Surfriding Can someone help me with ordering the right piston??

[220gsean]

Here are a couple pics of my piston. I have to get the cylinder bored. I am wandering what pistons these are and what next size up I should get.

Thanks in advance guys.

 

[Waternut]

They are 0.5mm oversize right now. The next size will be 1mm or 0.75mm over. You may be able to search the number on top and see what brand they are but that doesn't really matter.

 

[Freakrider]

my machine shop does a prelim bore to see when the cyl cleans up at...then you order piston at .25 or .50 next size. Once they get your pistons they finish final bore/hone at 4 1/2 thousands piston to cyl clearance (little more if riding warm waters) and make sure they chamfer the ports good so you don't snag a ring.

 

[220gsean]

That is what happened to me(snagged a ring I guess). I put a caliper on it and it read 82.2. That is why I was so confused.

 

[aramis]

Pistons aren't perfect cylinders, they are somewhat tapered. So, don't measure at the top, measure toward the bottom of the skirt. Yamaha manual says to measure diameter 10mm up from the bottom.

And, are you sure you snagged a ring? A quick glance at the photos and my first impression is that you are running a bit on the lean side...but I'm far from an expert on that.

 

[220gsean]

I bought this ski in this condition... I agree the wash looks way lean, but the cylinder looks to have been "home ported' and exhaust port/sleeve has a nick in it and the piston is gone all the way to the ring on that side... I will take the measurement again from the skirt side. Thanks for all your help! I'm sure its a noob question but id rather get the right piston the first time!

 

[Yamer-Jammer]

... and make sure they chamfer the ports good so you don't snag a ring.

Great info on chamfering the ports. I just had a rebuilt 701 last half a season from snagging a ring. I have been pondering if I forgot to align the ring opening with the cylindrical place holder inside the ring groove on the piston. I think it was because the ports were not chamfered... Anyone have pics on what the chamfered ports should look like?

 

[Nuklear6]

final bore/hone at 4 1/2 thousands piston to cyl clearance (little more if riding warm waters)

Believe it or not its actually more important to have a more tolerant piston to cylinder wall clearance when running in extremely cold conditions more then extremely warm. The difference in mass between the piston and cylinder is great. Therefore when running in 40 degree water the cylinder stays so cold because it is a larger mass and has 40 degree water being forced around it, and yet the piston is getting much warmer due to exhaust gas temp and a smaller mass. In short pistons gets warm and expands and cylinder doesnt, aka cold seizure......