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Discussion in 'Airheads' started by cbi, Oct 31, 2012.
With a long-stroke crank you can even get 1160cc.
A friend has an engine with 102mm bore.
Full story (in german) here.
Pistons were from Werner Fallert, cylinders are custom made.
No long term experience yet.
That goes for polishing various outer parts as well; some may think it looks good, but it makes the engine run much hotter.
Not much room between the bore and the studs.
The 98mm Siebenrock cylinders have a very short and thin (1mm) skirt.
Close to none. I'm wondering if they last ...
Yes, they do last... for 10 hours. For race motors only.
It's not a racer but a normal street bike.
And it's intended to last.
I'll report, if it doesn't.
Sign me up for a set.
I've never seen an open oil ring like that. It seems like you'd get some accelerated wear where the oil ring enters the grooves on both sides of the wrist pin. Is he using a normal 70.6mm stroke? Longer rods or shorter barrels?
I don't regard that open ring as a good idea, too.
Stroke is stock, rods are 148mm long, so that the piston stays completely outside the crank case.
Is there no teflon or aluminum button behind the oil ring?
According to the owner there is some kind of button to stabilize the ring and to prevent it from rotating.
Never seen it or an image of it.
It's certainly a small pin in the ring groove, like what can be found on 2strokes pistons.
See more here :
The gasket surfaces don't only seal the combustion chamber and crankcase. They also seal the rocker arm oil and oil return in the pushrod tubes. Therein is the trouble. I have worked on race bikes with that ring setup. They ran like scalded apes for the longer rods. What I don't know is if it is better to go that route or just two rings.
I know the 97mm and 98mm kits don't make trouble here. No personal experience with 100 or 102mm bore.
The Siebenrock kit comes with 151mm (standard kit, additionally versions with - 0.5 mm for the touring and +0.5 mm for the race kit) rods and three-ring pistons in standard setup. I doubt there is the need for such an open ring setup anyway.
Another effect of the long rods in combination with the lighter pistons: these engines run very smooth.
In my former engine (SR-Replacement) all parts were balanced and it ran really smooth. But compared to the SR BBK I have now there's still a big diff.
151mm rods are 16mm longer than stock for a 2.13:1 rod length ratio. Standard three ring setup? There is only so much room. 16mm less than stock. Three rings above the pin will have to be a lot higher and closer together than BMW's standard. You can only run them so high and so close together. I do know from experience that that setup pictured works. Lots of tuners have run rings like that with good results.
With "standard" I meant, that the rings don't go through the pin hole. Of course the rings are different and closer together than in stock arrangement. Have to be to gain the needed mm.
This is the Siebenrock BBK piston
Moorespeed have also gone down this route: