WHAT DID I FIND IN THE BOTTOM END?! SEADOO 300 RXT

SEA-DOO RXT 300 REBUILD — PART 2: ENGINE OUT, BOTTOM END TEARDOWN, & THE PLOT TWIST 🌊🔧 Welcome back to the $3,000 Sea-Doo saga. Last episode I bolted the head back on hoping for a miracle — cylinder 2 was still 85 psi. So the motor's coming out. In this one I strip the engine to the bottom end, find something the factory got wrong, rebuild it, put it back in the ski, take it out for a wide-open-throttle sea test at 65 mph… and end up with a completely different diagnosis than I expected. 🔧 In this video: • Head back off (10 mins — reusing the fresh gasket) • Prop shaft & carbon seal removal — plus my trick for pulling that C-clip when the shaft won't stop sliding • Engine out (light enough you could almost lift it by hand) • PTO housing off — check the starter drive UFO washers before you lose them • Split the case → bearings look mint, crank journals clean, no metal, no copper. Bottom end is healthy. • Pull cylinder 2 piston — and find the smoking gun 😳 The factory screw-up: the ring end-gaps were NOT staggered. All three ring gaps stacked on top of each other on cyl 2 — that's the mystery line I saw in the bore last episode. Then a side-by-side ring compression test showed the old rings had lost tension vs the new set. Cooked rings from the overheat. 🛠️ Full bottom-end rebuild: • New rings on all three cylinders • New main bearings, balance shaft bearings, rod bearings • Case pressure-washed and blown out, oil galleries clean • Careful sealant on the machined case surface (over-sealing shifts your bearing clearance — no gasket needed) • Head back on, cam timed, torqued down 📊 Compression re-test on the bench (with a squirt of oil for un-bedded rings): • Cyl 1 → 145 psi (was 125) ✅ • Cyl 3 → 135 psi (was 125) ✅ • Cyl 2 → still 105 psi ❌ • Leak-down test → head gasket not leaking, only minor valve leakage across all three 🧠 So into the ski it goes for a heat cycle and ring bed-in. Then things get weird. 🔥 On first fire-up the exhaust water box overheats and throws a dash light — even though the flush hose is running. I trace the cooling routes and discover the water is taking the path of least resistance and completely bypassing the exhaust. Massive stupid design (or a previous owner's plumbing screw-up). Sorted it, back to the water. 🌊 Sea test: • 62–65 mph / ~105 km/h WOT • 7,200 RPM (rev limiter probably 7,500) • Coolant peak 180°F even with fully advanced timing • Zero blow-by, zero smoke, runs healthy • These skis normally do 100–120 km/h so it's holding its own 🧪 Post-ride compression check (rings bed in): • Cyl 1 → 125 | Cyl 2 → still 90 | Cyl 3 → 110 • Still low on cyl 2 — but the ski rips. 🎯 The verdict: It's NOT the bottom end. It's NOT the rings. It's the cylinder head/valves. I've got a spare head in the container — send it away, get the seats re-cut, swap it in. Meanwhile the ski runs fine and I'll enjoy it while the weather's good. 💡 Big takeaway for anyone buying second-hand jet skis: If it's got a supposed blown head gasket or low compression, the bottom end is probably fine. The bores looked clean, bearings were reusable — save yourself the strip and get the head serviced first. And always compression test before you hand over the cash. ⏱️ Chapters 00:00 Recap — where we left off 00:45 Head back off (again) 01:30 Ripping the engine out 02:15 Additional oil cooler + supercharger removal tip 03:00 Jet pump + IBR removal (not in maintenance mode, don't do this) 04:00 Carbon seal & prop shaft — my C-clip trick 06:00 Engine lifted out 07:00 PTO housing + starter drive UFO washers 07:45 Main oil pump screen — clean, no metal 08:30 Case split — bearings, crank journals 09:15 Cyl 2 piston out — the un-staggered rings discovery 10:30 Ring tension test — old vs new 11:30 Full bottom-end rebuild (rings + all bearings) 13:00 Case sealing tips (don't over-sealant) 14:00 Cam timing + head torqued down 15:00 Bench compression test — the mystery deepens 16:30 Leak-down test 18:00 Reassembly time-lapse (1h45m turn-key) 19:30 Coolant fill + air bleed 20:15 First fire-up 21:00 The exhaust water box overheat mystery 22:15 Tracing the bypassed cooling routes 23:30 Boat ramp sea test — 65 mph WOT run 24:30 Post-ride compression check 25:15 The verdict: it's the cylinder head 26:00 Takeaway lessons for jet ski buyers 🛠️ Parts & Tools: • New rings & main/rod/balance shaft bearings (kept in stock — always handy) • Copper spray for the head gasket • Compression tester (12mm + 14mm adapters) • CD Tools leak-down tester • Prop shaft C-clip removal trick (washers-behind-the-pump) 👍 Like & subscribe — head swap coming in Part 3 once the machined head is back. 💬 Ever pulled a "blown head gasket" ski and found the same? Drop it in the comments. #SeaDoo #RXT300 #JetSki #Rotax #EngineRebuild #JetSkiRepair #Nikasil #BottomEndRebuild #CompressionTest #LeakDownTest #JordansGarage #ProjectSki #JetSkiFlip