Piston crown crumbling away...?

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Fruitjuice
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Last time I rode the lammy (Monza 225 / TMX30 / Adspeed ADS3) I lost compression just before I got home.
Yesterday I took off the kit to find the bottom part of the piston crown is crumbling away, the rings are seized in the grooves and a part of the Nicasil above the exhaust port is gone... No further seize marks or obvious air leaks.
Engine has been running for a longtime without problems and been on track a couple of times too. What's the cause?

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Life shouldn't be a journey to the grave with the intention of arriving safely in a pretty, well preserved body, but rather to skid in broadside, in a cloud of smoke, thoroughly used up, totally worn out, loudly proclaiming: "WOW, WHAT A RIDE"
hendy
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I'm certainly not an expert in this area, but I was suspicious about an engine I was building and was advised that the piston crown might crumble away. this was due to the exhaust port being too low on the bottom edge.

take a look at the thread, maybe it's your problem?

http://scooterotica.org/forum/viewtopic.php?f=7&t=33151
eden
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most likely due to the plating coming off first, that then fecked the rings and the piston began rocking into the exhaust port and the missing plating.
or the rings worn so piston rocking into the exhaust port and eventually gouging out the plating.

Has the exhaust port been widened or lifted? did the port have a good chamfer on it to the top edge?

On cast iron barrels worn rings and/or exhaust ports to wide lead to the same thing only rather than plating coming off, as they don't have any, the barrel wares heavily just above the exhaust port.

What ever the reason for it doing it, the piston has been rocking into the port!



Edit:

also if the inlet is huge AND the exhaust port is huge the tendency to rock into the exhaust port is increased even further!
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wack 63
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I'd say an air leak on the head probably caused this as the piston has detonation damage. Also check your exhaust is clear .
B-Race Tuning
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Humour me, how soon after refuel was this?
Fruitjuice
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B-Race Tuning wrote:Humour me, how soon after refuel was this?
I'd say about 30 miles, of which 15 were motorway...
Life shouldn't be a journey to the grave with the intention of arriving safely in a pretty, well preserved body, but rather to skid in broadside, in a cloud of smoke, thoroughly used up, totally worn out, loudly proclaiming: "WOW, WHAT A RIDE"
Fruitjuice
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eden wrote:most likely due to the plating coming off first, that then fecked the rings and the piston began rocking into the exhaust port and the missing plating.
or the rings worn so piston rocking into the exhaust port and eventually gouging out the plating.

Has the exhaust port been widened or lifted? did the port have a good chamfer on it to the top edge?

On cast iron barrels worn rings and/or exhaust ports to wide lead to the same thing only rather than plating coming off, as they don't have any, the barrel wares heavily just above the exhaust port.

What ever the reason for it doing it, the piston has been rocking into the port!



Edit:

also if the inlet is huge AND the exhaust port is huge the tendency to rock into the exhaust port is increased even further!
Bog standard Monza 225 barrel with nothing done to porting.
If this was the reason, then about every single Monza/TS1 would show the same signs.
Life shouldn't be a journey to the grave with the intention of arriving safely in a pretty, well preserved body, but rather to skid in broadside, in a cloud of smoke, thoroughly used up, totally worn out, loudly proclaiming: "WOW, WHAT A RIDE"
eden
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You said it had done many miles. Hence worn rings or was that fiction?

Have you taken the rings out and checked the gap or just looked for anything other than the obvious?

If it was due to detonation there would be signs of it around a wider section of the rim of the piston and head which there isn't. The pepper marks on the head are just from the bits of ali coming off the piston and hitting the head every time the piston is at tdc.

in the top photo it also looks like there is a lip at the top of the barrel which would be due to the plating warring but thats unlikely due how hard the plating is. is that lip just coke stuck the barrel above the height the rings run to at tdc?
or is it a lip?
if its a lip and if you have warn bearings the rings would get damaged by that lip, and as I said before, if the rings are badly worn the result is the piston rocking into the exhaust port!

if it has been rocking there will be access ware to the bottom of the inlet skirt on the piston too!

Maybe you misunderstood what I was saying, I wasn't having a dig at ali barrels just comparing how the same problem with very worn rings affects both types of barrel. I mentioned the port sizes as that results in the same kind of damage, if the ports are standard and you have indeed done many miles on the barrel and piston and rings my money is still on worn rings and worn piston making the piston rock too much. Could you hear piston slap when it was ticking over?


post a pic of the inlet skirt of the piston and also one of the inlet on the barrel!
gaz_powell
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I had the same on a mates 172 T5 which I look after - done loads of miles no problems and had a ring change not long before it happened - when I stripped it there were no leaks evident - rebuilt it with a new kit and same head and its still running today without skipping a beat since

It happened not long after a fill up - only thing I could blame was the petrol - rightly or wrongly
fairspares
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its more than likely exhaust gas/ heat eating away at the piston due to incorrect piston/barrel position at bdc, were did the piston line up with the transferes and exhaust port at bdc?
Fairspares Lambretta workshop.
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