Whoa, sorry guys I've been out for a while.
The picture was from November 8th actually. I had to stop riding when the ice was half an inch thick on the hull and the throttle froze open.
My experience so far, if it can help.
All my cold weather riding is on my 94 xp with the 657x. That machine has the cylinders bored over almost .002" of the suggested bore to match the pistons. The machine shop I have do my bore work always takes any pwc/outboard work a minimum of .001" over just because we do run on cooler water temps up here.
The coldest I've run is +1F air temps with water right above freezing. I didn't run wide open for longer than a few seconds and didn't have any engine seizure issues.
That's with the 657x.
My, limited, experience with the 951 is with my 98 XP LTD. I would never run that in the cold temps like the 94 does. I have run it in 50F water temps though. It didn't suffer any seizures or engine troubles and that is running it wide open for over 10 minutes at those temps, and we are talking wide open solid for ten minutes doing river running. I forgot if I am using cast or forged pistons, but again, the cylinders were bored .001" over when I rebuilt the top end since I bought the machine with water damaged pistons.
Now one thing I did run into that I have never experienced before was, I think, related to the cooling routing between the two generations of motor. And anyone feel free to correct me since we're trying to help each other here. From what I learned the 951 cooling is fed directly into the head/cylinders first where as the 580, 657 and 717 have the cooling water preheated a little by the exhaust before going into the cylinders and head. This would obviously keep the cylinders colder, relatively, on a 951 when compared to the 657, something I noticed as I was learning about the 951 while working on it.
Now.......
I had some headaches with the 951 motor and ended up having to rebuild it, due to water ingestion, over the winter and only was able to do the first start on a new motor in the spring. I did not do a break in until the water was ice free and had warmed up a bit, roughly 50 degrees but I never measured it. Within the first 1/2 tank of the break in I had the motor loose power and "fail", although there wasn't any damaged metal. I pulled the head and found my freshly bored and honed cylinders had almost zero visible cross hatch after half a tank of gentle riding with a little premix, they were glazed over like the cylinders had been run smooth????
Long story short. After talking to everyone I trust to give input, including the good people here I ended up getting some insight from "Group K", a PWC performance company I had never heard of before. I described my situation and the first thing the guy asked was "did you oil your cylinders on assembly and did you run premix for break in, how aggressive were you with the break in?". They said the cooling system feeding the cylinders first, the abnormally colder water temps and low throttle (gentle break in) could be causing a situation where the extra oil was leaving deposits in the cross hatch because the engine simple wasn't getting warm enough for complete combustion of the compounds in the oil. I did more research and found that this phenomenon is common in any type of motor if they are run too cool, little two strokes all the way up to industrial diesels.
I know the oil issue isn't what you are running into, but I'm emphasizing that the cooling style of the 951 (and 787) is different and has proven to be a hidden problem for me in the colder environments I run in. To compare, I have done tons of 587, 657 rebuilds in the exact same water temps and had zero failures or issues.
As for the 951. I pulled the cylinders to give them a hone, it litteraly took maybe two seconds to get rid of the glaze. The metal wasn't worn, the grooves were filled just as I was expecting. For fun I used the finest grit wet sand paper I had to just rub the faces of the rings incase they had contamination as well. Group K strongly suggested assembling the top end dry, no coating of oil on the cylinder, not running premix and being aggressive on the break in. This is NOT the way I ever have approached doing a rebuild, but I tried it anyhow. I warmed things up on the trailer, did a few minutes of mellow riding and was riding it pretty aggressively right off the bat. Compression check showed 140 psi dead on the next morning on a cold motor and a look down the plug holes showed perfect cross hatch, it's been two years on and it runs strong. I did the the hone, ring cleaning and second break in attempt within 48 hours of having the cylinders glaze over, so the water temps and running conditions were the same. The only difference was no extra oil during assembly, in the gas and hard running, the end result was a running motor as compared to a dead motor. Being TOO gentle and cautious actually led to a motor failure.......
As for why you are experiencing the piston seizures, I can't say for sure. But after what I experienced I will only take my 787 and 951 out later in the season once the water temps have come up a bit. But my "warm" is your "cold" and for regular riding you should not be experiencing piston to cylinder clearance issues, as long as you have proper machined clearances and the machine (carbs, fuel system) is in good working order, at 50F water temps.
I hope this helps.
