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 Post subject: Look at these pistons ............
PostPosted: Wed Dec 27, 2017 1:07 am 
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Joined: Thu Oct 14, 2010 11:23 am
Posts: 1180
..........and, what are your thoughts? I have mine, but would like to hear others. 88mm set of Pistals used one year on the track. These are not mine. MikeV

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 Post subject: Re: Look at these pistons ............
PostPosted: Wed Dec 27, 2017 3:14 pm 
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Joined: Thu Oct 14, 2010 7:30 am
Posts: 140
If the skirts are not torn up and measure out ok they should be usable. Inspect the pin and pin bores for wear/galling and see if the ring lands are beat up or worn. I don't have the specs but they shouldn't be hard to come up with. I've used Gio ca Motos for a few seasons without a failure, sure a little loose but whats a little oil use?


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 Post subject: Re: Look at these pistons ............
PostPosted: Wed Dec 27, 2017 5:14 pm 
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Joined: Tue Mar 18, 2014 10:03 am
Posts: 188
Hey Mike...
If it wasn't for the one valve pocket being clean, I would guess a very poorly executed squish band. Improperly torqued heads, bad ignition or cam timing? Something was retarding full flame travel.


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 Post subject: Re: Look at these pistons ............
PostPosted: Wed Dec 27, 2017 9:15 pm 
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To me it is a good example of why hemi head combustion chambers with high compression pistons really need a plug in the center, or one on both sides of the bump. Seems the flame can just make it over the hill from the plug side, but not quite to the plain below on the other side. Makes me wonder how the piston feels getting hit from one side all the time - ouch.

I remember a “debate” with another Guzzi owner after I had dual-plugged my big-bore Lemans. I was then able to retard the ignition several degrees, it felt like it would wheelie just leaving the garage, and the MPG went to around 52 from about 48. Oh, and I had installed even bigger valves at the same time. Yet, he insisted, others had proven dual plugs added nothing.

Plenty of old race Ducatis had them, and even many modern two-valve Ducatis now have them. I wonder why we don’t do it more when we build engines.

MikeV


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 Post subject: Re: Look at these pistons ............
PostPosted: Wed Dec 27, 2017 10:42 pm 
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Joined: Tue Mar 18, 2014 10:03 am
Posts: 188
Hi Mike...
I've read about the shadow effect high domed pistons can have on flame travel, but didn't realize how severe the results can be. The clean areas of the tops look like they just came out of the box. You would think there would be at least a little carbon build up across the entire crown.
Regards...
Flattop


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 Post subject: Re: Look at these pistons ............
PostPosted: Thu Dec 28, 2017 4:17 am 
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Joined: Sat Feb 25, 2017 11:51 am
Posts: 191
How about too fat on the jetting. In 2 strokes we called those clean areas piston wash. Around the intake/ports you would see this, the coolest parts of the piston.


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 Post subject: Re: Look at these pistons ............
PostPosted: Thu Dec 28, 2017 12:43 pm 
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Joined: Thu Oct 14, 2010 11:23 am
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Another possibility Cal. But why aren’t they coated with carbon? Like many engines i have disassembled. Maybe (probably) they were and got cleaned up. Would be interesting to see the combustion chambers. Still, you can see how far the flame burned with enthusiasm. MikeV

PS: These are now listed on ebay with a few days left. The walls are also pretty well scuffed. Maybe they were drowned in fuel.


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 Post subject: Look familiar?
PostPosted: Tue Jan 09, 2018 11:09 am 
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These are pistons from different bikes (both 620cc 80mm ) for sale on ebay. The first three pictures show the incomplete flame travel and resulting damage. The last picture shows normal combustion, if not a little rich. It would be good to know what causes this and how to avoid it.
Regards....
Flattop


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 Post subject: Re: Look at these pistons ............
PostPosted: Tue Jan 09, 2018 9:39 pm 
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Joined: Thu Oct 14, 2010 11:23 am
Posts: 1180
I am often surprised at how many of these show up on ebay — pistons out of modern computer-controlled fuel-injected engines that are obviously too rich. I thought the modern parts and electronics had this under control.

Not sure this is the same problem as the photos I posted, but still interesting.

“Lean is mean”.

MikeV


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