I had always said that running your ski close to shore would pick up bits of sand that would build up in the water jacket of the engine. Well, today I got first hand experience with seeing it. I was working on a 650 cc Yamaha engine, replacing the magneto, when I noticed some white powedery residue on the PTO cylinder. Upon further inspection, I noticed it was cracked and the water had been seeping from it. The crack was in the shape of a “U” and about 1 ½ “ to 2 “ long. I spoke with the owner and we decided to remove the head. I was curious as to why the crack was there, the piston still firing, showing a good plug. What would have caused it. I could see it if a piston had been pushed through the side while seizing up, but this engine spun over freely.
After removing the head, I was shocked. The outboard (furthest side to the water jacket’s inlet) side of the water jacket was full to the top with sand; beach sand! I can only speculate that after the sand had built up, the water became trapped. As the piston heated the cylinder walls, the sand caused insulation that caused the water/grit mix to expand and crack the botton side of the water jacket. I had a spare upper casing that we honed only enough to scrath up the surface and put it on. At days end, I haven’t torqued it down yet to do a compression test. This isn’t normally done. If you change out the cylinders, you need to change out the pistons. But since he said he just bought these last season and the “F” was still visible, I’m hoping for good results on the test.
This is a very good reason to be aware of how deep you ride your PWC. The older ones especially. The newer 4-TEC’s don’t have the same cooling system, but none the less, they do have a wearing ring and open water system to the exhaust.
The first pix is of the crack, the second is off the water jacket, after I cleaned it out. Had I known I was going to write this thread, I'd have left the sand. But you can see the measurement between the piston wall and the water jacket. It's about 1/4".......easy for sand to build up.
After removing the head, I was shocked. The outboard (furthest side to the water jacket’s inlet) side of the water jacket was full to the top with sand; beach sand! I can only speculate that after the sand had built up, the water became trapped. As the piston heated the cylinder walls, the sand caused insulation that caused the water/grit mix to expand and crack the botton side of the water jacket. I had a spare upper casing that we honed only enough to scrath up the surface and put it on. At days end, I haven’t torqued it down yet to do a compression test. This isn’t normally done. If you change out the cylinders, you need to change out the pistons. But since he said he just bought these last season and the “F” was still visible, I’m hoping for good results on the test.
This is a very good reason to be aware of how deep you ride your PWC. The older ones especially. The newer 4-TEC’s don’t have the same cooling system, but none the less, they do have a wearing ring and open water system to the exhaust.
The first pix is of the crack, the second is off the water jacket, after I cleaned it out. Had I known I was going to write this thread, I'd have left the sand. But you can see the measurement between the piston wall and the water jacket. It's about 1/4".......easy for sand to build up.
