Radiusing/polishing piston crowns? *Now with added fail*

[stinkwheel]

Been having detonation issues on my bigbore, high compression enfield bullet. A long ride up a pretty steep hill on motorway on my recent Spanish trip was causing intermittant detonation and lead to a seizure which did free off quickly and continued running for about 1000 miles. I threw a sticking plaster at the issue at the time by adding some royal purple octane boost* to the fuel which pretty much stopped the detonating.

Going to do a teardown tomorrow to see if the piston is toast or not, check bore, ring gaps, land clearances, etc. Hopefully not marked the bore too much and can give it a hone and new rings

Obviously checking timing and mixture is the main event but I've seen it suggested that one issue with these accralite forged pistons is that they have pretty sharp edges around the valve cutaways which can make them both noisy and more prone to detonation and that radiusing off the edges can be beneficial.
https://lh3.googleusercontent.com/pw/AM-JKLWXcey8AYmu28N5QbKXZqF4lPx7KSoEKHAjFwcPyZlz_iq8-sll0HfNRxYQFZVCtVl3W244YyUqDDKttqZ5DBA_0XkrasEZNb9-rJJPXLp5FqKyof1FwIwCLxAA2wlGXtzzZoxfHQFuV-TW5WCsPwpg=w1182-h886-no

The question is how? Someone warned me off using emery cloth because abrasive particles can get embedded in the aluminium.

I was thinking I can rough it in with needle files (going in at 90 degrees to the "point" then at 90 degrees to the two new points in the same way you'd round a square by going square, octagon, round), then polishing out the file marks with increasing grades of abrasive.

Would using a rotary tool be a stupid idea?

In terms of abrasive, wet and dry? Abrasive stone? Grinding compound? Polish on wire wool?

Obviously it'll get a damned good clean in the ultrasonic, soapy water then solvent afterwards to remove any residues.

*I took some with me in case I couldn't get super unleaded locally.
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[MCN]

Abrasives in the aluminium can be brushed out with wire brush or picked up using heavy steelwool (Or a Brillopad). Anything that left most likely won't be of concern.
Washed off in your mum's dishwasher when she's out at Tesco.

Clash it together, run it for a couple of hundred miles and change the oil and filter. (As per the norm after a deep service.)

A carbide cutter with Turks Head profile would be an option.

Carbide cutters are the goto for removing material during critical welding jobs as there is concern that abrasives left by grinding can cause 'inclusions' in the welded joint leading to stress cracking later.
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Disclaimer: The comments above may be predicted text and not necessarily the opinion of MCN.

[stinkwheel]

*Wonders if work will notice one of the carbide dental burs going missing.*
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Easy-X]

I hate to say this but another approach might be to lower the compression a tad, slip in a thicker gasket for the head?
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Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter

[stinkwheel]

[stinkwheel]

Well. Piston is toast. There are some marks on the bore but having run a hone down there, while still visible, I can't feel any of them with a fingernail, they're no deeper than the honing marks.

Here's the damage...
https://lh3.googleusercontent.com/pw/AM-JKLXLII3xc43v-Gh6k1B8WYPXG6LOj1hTIouUgfd77bzFWMk8puRlOmw2Wwak9d3KwJkS0AeFWxkkvlamtdLaJRIZI1n5BeqxDs8rQbND4oWjHz4-FGkbzYClZButiOrpE11DajC7p5TfbqBLn2m2xG0w=w1576-h886-no

As well as the seizure you'll notice the piston has cracked through the gudgeon pin boss.
https://lh3.googleusercontent.com/pw/AM-JKLWwlFNpy4uAsr4UN2MylKHLxhuu4AnA3jgvt5UJ2QNTaph5lo8syLx76MycHsVrVTyFGheUw48YVZvxPENafPFU2XmVu1BN8PhfoIxJmp24PhnR_F3B9O4plA_f2R0KfDAzjk-I2h88V2uDrzfxDhUy=w1576-h886-no

Interestingly, when you look at the underside, none of the heat from the seizure has transferred across the crack. This suggests to me that it seized because of the crack springing the piston skirt out rather than cracking because it seized. The crack must have been there before the seizure occurred or there would be scorching both sides of the crack?
https://lh3.googleusercontent.com/pw/AM-JKLVmi4gd9KYcL0ABdEbtmjy5CMAXasqpwiqoa4qVR1jZKNEDQL08_zUYkE6lFZ5wzBPN73OZNlogY75wK4vOtIJ4jlqaDMCVIh9tIJdAYE1L-y2vmInJI9droj3XqgVKzVZUxFay6jApKFZY8Z_oFD4V=w1576-h886-no

