When Larry Witmer (Top ranked US Vet mxer) raced RMs we tried al sorts of combinations of top end configurations. We tried the 2mm oversize Wiseco (#723MO6850) the 2mm oversize piston from a Honda 4-wheeler (#526MO6950) a Pro-X cast piston (69mm) and a custom 74mm that I designed as a universal 300cc for Max Power when I was employed there.
Overall the forged pistons are far more audible at engine idle and that freaks people out initially, but the sound goes away when the engine gets hot and most people get used to it. The 3mm has some serious bark and uses a domed piston which is more durable than the standard flat top with regard to sustained thermal load. I set up lots of these engines for mountain riding and desert riding where fuel quality is marginal. The domed piston will take a lot of detonation before it causes any damage to the ring groove. I've also converted a lot of the RMs to 300cc using a 74mm Wiseco custom. Thats a difficult task that I won't do anymore because on the Suzuki the inside O-ring groove must be welded over and relocated farther from the bore in order to have adequate margin spacing. The casting alloy that Suzuki uses is high in silicon content, and even when using a compatible filler rod during heli-arc welding, its tricky to get the material to flow correctly and I suspect that the parent metal's properties are altered due to the fact that after plating, theres inconsistent platletting at the top of the bore where the weldment is. Also I've seen a higher percentage of adhesion failures at that point in the cylinder wall.
The latest project is to combine the 69.5mm bore with a 76mm stroke. Crank Works in Tempe Arizona performed the stroking (
www.crankworks.com) and they did an excellent job. Cometic is now making aluminized stainless steel base plates to shim the cylinder for stroker engines so we're testing some of those too. The current practise by companies like RPM is to just cut the head recess to the squishband deeper to accomodate the additional stroke distance. But I strongly disaggree with that method because it positions the ring to the very top of the bore which changes the heat path and shortens the ring life. Shimming the base allows for quick changes in compression ratios and timing. Our test bike is being raced in dirt track including half-mile ovals which put a huge thermal load on the bigger displacement 250 engines. I'll keep you guys updated as things develop.
I've been getting a lot of calls this year about stroking and crank repair so we're getting geared up to offer a wide range of combos for all the different models. Right now I'm working on a tech article that I'll post here, which will give an overview of crankshaft stroking for the average rider to ponder.
Happy trails!
var ska jag vända mej, helst en svensk sida
