250-500cc GasGas piston in WR 300

Great thread

I'm on the hunt. Got messages into Wossner, Vertex and Wiseco.
Hopefully one of them has a piston with the same dimensions as the stocker (Vertex #23457), but with a dome.

If not, I'll buy a stock replacement and have the head cut to raise compression.

You might take a look at your air boot to box seal and make sure you're not sucking dirt. That sand blasted look on the intake skirt might be a sign.

Just saying.

Hello! I think i have to learn to count, because when i checked in my book i had maybe around 55 hours on my piston, and as i use it for some streetriding i don´t use WOT so much, so i think i will wait until i change it. But the Wössner piston looks good! 71,945mm in diameter and tefloncoated.

Johnny:sweden:

Hi guys!

I dont care too much about more power, squishbands, domed pistons and so on, the bike already scares me sometimes.

I just need a new piston. Tried to order on yesterday, but the mechanic asked wich size I wanted?? For a WR 300 of course was my answer, but that was not enough.
According to him there is supposed to be two different diameters. He said that there should be a stamped 1 och 2 somewhere on the cylinder. I can´t find that. Does anyone of you know where this stamp is supposed do be located??? In whorst case it can be placed so that you have to remove the cylinder head he said?

I dont want to take it apart, measure and then wait for a piston for a few days. I want to ride, change piston some night and go riding again next day....

Johnny,
can you post a picture of the intake side of the piston please?

Foggen33,
there's an A, B and C sized piston. A is 71.94mm, B is 71.95mm and C is 71.96mm.
If you remove the head you can see what size piston it currently has.

If anything it was from the Mikuni sucking dirt before I got the 7602 filter on there. There was always dirt in the carb when I'd pull it.

Although, I think the flash of the camera makes it look worse than it truly is. The intake window wearing a 1/32" ridge into the piston was more worrisome to me.

Hello Swedish friend! As i have a few hours left on my piston it will take a while before disassemble it. I don´t know if it makes any special differenc if the piston is 71,94 or 71,95mm as standard when my piston is in between at 71,945mm. Thats 5 thousands! I guess rhey make the piston like that to fit both sizes.
What do you think?

Johnny

Thank you!
Exactly what I didn´t want to hear. But there is no other solution then grabbing the tools with a smile on my face..

I have run my piston approx 100 hours now, very long according to my local mechanic, but most riding have been on gnarly endurotracks with very limited time och full trottle. If it would have been a forged piston I wouldnt have changed befor 150.

I actually don´t have a clue if its true that they make pistons that way, but it wouldn´t be such a wild guess maybe?

Question: Is the head sealed with o-rings or what. Do I need to change anything or can i just lift it off and put it back without risking any leaks if i ride for a couple of hours? This is my first 2-stroke ever so I am not so familiar with them.

And remember to drain the coolant first

Brandon - With how fast the 300 is stock I can't imagine wanting more, but I am not a power junkie. I hope it does not transform into another big bore YZ

Not really looking for more power. Just want to shift some down low (where I spend more of my timer riding this bike) and less up top. This bikes makes my YZ feel tame on top and the YZ makes this bike feel tame on the bottom.

And I am a self proclaimed power junkie and I cant leave well enough alone. No point lying!

Johnny,
any chance of getting a picture of the intake side of the piston? Please!

I´ll try to post up some pictures as soon as i get my new phone working with my mail! Promise!

GasGas piston for WR 300

Here it is!

Heré´s another one! Showing intake side.

Here´s the box with partnumber and stuff. Hope it´s some help for you!

Johnny:sweden:

Thank you!

The Wossner piston looks like it has a wider skirt on the intake side than the Wiseco. If you have a chance can you drop the piston into the cylinder and see if the skirt is wider than the intake port at its largest opening? Or more simply can the intake skirt fit into the intake port as it passes?

The Wiseco comes to a narrow point and it can fit into the port, which worries me.


Against my better judgment I installed it last night. It turns over just fine and feels great, but that intake skirt makes me nervous. I'm going to run it for a bit and then change it out for a Vertex on an accelerated time frame (assuming the Vertex intake skirt clears the port).

On a positive note the compression is up quite a bit. I can really feel it in the kick starter. And I can actually get a squish measurement with .125" solder. My caliper is a Harbor Freight hunk of junk that only measures down to mm's, but I believe it to be around 1.6-1.7mm's, which puts the squish around .067-.070". Not ideal, but a lot better than .125"+.

Need a new head bolt and exhaust o-ring before buttoning her up, but should have those today and hopefully go for a ride tonight. Will report back.