Kawi2strokes.com Forum

Later no skirt 4 stroke pistons, as far as factory Honda went, were only two ring, top and oil. they do have something Steve and I have been doing for decades, we cut the piston land from the top of the top ring, to the deck, 1 degree angle, smaller at the top of the bore, than at the tip of the land. Arias does this to all their pistons as well.

Why? The piston still rocks over, TDC to BDC. This angles the piston in the bore, and, if the top of the piston is holding the piston at a strange angle to the bore line, instead of letting the ring guide the piston, wear becomes accelerated. The 1 degree angle gets the top of the ring land out of the way of the bore, allows better mixture migration to the top ring for cooling, and more liquid mix for a better ring seal.

Works for both two and four stroke engines.

As far as piston skirt "collapse", that is exactly what we saw on the factory forged pisgtons, but, NOT on the later castings Kawasaki did, NOT bore wear, not skirt wear off, it was collapse. We used flat edges along the cut of the skirt, and when you see the angle OK from the bottom of the ring lands, to to just below the pin bosses, than, go backwards, to the center of the piston, that is definitely collapse, NOT wear.

I haven't seem any collapsing of the Yamaha full skirt pistons unless they were the original old school TA/TD/YDS series ones. The old school ones were forged, the new designs are all cast.

What Steve, Erv Kanemoto, Russ Collins, Nick Arias and I figured out was that on the factory forged pistons, they collapsed because they were literally beaten into collapse, from those forged pistons demanding large clearances for expansion. By the time the forged pistons, at large clearances finally got to temperature, they were already collapsed from severely rocking, and knocking around in the bores.

Back then, forged pistons, large clearances, cast pistons, accelerated wear, almost instant collapse. First design cast pistons of that day, much better rock over thrust control from much tighter clearances, even in race engines. Even better today, as cast piston metallurgy and technology have evolved.

I would like to see a metallurgy spec sheet for the Wossner pistons, I figure they are both austinitic and autothermic, and that should make them candidates for a lot less clearance than old school forged pistons like the Wiseco's were back in their early days. It is all n the temperature and expansion rates between the cylinder and piston.

Wossner clearance is .0027, Wiseco is .003

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

Now that's splitting hairs!

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steve

Whoa, way too large for me.

If someone here would like to post a picture of the Yamaha OW31 Replica piston, please send me your real, live e/mail address, and I will send one to you for this topic. For the life of me, I still don't know how, nor understand how to post pictures on web sites. Send them by e/mail, got that down, post on forums, not a clue. I did figure out how to do it on craigslist, that is simple, everywhere else, uh...NOPE.

Now I can see how Yammi got away with it.......

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

I would be honored to help you with that ... PM me if your interested ...

Darth

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

The ribs are very impressive; does that skirt look extra thick?

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

Bummer but on the flip side some new videos to watch.

Darth, thanks for the offer, but Moo beat us both to it.

Those be the pistons, and they are no thicker in the skirts than an ART for an H1 or H2. I have even opened the cuts up to the center of the wrist pin diameter area, with NO adverse affects. The picture is deceiving, as you see a cross section of metal, instead of straight on the cut. I've used these things, and a lot of other pistons cut like these, and more radical, in all sorts of two stroke reed valve engines, and not had one problem with any of them. Look at the bottom of the skirt on the exhaust side, and you will see that the piston thickness isn't larger than conventional "full skirt" pistons.

The first ones of these I saw were on Roberts', Baker's and Cecotto's factory race engines at Daytona, before the OW31 Replicas were available, machine milled into the regular full skirt pistons. The piston in the picture is a factory replacement one, from later stock, made/cast just the way it looks.

These were done on Yamaha engines because Yamaha engineers knew that the intake skirt simply became an obstruction to the port and mixture transfer when it was either left long, and/or drilled/slotted, so they designed this cut. This let the reed dictate the inlet operation, NOT the piston skirt, and was used along with a port at the top of the inlet port, that aimed mixture up to the spark plug. That added port was literally an added transfer port then, as the inlet port remained open to the bottom end at all times. All the reed valve TZ750 cylinders, and especially the best factory TZ750 cylinders, both factory and privateer, has one of these ports at their inlet side, off the inlet port. Privateer cylinders had 2 transfer ports per side, making them "5 port", and the factory team cylinders had this port, and 3 transfers per side, making them "7 port" cylinders.

On the cylinders I did for Harry Klinzmann's, and others TZ750's and other cylinder reed valved engines, I also added the finger port type that was above the intake port, to the bottom of it as well. The inlet port then became a boundary layer area when the reed closed and the piston came down, making both cuts into their own full transfer port, with no piston materials in the way to obstruct transfer port mix movement up the back of the cylinder. Imagine hour hand standing straight up. factory piston port cut would be the palm of your hand. Added upper port would be the two center fingers, standing curved, upright. Now, imagine another set of two fingers, going down from the bottom of the inlet port, into the crankcase, that is the extra port into the cylinder I cut. When the inlet port is closed off by the reed, mixture is pressured up the bottom added port, through the inlet port as a guided flow, through the upper port, to the cylinder and spark plug. Added transfer port, no obstructions, works well.

As I said, if the skirt isn't there in the first place, it can't break off.

But the skirt is still there on the sides, just much smaller contact area which translates into higher stress in the piston.
Again, if the pistons are changed every weekend, there is no concern for fatigue...
John's motor blew up on the last race of the season, not the first one. Since we are talking street bikes for about 90% of the guys here, I'd still err on the side of caution.

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