Kawasaki Stx 12f
Thread: STX-12f dilemma
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07-03-2018,01:39 PM #1
STX-12f dilemma
I have this stx-12f that I have never heard run. PO said it blew the head gasket, had gotten water in the oil, and started disassembly. Compression was poor across the engine when i jumped the starter. Pulled it down, head gasket was not really blown but the exhaust was eaten. All four cylinders showed signs of water infiltration and the cylinder is a write off due to pittting from water sitting in it. One piston has crown pitting, and down into the ring lands. The head has pitting in the combustion chamber on that hole and it needs all eight intake valves. I have a source for a good used head and cylinder so not a big deal.
The boat is pretty mint looking, I got it cheap (really was buying the trailer), and it appears to have had the whole pump assembly replaced recently.
Now for my dilemma. The cam surfaces and retainer looked great. No sign of oil issues. So I am debating splitting the case to look at the bottom end. There is no end play on the crank (well there is but i would have to dig out the dial indicator to measure it). Should I split it and check out the mains and rods, or put the used top end on and hope for the best. Its mine, and I wont be selling it off so I am not patching to pass it off or anything unethical. I am kind of thinking buy one new piston and ring sets for all four holes, gaskets and let it ride. Thoughts?
(i am not paying anyone to do the work so its literally just costing me a little bit of time and used bits and new gaskets to put it back together, i try to introduce a few folks a year to PWC riding so i think it would be a good beater boat for folks to ride)
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07-03-2018,06:33 PM #2
While you can pull the cylinder out with the engine in the ski, it will be really difficult to put the cylinder back in, compressing two sets of rings at a time into the bore. I have never tried it, though.
I can only assume that you will remove the engine, which, in that case, might as well split the block. Because of the corrosion in exhaust, you should check the corrosion in alternator cover. Replace the thrust washer and check/clean out the bottom of oil pan(metal dust). Reuse the oil pan gasket, sealant for the rest. Then you'll have a good bottom end you would not have to deal with again. Very reliable engine. It would only cost you time. But replacing the cylinder, head, exhaust, rings x4, can cost a bit. So the extra time just knowing the bottom end is good is worth it. I'm pretty sure you can split it without removing the alternator completely, just a few select bolts.
I recently purchased a 2006 12f, the rings where fused into the pistons and sat. Some pitting in bore. So, to rebuild would involve replacing all pistons, I soaked and heated ; couldn't get the rings out. Bottom end was perfect and I sold the matching block halves, to recover cost. Just make sure the display works, its expensive.
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07-03-2018,06:36 PM #3
Assuming water leaked in exhaust, cause water to go into cylinders, where it sat, rusting away. Engine won't run due to hydro locking, owner assumed leaking head gasket. In this case.... a possible bent rod as well, so check that too while you are in there.
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07-05-2018,07:47 AM #4
I think you are right. I already have the engine out and torn down to the case. Has anyone experienced any issues with the rods needing resizing after installing new rod bolts?
This is one of those "grab used parts when you see a deal" sort of projects.
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07-05-2018,09:19 AM #5
Rods are not a standard size. They are different based on a letter stamped on it, sized to match journal at factory, from what I understand from past research. I have never replaced rod bolts before, i have always reused them. Sorry guys, I hope I don't get flamed for this no-no(bad advice).
I kept hydrolocking #1 cylinder while testing the ski when I first bought it, and thought to replace the rod(just for peace of mind) when the replacement rod didn't feel right, bought another rod(same results), and I finally bought a "matching letter" used rod and std bearing, and it worked. I never thought of thrust bearing being bad causing this particular motor to be bad(unreliable). So after a year of messing with this particular engine, I split the case, and went through it, found the bad thrust washer, and it seemed to resolve all my issues. I must have pulled the engine out 3 to 4x's thinking stator/charging problem. I can't offer any more help as far as the matching process or replacing rod bolts. Sorry. You'll just have to do the plastic gauge thing and check clearances manually.
Last edited by Chinaman4u; 07-05-2018 at 10:14 AM.
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07-05-2018,12:05 PM #6
I bought a pair of 04 12Fs from a guy I know, he swore they were stored properly before they sat for 5 years. Long story short, one motor had water intrusion in all cylinders and motor was locked. Got the motor freed up with marvel soaking in the cylinders overnight but when I got it started it was blowing the oil filter off at 2K RPMs and wouldn't idle. Rings were rusted in place and the steel oil pressure relief valve was rusted into a chunk. I pulled the motor, put pistons/rings in and replaced the oil pressure relief valve, has ran like a top for a couple of years now.
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07-09-2018,10:27 AM #7
I think I am gonna split it and look at everything. Otherwise every time I have the throttle pinned I will be wondering if there is a rod/main/thrust bearing about to wipe out. lol. I also dont know what kind of gunk is in the crankcase unless i take it all the way down. I am at a point where I will be buying a full gasket set anyway so its just time.
My question about the rods is, often when you replace rod bolts, the new bolt stresses the rod differently and the big end can be nudged out of round. This is especially a problem with aftermarket bolts like ARP. I was wondering if anyone has ever checked their rods after replacing with OEM bolts and if it was an issue.
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07-09-2018,10:40 AM #8
Just keep everything numbered, in order(particularly the rods). I thought you already took it apart! Good luck to you.
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07-09-2018,04:30 PM #9
I just have the head and cylinder off right now.
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07-09-2018,05:29 PM #10
My experience is little on this jet ski engine, I do mostly cars. But take apart first, I think the full gasket set is uneeded, cheaper to buy separate(you may not use evryy gasket) examine the main bearings and if they are good, then
You might avoid taking apart the rod cap/bearings and leave them installed, thenn avoiding the whole rod bolt thing, the rod bolts dont come out easily either. only a suggestion. Dont use heat on the crank to remove bolts, you will ruin an oil seal, which is diffucult to replace. And the oil pump timing teeth on the crank can break, too.Just check the thrust bearing clearance or replace that part only. The manual helps alot! Gives you all the procedures and clearances. If the part is bad, the crank counter balance will rub against the block, wearrubbing marks will be evident.
Remember, this is a cheap, junk beater ski to run(as you stated), parts $ add up very quickly. A new engine isnt that much more expensive.
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