Relieving Side of Piston
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Hooked
2007 Stella 150
Joined: 02 Nov 2019
Posts: 218
Location: Rochester, Minnesota
Mon Feb 10, 2020 5:16 pm quote
Can someone point me towards some reading or tell me why I see people relieving or cutting away the area wrong a piston wrist pin? I see it on some o-tuning videos.

Ossessionato
LXV 150 3v ie. Midnight Blue (Sold)
Joined: 06 Dec 2010
Posts: 3131
Location: Bangkok
Mon Feb 10, 2020 6:17 pm quote
Will you add the Yuotube link for us who use Chrome?
Hooked
2007 Stella 150
Joined: 02 Nov 2019
Posts: 218
Location: Rochester, Minnesota
Mon Feb 10, 2020 6:43 pm quote
https://www.google.com/search?sxsrf=ACYBGNTzwJMrX7gm35WnhDo8mqqIisso9Q%3A1581388923338&q=o+tuning+vespa+polini+177+piston&oq=o+tuning+vespa+polini+177+piston&aqs=heirloom-srp..

I can't get the YouTube link to work in Chrome, funny part is I have Chrome on my tablet also. I also have the YouTube app and that is the link it creates.
Addicted
Joined: 26 Jan 2019
Posts: 871
Location: california
Mon Feb 10, 2020 9:41 pm quote
Christopher - here are the basics.
When the piston goes in the down direction - it compresses the fuel oil mixture in the case. The transfer ports are like tubes connected motor case to the cylinder above the piston - so as the piston comes down it acts as a pump - gas/ fuel is pumped through these channels and to the area above the piston in to the cylinder. This is how the cylinder is "charged" with gas fuel.

If you want to see it in action when you have your motor split - its kinda cool to put the cylinder on half the case and rotate the crank - watch as the piston comes down. You will see that those holes above the piston pin and relief on the side of the piston become pathways for the gas fuel to enter the transfer Chanels.

Remember - the area below the piston is compressed by the piston lowering - so the gas is under pressure. It uses the holes in the piston above the pin and those received channels to escape the crank case and go in to the transfer ports.

The cut out at the bottom of the piston below the piston pin is there to do the same. But as the piston skjirt comes down, it starts to close off the transfer channels - so having the relief on the side of the piston allows gas fuel to continue to flow in to the channels even after the piston skirt is closing things off.

Modern pistons are designed with this "relief" built in.
Hooked
2007 Stella 150
Joined: 02 Nov 2019
Posts: 218
Location: Rochester, Minnesota
Tue Feb 11, 2020 4:59 pm quote
Would it be fair to say relieving the sides basically enlarges the transfer ports? Looking at my DR177 it never closes off the transfer ports do to the size of the hole in the skirts. Wondering if it is worthwhile to do this to a DR177? Makes me curious when I see new pistons, such as the new DR177 which arenít relieved and the VMC that is. Also see the BGM piston is relieved.
Addicted
Joined: 26 Jan 2019
Posts: 871
Location: california
Tue Feb 11, 2020 6:19 pm quote
Yes - it would.
In my view - the BGM is one of the better designed pistons.
Really optimized for flow to transfer ports with deep indentation.
There are limits to what you can do with a stock piston when carving in to the side due to thickness of material.
Others can comment on what has worked for them on the DR177 - I don't know it very well.
I like the holes above and to the side of the pin - keeping a horizontal full thickness connection between the piston pin and the piston skirt. If you watch more of FMP - you can see a lot of different carvings he's done.
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