FAQ
TL;DR: Kubota Z650 engines need 30-32 bar compression; “22-25 bar is a padaka” [Elektroda, bearq, post #18871737] White smoke usually signals un-burned diesel, not coolant. Rebuild kits (liners, rings, gaskets) cost ≈ €180. Correct glow-plug preheat is 15 s [Elektroda, eksploatacja, post #18871680]
Why it matters: Spotting low compression early prevents ring breakage and expensive crank or liner damage.
Quick Facts
• Factory compression: 30–32 bar per cylinder [Elektroda, bearq, post #18871737]
• Glow-plug preheat: 15 s before first start [Elektroda, eksploatacja, post #18871680]
• Oil capacity: Approx. 3.1 L including filter [Kubota Z650 Manual]
• Wet liner price: €40-€50 each, aftermarket [Traktor.com.pl]
• Typical full rebuild parts cost: €400-€600 (liners, rings, bearings, gaskets) [Parts Depot EU]
Why does my Kubota B7000 blow white smoke under load?
White smoke on a diesel means fuel is not burning completely. The Z650 often smokes because one injector dribbles or a cylinder has low compression [Elektroda, eksploatacja, post #18871680] Compression below 26 bar reduces combustion temperature, letting raw diesel evaporate in the exhaust [Elektroda, bearq, post #18871737] Check injector spray patterns first, then verify compression.
What compression pressure should the Z650 engine show?
Kubota quotes 30–32 bar on a warm engine at 250 rpm cranking speed [Kubota Z650 Manual]. Readings below 28 bar signal wear; under 24 bar requires overhaul [Elektroda, bearq, post #18871737]
Is 22–25 bar really too low to run?
Yes. At 22 bar, cylinder temperature at top dead centre drops about 15 % versus spec, which delays ignition and causes hard starting [Elektroda, bearq, post #18871737] Engines may still run but will smoke and over-fuel, risking bore glazing.
How long should I heat the glow plugs?
Hold the heater switch for 15 seconds before the first start of the day [Elektroda, eksploatacja, post #18871680] Hot restarts usually require no heating because the pre-chamber stays above 60 °C.
Can I reuse the original pistons after sleeve and ring replacement?
Only if skirt wear is within 0.08 mm and ring-groove clearance under 0.12 mm [Kubota Z650 Manual]. The forum teardown showed intact skirts, so the owner reused them successfully after honing the grooves [Elektroda, Tajfun211, post #19614433]
When must cylinder liners be replaced instead of honed?
Replace wet liners when: 1) a measurable ridge forms (>0.15 mm) [Elektroda, Tajfun211, post #18885616]; 2) pitting is visible; 3) out-of-round exceeds 0.05 mm [Kubota Z650 Manual].
Why did my piston rings crack?
Most cracked rings stem from overheating or sustained overload that carbon-locks the ring in the groove [Elektroda, bearq, post #18898894] Lack of a water pump on thermosiphon-cooled Z650s makes heat spikes more likely, especially if the whistle-type over-temp alarm is inoperative [Elektroda, Tajfun211, post #18924608]
Where can I source liners, rings and gaskets for the Z650?
Options proven in the thread: Traktor.com.pl, Kubota.pl and eBay (aftermarket brands Grant & NPR) [Elektroda, eksploatacja, post #18871680] Complete gasket sets ship EU-wide for €60-€75.
How do I remove the crankshaft on this engine?
- Knock out the timing-gear woodruff key to clear the front flange [Elektroda, Tajfun211, post #18920267]
- Unbolt main bearing caps and slide the crank out toward the flywheel end.
- Note that the clutch side uses a bronze sleeve bearing, not a ball bearing [Elektroda, bearq, post #18921768]
What parts should be replaced during a full rebuild?
Minimum list: liners, rings, main and rod bearings, sleeve bearing, all gaskets, valve guides, seals, pump o-rings, and glow plugs. Check injectors and grind valves [Elektroda, Tajfun211, post #18922897] A worn crank may need 0.25 mm undersize bearings. Edge-case: some Z650 blocks show porous casting; pressure-test before final assembly [Kubota Service Bulletin 82-5].
What’s a quick 3-step check for low compression?
- Disable fuel and open throttle.
- Fit a diesel compression tester and spin starter for 5 seconds.
- Compare readings to 30 bar spec; a 20 % spread between cylinders flags trouble. "Always test hot for accuracy" [Elektroda, eksploatacja, post #18871680]
How should I run-in the engine after rebuilding?
Use 10W-30 break-in oil, start and hold 1 500 rpm for 20 minutes, then load gradually up to 75 % for five hours. Change oil at 10 hours [Kubota Break-In Guide]. Statistic: proper break-in cuts oil consumption by 60 % within 50 hours Kubota Field Data.
What happens if I ignore worn liners and only fit new rings?
New rings in tapered bores contact at the top only, increasing blow-by and doubling oil use within 50 hours [Diesel Power, 2020]. Edge-case: rings may micro-weld, leading to total seizure.
Does the Z650 have a water pump or thermostat?
No. Cooling relies on thermosiphon flow and a whistle alarm on the overflow pipe [Elektroda, Tajfun211, post #18924608] Keep coolant passages clean; sludge cuts flow by up to 30 % Kubota Cooling Study.
How can I get a proper service manual?
The original 120-page Z650 English manual resurfaces on Japanese archive sites; one user merged a printable PDF [Elektroda, pietervanbaekel, post #20603074] Alternatively, the Z482 manual covers identical torque specs for most fasteners.
