Please let me clarify, All:
The person asking about the pistons (it started with a PM to me, asking to reply in his thread) was not an experienced engine builder, but a first-timer. The things that happen with forged pistons, especially big-bores on the SOHC4 engines, because of machine shops that have their own 'opinions' about how they should be built, can result in very expensive problems. The novice won't know to stop an unknowing machine shop from doing it wrong.
This is not to say that someone who knows how to use them would have troubles: far from it. But, they are quite different from the usual cast pistons, exhibit different operating traits, and generally do not go 50k miles trouble-free, in my experience.
What they do well is: make lots of torque, relative to similar cast pistons. This is partly due to the extra heat they generate with their high compression (usually 10:1 or more) and, once warmed up, their tighter fit in the bores. They also manage increased top-end pressures better than cast pistons, with less distortion and the attendant drag across the wristpins that can develop with, say, a turbo attached.
Here's what I have experienced with forged pistons in the 750 (so many times, now, that I actually shy away from them for inexperienced rider-rebuilders): the bores are supposed to be clearanced about .0022" to .0024" for the Wiseco 836 or [old] Powroll 836 forged units. This is 2x the clearance for Honda's own cast pistons, which use .0008" to .0012" - even the cast 65mm pistons use .0010" to .0014". If this is done precisely for the forged units, they work well IF broken in properly. And, they make for a very oil-sensitive performer, particularly during break-in. During this break-in cycle, my experience has shown the engine should not be run for short periods of time, then stopped for [very] short periods, and restarted (like a 2-mile ride to the gas station, and refuel-restart): rather, it should be started up and run for a while, at varying speeds (not idle), then allowed to fully cool down, and the cycle repeated a couple more times. If this is not done pretty well, the aluminum cylinders cool so fast, relative to the pistons, that they reduce the clearance considerably on the warm restart, often causing a difficult startup, which leads to scuffed-skirt problems. These issues lead to ring damage.
In the last 4 years here on these forums, I have been thru 3 instances with rider-builders who installed the Wiseco kits (not by me) and then experienced similar problems: they either fired it up at their shop/garage and ran it until hot, or rode to their local fuel stop, then let it cool just 5-10 minutes, and afterward found it impossible to turn over the engine, as if seized. After a much longer cool-down, it (they) started back up. This cycle was repeatable for 2 of these guys until the engines had almost 1000 miles on them: the 3rd one took it back apart and discovered his machine shop had installed .0040" clearance "because that's what forged pistons need" [sic]. This one had already fractured a compression ring, with less than 10 minutes run time. After being careful to run the [other] engines longer before shutoff and either keeping the shutoff very short (2-3 minutes) or much longer (20+ minutes), the other 2 engines settled down, but both are still hard to start if hot, on a cold day. They simply drag a lot on restart because the sleeves shrink so much faster than the pistons.
I used to have a box of these (broken) pistons that I kept around to show those who wanted to build with these pistons, to try to 'drive home' the importance of knowing that the old standard break-in routine needed to be modified for these types of pistons. This issue first came to my experience with the old Yosh forged 811cc 10.75:1 CR pistons for this engine (circa 1972), which would often get installed with too little clearance. They acted similarly, back then.
So, please know my comments are not a bash of Wiseco products: rather, they were intended as a note of caution to a rank beginner who doesn't understand [yet] the 'whys' and 'whats' of the clearances, materials, and combinations of parts that could make for a bad experience if not correctly synch'd. I build engines with these Wiseco pistons for quite a few folks, unless they are planning on long-distance touring with these bikes. For those, I caution them that, in my experience, forged, hi-CR pistons used under touring conditions lead to early oil leaks (like, less than 20k miles), and the thin Wiseco rings don't last nearly as long as the somewhat over-thick cast rings usually found on these pistons.
I am still wondering about the 'things not said' (maybe not understood?) about the CycleX debut of their super-"F" bike: if you read their story about the problems they had with hot startup on the track, it smacks of this very symptom. I haven't written to talk with Ken about it, as he doesn't know me from Adam, but I would be interested to know if their non-start was due to 'stuck' forged pistons. Often, this issue gets blamed on "too much compression" (which appears in the online articles about the event), but I wonder about that when even a Powroll 11:1 set can be whipped over (when cold) with the stock electric starter, unless it is 'caught' in this cooling-period situation?
Thanks, for listening...
