Wossner arrow direction and Cylinder gasket Q's
 

Wossner piston, arrow direction- To exhaust port?

Also, I am getting ready to set my cylinder on my rebuilt engine (300) but I can't get the top of the piston to the bottom of the exhaust port at BDC. I will need lots more gaskets to raise the cylinder enough to effectively lower the piston, are there any significant disadvantages to having the piston too high? I am at 1.5mm gasket stack, anything more and I will have to use old gaskets. Advice or suggestions?

Arrow towards exhaust port - check! Might want to double check which way the rings go on too. I can't remember if its the marked side up or down off the top of my memory. The ring gaps + pins face the intake side of the engine.

How far off is the exhaust timing? You could consider having a machinist make up a spacer if the gap is quite large. Then run a gasket each side of the spacer.

See this thread for some general info on how port timing, squish and porting modifications alter the power curve. http://www.gasgasrider.org/forum/showthread.php?t=10113

Rings face up with the letter or writing on it.

If the edge of the piston is slightly off, but it clears the rings, don't stress, and DONT use old gaskets.

Writing on rings up, check. Pins aft (intake), check. Arrow fwd (exhaust), check. Ring gap at 0.5mm 1" below cylinder deck, check. Piston edge at BDC is about 0.8mm (roughly) above exhaust port floor. Final gasket stack is ~1.4mm.

I read the link regarding gaskets the other, Jakobi, hence my questions (made me more confused, per the usual). Looking through it now, I do not see a reference to exhaust port/piston, just gasket stack. I just had the cylinder lined so I don't particularly want to start running a dremel around the ports, plus I wouldn't know where to start. I can drop the cylinder a bit, but I don't know if it's worth it.

Keg- What do you think regarding 1.4mm gasket stack? 2002 EC300, I don't know if the porting is stock or not. It was allegedly built by Eric Gorr many moons ago but there was no build sheet with the bike. I don't know the squish, I'll measure tomorrow.

You'll need to RAISE the cylinder, best power setup is piston edge even with exhaust port floor and THEN measure/adjust squish. You'd end up with 2.2mm gasket stack based on your numbers

Yeah as per the link.

"The Black line is the stock engine, in this case 2.5mm squish, 11.9:1 comp, 1.3mm base gasket stack, piston level with port floors at bdc"

From here they speak of dropping the cylinder (less gaskets).
0.7mm in this scenario boosts overall with a slight drop at top end, however going to 1.0mm results in a major sign off up top.

The figures will change depending on your bike, your compression ratios, squish etc, but basically regarding the port timing having the piston sit higher above the exhaust port results in exh duration drop, transfer duration drop. This increases bottom end at the expense of a good flowing top end. More back pressure, more grunt.

Increasing compression has the same effect on an engine. Adds to bottom end, but will be less willing to rev as high.

So by running less gaskets you end up increasing the compression and changing the ports to all focus on improvements in the bottom to mid. The top end is sacrificed.

If you time ports flush with piston at bdc you get a softer bottom end more inclined to rev out. Add some compression by changing your squish and you pick some up all over.

Make sense?

That helps immensely. Keeping the piston above the floor effectively lowers the top of the exhaust port (reduces exhaust port duration, right?), thereby "choking" top end. Reducing squish "recovers" some top end. My numbers equate (roughly) to the .7mm reduction in gasket stack, so I just need to measure squish. If my squish is ~1.8mm, I'm in the sweet spot. Sounds like I'm pretty close, now that I understand the basic concept. I'll measure the piston/floor difference more accurately tomorrow and go from there. 0.7mm of piston showing (roughly) and ~1.8mm of squish. I want bottom end right now, not worried about revving it out. When I get more experienced I'll chase the RPM's, but most of my riding is single track with limited transfer areas. Sounds like a plan. Thanks for your help.

Mostly on target :)

You're right that it reduces the duration the exhaust port is open for, and at bdc due to it not completely opening the port you have also effectively made the port smaller.

The squish doesn't really recover top end. The squish will provide a more efficient burn which nets better economy and stronger more responsive power. Thats it in a nutshell. The physics behind head design are way way way beyond my knowledge. RB actually reshapes the band, reduces the height and cc's the bowl to achieve a specified compression ratio. You can get away with simply shaving some material off the head if its not too far out. The bike will still run with squish way up around 2.6mm but it becomes less stable 1.6-1.8mm is probably alright for a trail riding setup. Going tighter you will get snappier power but may require better fuels (more so the tighter you go).

squish at 1.4 or so is optimal for racing, probably need higher octane at that point (mine does). you get much better fuel consumpiton, and a strong bottom end. you seriously lose top end once the piston is above exhaust port at bdc. a little bit (.5mm) is not bad, any more and no overrev. squish is set very very loose from the factory - my 07 was over 3.0mm....

so without cutting the head, try to find a happy medium. as tight a squish as you can get, and still keep piston right height.

if you change squish/piston height you will most likely need to rejet.

don't grind ports on nicasil - it will chip/peel. you gotta port before relining.

I'd move with a bit of caution. If & I mean if it was built by Eric then he would have built it right & possibly moved the barrel to suit what the tune was for.

1.4mm squish is still waay too big for 'optimal', but to go lower you'd have to lower the compression quite somewhat, so maybe that measurement is the one if chamber hasn't been modified to correct the compression ratio. They don't rev high so even though 72mm stroke you should be able to run them to 0.9 with the piston just before kissing, maybe 1mm to be safe.

If the head has been machined there will be exposed ally rather than the black plated std. This should indicate if the head has had chamber modified. If not then Eric hasn't touched it or they bought some bits off him, but he didn't tune it.