go from an A to a D piston, your dia would increase (and clearance decrease) by only .03mm.

 

[wence]

Thanks to everyone who replied . I have it all sorted now although /I am not sure what piston was in it originally as it is 71.884 in size.It has either worn about 1?10th of a mil or it was the wrong one when the last bloke got it rebuilt.
I found the link to the workshop manual finally , Here it is:

http://www.gasgas.com/manuals/ec_shopmanual_06.pdf

 

[moto9]

No worries Jeff,
I had a bloke measure it tonight( does it for a living), and he measured 72.01 basically everywhere in there .
It is a B cylinder by the way.
Piston had about 5 thou difference(tolrance), is that about right?
I cannot get a workshop manual from the gasmoto link(only the handbook).
Cheers Mark

From what I remember 2 to 3 thousands clearance is about right for a forged piston.
So, if your cylinder was 72.01 and you use a C piston @ 71.95 you'll be at a 0.06mm difference...which falls between 2 and 3 thousands clearance.
"I think I got that right" ??
I have not seen a "D" piston anywhere, who makes one for GG ?

I also have a "B" cylinder and I plan to order a "C" piston

 

[noobi]

Wossner make a D (66.36mm) size forged piston for the 250, but the pistons for the other models only go to a C size.
But when I bought a wossner for my 125 it was a domed piston and my bike runs flat pistons, they list 03-09 for all their gasgas pistons, which I think is wrong.

 

[moto9]

Wossner make a D (66.36mm) size forged piston for the 250, but the pistons for the other models only go to a C size.
But when I bought a wossner for my 125 it was a domed piston and my bike runs flat pistons, they list 03-09 for all their gasgas pistons, which I think is wrong.

really, I wonder if the 300 piston will be the right one ?
I'll have to check into that
Thanks

 

[noobi]

really, I wonder if the 300 piston will be the right one ?
I'll have to check into that
Thanks

Yea, I thought it was weird that it was a different type of piston altogether....
But I run wiseco CR 125 pistons for the 00-03 model, and I can get a wossner for a CR and that piston is a flat top, strange, the wossner is cheap too, well for NZ it is, its even cheaper than the nasty mitaka piston kits.... and about $60 cheaper than the wiseco piston.

 

[moto9]

Yea, I thought it was weird that it was a different type of piston altogether....
But I run wiseco CR 125 pistons for the 00-03 model, and I can get a wossner for a CR and that piston is a flat top, strange, the wossner is cheap too, well for NZ it is, its even cheaper than the nasty mitaka piston kits.... and about $60 cheaper than the wiseco piston.

I checked the Weisco website and found a piston for GG in the european section but there wasn't much info and it was at $200.00 for just piston, rings,clips and pin.
I found a "B" piston kit (wossner) with everything including gaskets and bearing for $229.00
And it's a flat top.
I didn't see a vertex piston kit for the 300

 

[noobi]

I checked the Weisco website and found a piston for GG in the european section but there wasn't much info and it was at $200.00 for just piston, rings,clips and pin.
I found a "B" piston kit (wossner) with everything including gaskets and bearing for $229.00
And it's a flat top.
I didn't see a vertex piston kit for the 300

A 300 vertex does exist http://www.454.co.nz/ go to engine parts, pistons, and search 'gasgas'. The prices are all in nzd, which is too expensive anyway, but just so you know there is a vertex kit.
229usd for the wossner kit sounds expensive, I can get the piston for ~200nzd