How does this piston look?

[jan_b]

Bought the bike used and rode this season. Thought to open her up and have a look to put in a new piston. The piston that was in it was an A and the cylinder is stamped B. I was surprised that the power valve looked pretty clean. Cylinder and head look good. Will send the head out for an RB design mode. this winter.

How bad does this piston look?

[Jakobi]

Bit hard to tell with the pics but the burn patter on the dome looks about as close to a perfect specimen as you can ask for. Looks like some wear on the skirt which could have occurred faster due to the undersize piston/cylinder match. Still a decent amount of coating on the piston visible, and doesn't appear to be much blow by past the rings.

What oil have you been running? Jetting specs? And approx how many hours in the season? Just curious.

[jan_b]

Not sure on the amount of hours I don't have an hour meter.
I've gone through about 60 Liters of fuel this season, I have no idea how long that piston has been in the bike before I bought it. I just want to make sure everything is in good order for next season. Ride a mix of MX / Trails and harescrambles.

For oil I don't know the brand I ussually buy syntetic 2 stroke oil for bikes at the local car parts store.

Jetting:
PJ: 45
Main: 178
Needle: DDK second slot from the top.

Squish was at 1.97mm before I took the piston out. bike had one green base gasket and the piston did not alow the transfer port to full open in BDC

Bike had good low throttle response but felt a bit flat on top with little to no over rev. I'm hopen that running a thicker stack of base gaskets + BR designs head mod will recover some of the top end. PV was nice an clean no carbon just oily will check the adjustmen when I get the new parts and put things back together.

[andoman]

I believe that increasing compression will make the lack of over-rev worse.

[motopsycho87]

That piston shows a lot of ring blow by. Either worn rings or not enough oil to aid sealing (which would in turn wear the rings faster). With your new piston use 32:1 minimum! Ignore anyone that says less, so many studies have been carried out all proving benefits to power and engine life all the way to 16:1, and with correct jetting you will not get spooge. Nurture that new piston

[Rick]

Originally Posted by motopsycho87

That piston shows a lot of ring blow by. Either worn rings or not enough oil to aid sealing (which would in turn wear the rings faster). With your new piston use 32:1 minimum! Ignore anyone that says less, so many studies have been carried out all proving benefits to power and engine life all the way to 16:1, and with correct jetting you will not get spooge. Nurture that new piston

+1 On the ring blow by. I think you stated that you had an "a" piston in a "b" cylinder, could be the reason for blow by?

[Jakobi]

Is the dark patch below the rings blow by or just the coating which hasn't worn off near the rings (where the piston rocks back and forwards less)? I had assumed the later, but on closer inspection you can see somewhere there was no coating. Ring end gap should tell the story.

Increasing comp will definitely reduce top end and over rev, however lifting the cylinder (more gaskets) to time the ports (exh to be full open at bdc) will get you a peakier power delivery. Having the head worked after this should correct the squish, put the CR wherever you want it, and also improve efficiency across the board.

[jan_b]

Originally Posted by Jakobi

Is the dark patch below the rings blow by or just the coating which hasn't worn off near the rings (where the piston rocks back and forwards less)? I had assumed the later, but on closer inspection you can see somewhere there was no coating. Ring end gap should tell the story.

Increasing comp will definitely reduce top end and over rev, however lifting the cylinder (more gaskets) to time the ports (exh to be full open at bdc) will get you a peakier power delivery. Having the head worked after this should correct the squish, put the CR wherever you want it, and also improve efficiency across the board.

That's the plan I figure 2 black base gaskets or 2 black one green will get the ports to line up, then headwork to get correct squish and compression.

The bike had only 1 green base gasket so I assume compression was high.

[Jakobi]

You never mentioned if it was a 250/300, but with the 2mm squish clearance the CR wouldn't have been too unreasonable.

[Elvis74]

Imho it looks like the coating is still on the piston.
I would measure it and when to diameter is still in tolerance, put new rings on and use it again.