Holed Piston

[tz375]

It's not an oil problem. That is the sign of persistent detonation - that "rattle" from 2,500 - 3,000 was slowly eating the crown away. That piston is very old and the crown shows signs of coming apart for a long time.

Looks like a combination of a material that has become brittle over time, and detonation that hammered its way through. It's probably slightly lean at part throttle and that combined with modern crappy fuel was probably just enough.

Forget theories about sparking across from the plug to the piston or small end play. They are not likely - check out how far the electrode is from the piston at TDC.

When you rebuild it, check the oil lines and retard the ignition slightly plus raise the needles one clip position or contact Ivan on this board for one of his jet kits.

[tricky1962]

Thanks for the response.

To clarify, the rattle was intermittent, and to seemed on occasion to be induced by bumps in the road, not throttle ie open the throttle would not necessarily induce rattle. So maybe, maybe not pre-ignition

If I could show you the spark plug I would, but I've lost it. However, that one had been in there for close on 2000 miles and showed no sign of pre-ignition (although as I've said, I'm gong to check the timing from first principles) and has always had a dark chocolate coloured electrode.

Are you suggesting metal fatigue? I could go with that given the way it has been removed in "flakes".

How far is the piston from the electrode at TDC ( waiting for my new piston)?

[Vintageman]

Bubbles in lne not good, but still did not look like you were lacking, so yes fix that.

It looked to me maybe that line was leaking and when you park it oil drips into the failed cyl and smokes heavy first start after setting a while until you swashed it all out? Or Maybe center seal?

No I don't think a little extra oil really changes mixture too much. Others may disagree

I think you got a lot of miles (And years!) out of that older style piston, failed slowly and time up. It ma have seen little more heat than others maybe for the reson ssuggested lean and /or type of riding. And years of chemical reactions/corrosion


Why I say a little richer on pilot is so you can run fast and cruise with less worry (Saab guy should do that too). what I mean by cruise is you are really given just enough throttle to hold steady speed (never did say what throttle pos you were at when failed). so If you were running at say under 1/3 a little more pilot will give a little more fuel to help cool things that's all (it helps). There also a period article out there who found the t500 was a little too lean on pilot. I agreee just one step is fine ( i think they said two?))

Since you are a part check everthing you and all have suggested, get fresh shinny pistons, wrist pin and SE bearings and you should be good to go.

Oh if you know you are about to cruise fast for long peiods warn up on BH7S and run the B8HS.

Look at that needle jet too! You get the 159 series new where as the 188 (182?) is obsolete. All cheep insurance and man it will cruise sooooo much better too

Thanks for sharing this experience and let us know what you do/find and how it works out!

[Coyote]

The piston coming apart in layers like that suggest a piss poor casting. One that made it by QC that should have been culled Almost like a double pour. May not have anything to do with mixture or timing. Might just be a crappy part that got hot and cooled down one too many times.

[tricky1962]

Here's a quick update as requested.

The timing was advanced by, at a guess about 5°. My mistake, I set it up wrongly before, now bang on the marks. This was on both cylinders, but only one seems to have been affected.

I lifted the head to check for pre-ignition after 200 miles with the new piston and everything looks good.

The intermittent rattle that could have been pre-ignition, was subsequently traced to a slightly loose front sprocket being held in place by the tab washer. Now all tightened up, everything sounds fine.

Fingers crossed

[Cliff]

What RPM were you running at? Was the bike overloaded (baggage etc.)? Lots of hills/inclines/long uphill grades? Was the motor working too hard and you should have downshifted? Was the motor lugging down/working hard just prior to the hole? These old girls will hole a piston if they're worked too hard for too long!

[tricky1962]

Not working hard really, 65-70mph, 4000rpm on at a guess 1/4 throttle.

I'm going for the metal fatigue/poor casting suggested above combined with slightly out timing as the cause

[smokin_blue]

I had a TS250 that I bought off a customer while I worked in a Suzuki shop. The guy came in and said it wouldn't go over 45mph and all of a sudden he had no power. The rest of the story of course was that he had just pulled it out of 2 year storage threw fresh gas in it and headed down the road. It had no compression so when the head came off it displayed a piston that had a hole just like yours. However there was one serious difference. Mine was definitely a case of running lean and detonating. The top of the piston was bright silver and the best part was the check of the spark plug. It too was bright silver with all the little speckles of aluminum that came exploding off the melting piston and stuck to the spark plug!

[Pete O'Dell]

That piston pic clearly shows ignition timing is out and you likely need a rebore.

[tricky1962]

Didn't get it rebored and the last 1000 miles have been covered without issue.

How would I know it needs a rebore?

It ticks over quietly, smoothly and steadily, and will cruise at 80 should I so demand