Hitchcocks Forum

Has anyone seen anything like this before? The circlip, or at least part of it is still in its groove but so deeply recessed into the groove that the gudgeon pin can pass through it. My cylinder barrel is now scrap because of the depth of scoring in the cylinder wall. The other side of the piston still retains the circlip complete but above and below the gudgeon pin hole is starting to look like the more damaged side. What's going on?

Sorry..........having difficulty posting the photo.

Thanks papasmurf

Aagh! Where did all that alloy go? Hopefully it's not all lodged up in the rocker spindles, courtesy of the scavenge pump. What was the little-end play like? It sort of suggests that the piston has been oscillating along the plane of the gudgeon pin. Why, I have no idea otherwise. Could just be poor quality alloy in the piston?



A.

The small end and the big end are showing no signs of wear. There were no outward signs of trouble. I removed the head to fix an oil weep from the pushrod tunnels The rest was a bit of a shock. I'm not yet certain but I think the barrel may be scrap it is so badly scored. And yet, the bike started and ran as well as ever. It didn't even smoke. Perhaps the calamity occurred on its most recent run???

I recently had a problem on my Mk2 Interceptor, this casting flaw appeared in one bore. The result was piston damage similar to yours but not exactly the same. I think what happened was the air pocket was hiding in the casting and the re-bore just touched the edge of it, you couldn't see it but it must have been wafer thin and when the thin layer collapsed it acted as grinding paste in the gudgeon pin bore. You said there was a lot of scoring, could that be covering something similar? An expert in iron casting tells me this is a result of an interrupted pour or the mould not being hot enough. REgards, Mark

Gudgeon pins do move about in service, that's what the circlips are for. I wonder if someone has used the wrong circlip? I've never known one erode it's slot like that.
As Adrian says, I would want to know where that alloy has gone.
I don't know what mileage is on the piston but it looks like quite a bit of blow-by around the gudgeon.

Has your piston crown collapsed too? Looks kind of like it might have given the scoring around the crown area and blowpast more on the sides than the front/back. Are the rings free to move in the lands or are there tight and loose spots?



It looks like the skirt of the piston around the gudgeon pin hole has bulged sideway then rubbed against the bore. Presumably the remaining damage is a result of the heat generated by this. I wonder if a collapsed piston crown front and back has lead to ovalling of the piston so it is slightly wider sideways or preventing the front and rear of the rings seating properly in their lands, then rubbing of the piston skirt sides against the bore could have lead to a chain reaction of wear and melting?



You can see smears of alloy between the top two ring lands immediately above the damaged area too as if the side of the piston has rubbed against the bore there too, be interesting to see if you can push the rings into the lands at this point so far they are no longer protruding. Maybe also measure the piston for ovality?

[img]https://i.post[url=https://postimages.org/]img.cc/QCQ8ZwKs/DSCF4362.jpg[/img][/url]