Kawi2strokes.com Forum

Zambia, actually, NO, no adverse issues as long as you keep the clearances down to a very reasonable level. I have also cut dirt bike two stroke cylinder reed pistons the same way, no issues. The only issues I ever saw with those pistons were the regular things one saw from any piston, from someone doing it wrong, as in seizures from too lean, holing them from ignition to far advanced, lean, etc. Never did see one, in anyone's race, nor street two strokes that did damage because of their reed valve cuts.

When these skirt-less pistons were clearanced correctly, you couldn't hear them, even in a silenced TZ/RD engine, and they weren't noisy in an air cooled Kawasaki triple either.

Just about everybody spun their OW31 Reps up back then, stock red line was 10,500, with some of us going up to 12,800 or so, and myself, up to 13,500. We got better life mileage from the OW cuts than full skirt pistons, with increased ring life as well. We used to re-ring after each last Saturday test session, and piston every other race with full skirt pistons. The OW cuts allowed us to get better ring seal for the same time between changes, and 3 full races on the same pistons, making them much more cost effective.

On the first series factory H2R piston port pistons we had originally, they were forged, full skirt length, and collapsed the intake skirt at least .003 in the first lap or two of testing. Plus, they weren't austenitic, nor autothermic, so, they needed .003 min when first fitted. So, one lap, they were toast, let alone a full race length. Factory went to a spin cast piston, and .0015 clearances, life became good with the new pistons, they'd live a whole race weekend, as long as yu didn't do something stupid and rip them apart inside the engine.

When it comes time for me to do the engine in my RZ500, it will get the same piston cut as the OW31 Replica pistons, and it has both cylinder and case reeds.

When I worked for R.C. Engineering eons ago, I built for our then general manager, a 4 stroke KL250 shrunken rod stroker, big bore single engine. The stroke was increased 8mm's, with the rod being heated, then shrunk eye to eye, and one of our big bore Z/KZ 4 cylinder pistons. To clear the flywheels, the piston had both skirts removed completely, flat on the bottom just below the pin boss. NO PROBLEMS AT ALL. Clearances were kept to .0015 for the Venolia made, forged piston. If you look at some of the single cylinder Hondas dirt bike 4 stroke engines of the 1990's thru 2010, they use a same design, NO skirt piston as well, and have NO problems with rock over, ring issues, as long as the clearances are adhered to, .001 to .0015. All the factory race team Honda RC series pistons for singles were no skirt design as well. First I saw of them was at Hollister, Ca., years ago, when I worked with my friend and cohort Steve Whitelock on the bikes for a dirt GP.

So, I have seen skirt-less pistons work in both high and low rpm two and four stroke engines, no issues.

I haven't done dyno testing. The extra power is probably in the low to middle single digit percentages. The piston skirt is a restriction, and causes turbulence in the intake path. The more area "through" the skirt, the better the flow.

Because of longevity on street engines, my piston template includes all three options, with just holes recommended for the most strength. There is notable increase on the butt dyno between no holes and holes in the skirt.

I have talked in length with a big time tuner, that has built 100's of very fast RD's/RZ's. He has tried everything one can think of on those engines. The half moon does make the best power, but piston longevity is greatly reduced in race engines that use them, One race weekend usually.

The new 4 stroke pistons do not have much skirt at all, but hey have a longer head because of the 3 rings, closer tolerances, water cooling and no cylinder distortion from having holes in them.

_________________
Twist the throttle, tilt the horizon, and have a great time. What triples are all about...........

Kind of a related question:
In a H series street engine, which wears faster, the piston or the cylinder?

_________________
There are only two types of motor sport racing:
Nitro and everything else
(Sometimes referred to as fast and slow)

The valves.

_________________
This is true. Where I grew up the hills were so steep and long, when your ball rolled down the hill you just said "screw it"...

After 14k on my top end, no noticeable bore wear but piston skirts collapsed about .002"

_________________


http://kawatriple.com/wtf/

Did they really collapse? Over the years my impression has been that a couple thou in that distance is just wear. I guess if you can still see the machining marks on the skirts, but the piston is smaller there, that's collapse. Or if you measured the skirt thickness and that's still the same.

_________________
If it surges, that's normal, upshift.

There was a shinny lip along the edge of the intake skirt about 1/16" wide, which looked like wear but the measurement was taken just above this area where machining marks were still present.
I believe it is collapsed but its only my opinion.

_________________


http://kawatriple.com/wtf/

thanks John..

I like these threads (not the blown engine part, the "techy" stuff)

That's what's cool, you never know what will come up in board conversations.

_________________
Twist the throttle, tilt the horizon, and have a great time. What triples are all about...........

Hey John.

I just watched the vids and read this entire thread … well, all but the boring stuff … I find it absolutely amazing , extremely unfortunate , but amazing.

The simple fact that it held together as well as it did just astounds me. When I saw that skirt/cylinder, I really though it would have locked up.

Glad your OK …

Darth

_________________
If you look for the bad in people expecting to find it, you surely will.