There are also two cracks in the ring land area between the two compression rings. Not sure what to make of these, cause or effect?
https://lh3.googleusercontent.com/pw/AM-JKLVUiaGhmMni6lQWhSU0BtoGeJw3ch21ogVoMTeiXpiDaQ_f6Afv7op0NYJNIJS15Ybc0pt6DsZkryEcKzgBPZq4-oNAR1wUBJAVLa5x21lKhwEqq-R-DwMGNvtwd6yRu5wqI4bzVYXdRemrwZ-vpvE2=w1576-h886-no

There is no evidence of nibbling/pitting on the piston crown like I'd expect with detonation.

I'm not sure but I wonder if one of the big backfires it was doing before I got the timing dialled in caused a crack to start and it's been slowly propogating until it finally let go in a bigger way.

I'm astounded it kept running for as long as it did. Look how much play there was on the piston! Miraculously, there doesn't seem to be any damage on the small end.
Link to short video
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[MCN]

That's a horse's daubur of a piston crown.
Over-designed a tad.
That probably prevented a punch through the crown.
But like your say, the forces have found the next weakpart and the thin section at the edge of the pin bore was the focus.

Amal carbs are fine if you don't need performance over a load range. They must have been invented before the wheel.
Murican engine tech is better coz their market is better.
And most stuff is cheap.
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Disclaimer: The comments above may be predicted text and not necessarily the opinion of MCN.

[Robby]

Is it running a needle roller or a brass bush in the little end?

If it is running a needle roller, could you run a brass bush? Looks to me like it requires a more brute-force engineering approach than worrying about little things like the right amount of oil.

[stinkwheel]

[Robby]

Then I get the feeling you may have discovered the limit of your cylinder head.

That piston is a solid-looking thing. Unless there was a hidden manufacturing defect, it must have seen quite a bit of abuse to let go like that.

I'm going to go with Easy-X and call you a pussy, dropping the compression seems like the next logical step. Would be interesting to know how it changes the feel of the bike going from silly compression down to just inadvisable.

[stinkwheel]

I'm thinking there may have been a single "event" that's at the root of this. Possibly dating back to however the hell it managed to bend the inlet pushrod a few months back. I'm leaning towards a particularly violent backfire on startup before I had the timing dialled in properly.

The compression isn't proper stupid, it's 10.3:1. The scrambles guys running on dope have them WAY higher than that.

I'm also now wondering if what I thought was detonating was actually the sound of the piston coming apart because the crown and inside of the head look fine, no pitting or hotspots.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[steve the grease]

Can't you knock the timing back a fraction? Were you at the Colombres rally?
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All the above is my personal opinion, you can see my lips move, but I'm talking out of my arse.
I've been riding, and fixing , bikes for 50 years, in that time the more I learn, the less I am absolutely sure of.....

[stinkwheel]

[Robby]

Too many variables. Seems your best bet is to reassemble on a new piston and see how it behaves.

Without knowing when the damage happened, and therefore what the noises may have been, its impossible to tell if you're at the edge of what can be achieved with that cylinder head on normal petrol.

Are you doing any experimentation with inlet manifold lengths?

[stinkwheel]

[MCN]

I'd buy two of those beauties and have one as a paperweight for the desk.
If you still use paper.
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Disclaimer: The comments above may be predicted text and not necessarily the opinion of MCN.

[colink98]

for some reason *Now with added fail* just drew me into this thread.

Sorry to see your piston went pop.
but it is interesting to follow.
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PCX125 (stolen) - CBF600 (current)
Ride it like you stole it.
ride sensible and not like an idiot and you wont get 6 points in one week.

[stinkwheel]

Hmmm. Interesting. The used piston that just arrived has already been mostly radiused, just needs a clean up and light polish. That's saved me a good 3 or 4 hours!

Presumably it's from a development engine because I can't see why they'd have a stock of low mileage, used pistons otherwise. So given these pistons were manufactured for the company I bought it from, radiusing seems to be the way to go.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[bikenut]

Interesting, X marks the spot perhaps....

Those numbers are stamped in ?
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nuts about bikes

[Pete.]

[stinkwheel]

[Pete.]