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Discussion Starter #1 (Edited)
Attention all you 998 turbo owners!
Starting when my 2017 ZR9000 Ltd was new, I have experienced coolant loss AND a corresponding increase in level of oil in oil tank. After about 1000 miles, the issue was getting worse. One day, after some wide open lake racing, the coolant bottle was empty (just filled it that morning) and the oil level was way over full. It was obvious the oil was milky indicating coolant in the oil. I stopped running it and took it back to my shop. I drained the oil and collected a sample that I sent to a professional lab. Indeed, the oil was heavily contaminated with coolant. So, the entire contents of my overflow bottle had drained into the oil sump, somehow. The cooling system never got pressurized by engine compression. There were no external leaks. I inspected every singe hose and clamp. No external leaks. My dealer and others told me the head gasket must be compromised. However, usually when that happens, you will get compression into cooling system. I had none. So, after debating with them (sled is still under warranty at this point) I decided to replace the head gasket. At the same time, I replaced the stock head bolts with a racing 8740 chrome moly stud set. I did this in April and winter was over.

Last weekend, I was testing for the first time and had the same problem, only WORSE. In about 20-25 minutes of running, most of the coolant from the overflow bottle were gone and the oil level was too high. I took sled back to my shop to diagnose. I knew it couldn't be the head gasket. I knew it had to be either be the head or the block. I knew it had to be in a place where there was no compression (cooling system never got compression). Therefore, it had to be on the top side of the head, IF it was the head. I decided to run a pressure test on the cooling system (again as I had done this last spring and saw nothing). This time with a high power magnifying glass. I took the valve cover off exposing the cams and top side of the head. Upon close inspection, I discovered a crack in the left side aluminum boss around the large plug. See pic! Conclusion: this head was cracked at original assembly at factory when they installed the plugs. If you look at the design, the plug is pipe thread. The wall section of the boss it's installed in is thin. When they installed the plug, they boss cracked. Since it's a pipe thread, the tighter they put it in, the more stress they put on the boss. Either they installed it too tight, or the boss is just too thin to withstand the amount of force the plug puts on it or this head casting was defective. This crack started small and got bigger. Now, the crack runs all the way down the outside of the boss to the floor of the head. Bottom line, head was cracked at original assembly. Warranty claim!
So, if your 998 turbo "uses" some coolant, maybe yours has a crack like mine. Pressure test will tell the story.
 

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Discussion Starter #2
Resolution to cracked head ...

Cat/Yamaha warrantied the parts.
Never have heard of or seen another one like it.

Whew. :smile
 

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Can you circle the crack in the picture? Maybe I’m blind. Sounds exactly what I’m dealing with with my 2019 Sidewinder!!
 

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Discussion Starter #4
As I say, not easy to see at all. If you open the pic, the crack can be seen immediately to the right of the red marker dot. Approx. 3 o'clock on pic. You can see it as a hairline crack on the edge of the core plug boss but it extends down the outside of the boss almost onto the floor of the head. When I pressurized the cooling system and looked at it with a high power magnifying glass, I could easily see coolant flowing out of the crack. I'll see if I can edit picture and re-post.
 
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