All text & images © The Flying Banana, 2007.

GS1000ST Engine Rebuild...

Well I'd had the old GS1000ST a few months, and I knew she was burning a bit of oil and lacking a bit of compression. Of course, with over 100,000 kms on the clock and 27 years under the belt, it was hardly surprising.

But it wasn't until my mate Paul obligingly took the GS for a ride in early Feb. 2007, with me following on the Katana, that the spectacular extent of the oil-burning became clear.

Maaaaate, talk about SMOKE. At highway speeds, there was a huge billowing cloud of blue smoke when the throttle was trailing, and a huge billowing cloud of black smoke when the throttle was wound open.

So, it appeared to be valve stem seals and maybe worn valve guides, and something up with the carburettors as well. Or so I thought...

Well, one thing was certain: I couldn't keep riding it in that condition. Because it was only a matter of time until plod would take more than a passing interest. Besides, the curiosity was overpowering — so Sunday night saw me stripping the top end of the engine apart.

Now the excellent thing about all these GS and GSX engines, is that they are simply a dead sinch to work on. I was able to completely pull it apart in about 3 hours without even taking a peek in the manual! OK, it would have taken me only 2 hours, were it not for the fact that the cylinder block was virtually cemented on to the studs, thanks to 25 years of grit, oil, road grime, and sand — of which there is plenty here in Western Australia.

Anyhow, enough chattering on for the moment: here's a few photos of the dismantling, step by step:

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The moment before the spanners descended. Note the Katana looking on with interest.

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Tank off, battery disconnected: set the controls for the heart of the sun.

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Carbies out (always a pig of a job), coils off, cam cover removed.

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Camshafts and cam caps removed; the camchain dangles above the abyss.

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Cylinder head came off without a fight. I'm pretty sure I'm the first to look inside those pots since the gents at Hamamatsu 25 years ago.

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The cylinder block, however, wasn't so easy to remove. The cylinder studs are exposed to the elements in quite a few places, so over the years a solid cement made of oil, grime and grit had taken a set around the studs. Of course, as I carefully wrestled the block off, a lot of this sandy grit fell straight into the cases — bother. My solution was to mix up kerosene and oil about 3:1, remove the sump plate, and flush it all out by pouring and squirting the mixture through. The sump plate was bolted back on, and using the oil can I squirted oil back over all the bearing surfaces, the crank webs — everywhere I could get to, basically.

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As you can see, things had got pretty crusty in there: rich mixture and burning oil had properly coked things up. (And yes, the intake manifold rubbers did get new O-rings too.)

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A freshly honed bore. A bit of wear was evident at the top of the cylinders, but not enough to justify a re-bore and piston kit.

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A rough calculation told me that this little beauty had done about half a billion strokes along its cylinder bore — and still we didn't need a new piston kit! Just a hone and a fresh set of rings.

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Well, if only it was that simple; rebuilds are never a matter of "just"! Because then there were all the gaskets (head, base, and exhaust flanges), the O-rings (cylinder base, cam chain tunnel), and the valve-stem seals. But by far the most costly items were the four OEM piston ring sets — they were just shy of AU$70 per set. Owwwwch.

I also lapped in the valves, which meant I had to get thinner shims to get the valve clearances back into spec.

Reassembly was pretty straight-forward. I topped up the crankcase with a basic mineral multigrade oil. (You mustn't run-in an engine using synthetic oil because, well, the metal-to-metal surfaces won't bed-in properly.) Then I made sure that the crankshaft could be rotated by hand a few times, without valves clipping pistons, etc. It's always best to find this out at hand-speed rather than cranking speed.

I put the battery on a slow charge for about 8 hours, as it had been sitting idle for over three weeks. Then it was time to wheel the old girl out of the garage, and thumb the button.

On only about the fourth revolution of the crankshaft, she fired up without a splutter and settled into a fast idle while I ran off looking for the helmet and gloves. When it would idle without the choke, off we went around the block for a quick run, up through the gears and then back down, and then switched her off to cool down while I went inside and had a victory cuppa!

After about 800kms I did the first oil and filter change. I could really tell the new rings were bedding in, because the rising compression meant that the tired old starter clutch started slipping, and letting out that characteristic 'clang' first thing in the morning.

Then I rustled up Paul and we went for another ride — but this time with me on the GS, and Paul on his new GSX1150EFE beastie, as I gave the rebuilt engine a good workout on the open road to Dwellingup. No smoke to report this time; just a willing, refreshed engine that didn't burn oil or foul plugs — ready for another 100,000 kms!

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The Katana's engine rebuild page is here.

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