compression testing a 2 stroke

[Dobi]

What are some good numbers to have for the 300xc? I tested mine just after new and can't find where I wrote the numbers down at......

[dirtyboy]

250 should be between 165 and 195 depending on model and modifications. I think the 300 is very similar.

I have mine tuned up for 195 when new and when it drops below 190 I replace the rings. Second time gets Piston, rings, wrist pin and bearing. But maybe I'm over cautious.

[Dobi]

do you use a #1 or #2 piston with your rebuild?

Do you get the gasket kit from KTM or just buy the ones you need individually?

[MotorEd]

do you use a #1 or #2 piston with your rebuild?

Do you get the gasket kit from KTM or just buy the ones you need individually?

should go #2, and I bought my gaskets individual....you will never use most of the thicker ones they give you.

[kawasakijockey]

whats a general hour mark for doing a ring/piston over haul?? 250 ktm sx

[crazy2wheeler]

If you can get an unmodified 300 with a fresh top end to give you more than 160-165 psi at this elevation, you have a VERY special bike!

D.

[cycleworks]

whats a general hour mark for doing a ring/piston over haul?? 250 ktm sx

On a motocross bike you will want to "check for wear" at 40, 60 and 80 hrs (according to manual), (and if you race before EVERY race). Piston should be changed at 80 hrs, though it probably wont make it that long if you are an aggresive motocross rider. Seeing as how this is the RMDRA forum, you might not be an aggresive motocross rider, so I would still check it at those points, and just look for signs of wear until about the 80 hour mark.

[Dobi]

I used to pull the pipe off and look for blowby to determine ring wear. I guess I have around 100hrs on this piston. It was down in the 155psi range and needed some love.

I am hoping to get some much bigger numbers now that I have a + x dimension and a clean and adjusted powervalve.

[crazy2wheeler]

I used to pull the pipe off and look for blowby to determine ring wear. I guess I have around 100hrs on this piston. It was down in the 155psi range and needed some love.

I am hoping to get some much bigger numbers now that I have a + x dimension and a clean and adjusted powervalve.

Let us know what you measure - I'm betting 165(ish) with a fresh piston and rings.

[steve-o]

I used to pull the pipe off and look for blowby to determine ring wear. I guess I have around 100hrs on this piston. It was down in the 155psi range and needed some love.

I am hoping to get some much bigger numbers now that I have a + x dimension and a clean and adjusted powervalve.

Let us know what you measure - I'm betting 165(ish) with a fresh piston and rings.

Derek is likely right.

155 isn't bad either so unless you plan on milling your existing head, or buying an aftermarket head and changing base gaskets to the thinest one possible, I don't think you'll get much more thatn 165 if that with stock piston and rings. Every 2T I've had was right around 155-165 when new, including my stand up jet ski which was a 750cc two cylinder 2T Kawi. Each cylinder was consitantly between 155-162. After I did the cylibnder head (high perf) and jacked up compression with different pistons too I then started to see 190 at this elevation. IMO you don't want it that high anyway as you have to start dealing with race gas etc... that became a nusance.

Don't just assume that a number 2 piston will fit when changing your top end. Measure it first.

[Brass]

When you get it apart, could you post a pic of the cylinder condition?

I'd be interested to see how the cylinder plating looks after 100 hours of use.

[Brass]

I almost forgot to ask.

What piston is recommened for replacement? KTM, Vertex, Wisco, Athena, other? Coated skirt or uncoated??

I've always been a bit fanatical about changing pistons and rings. Normally changing the rings every couple months and pistons at each 3rd set of rings. But that was on old tech cast iron cylinders.

The nikasil coatings are new to me so I'm very interested in knowing what works, what doesn't and what is considered "normal" wear indicators on the piston and cylinder and what requires replating.

[dirtyboy]

I like to use the KTM piston kit. I know it will fit. I believe Vertex makes it for KTM.

The other brand pistons seem to be confusing to buy for me as I don't really know the exact size that I need and I don't want to change the top of the piston shape as I've usually got it tuned by the point of replacement so I don't want to make any changes.

My understanding is to use the #2 piston for a rebuild due to slight wear in the cylinder and use a #1 if you buy a new cylinder and it is a #1 cylinder or if you re-coat the cylinder / follow the re-coater's advice.

[dirtyboy]

BTW, the wrist pin bearing and gaskets don't come with the piston kit. You'll need to add these to your parts order.

[dirtyboy]

2011 250XC in stock form just tested 200lbs but I was kicking like crazy. You guys hold the throttle wide open too, right?