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Old Jun 17, 2014, 04:37 AM
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ZAGNUT's Avatar
Tel Aviv, Israel
Joined Jul 2004
2,584 Posts
Dar, even in glow engines with bushed rods it's best that all movement be between the pin and rod but compromises are made to simplify manufacture.....by giving the piston bosses more combined surface area and a tighter fit than at the rod the movement is usually limited to the rod bushing. higher heat at the piston bosses resulting in varnish also helps. sometimes though the strategy fails and you get massively worn out piston bosses, some of the K&B .40 engines had this problem and wouldn't even survive break in if enough castor wasn't used in the fuel. some manufacturers like YS put in a bit more effort and made the pin a light press fit in the piston.
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Old Jun 17, 2014, 06:37 AM
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DarZeelon's Avatar
Israel, Ramat HaSharon
Joined Nov 2003
1,213 Posts
Let us calculate, Bert and Dave.

If we suppose the piston is not simply at top-stroke, but also extended-out...
The 'loose' wrist-pin is at the bottom of the bores in the piston, .08 mm below the top of the bores.
We'll also assume the following:
full combustion pressure of 80 Bar is applied to the piston crown immediately and that the piston weighs 50 grams with the ring...

The force applied to the 36 mm diameter piston is 814 kgf, causing it to accelerate at 16,300 g, or nearly 160,000 m/s squared; again over a distance of .08 mm, which are 0.00008 meters.
The calculated speed at which the piston will hit the wrist-pin is just 5 m/s.

This is hardly a blow that can wallow out the wrist-pin holes in the piston.

And many things that will reduce that speed, like the flexibility of the piston and combustion pressure that is being built - not all coming at once. ...And it is 'softened' by the oil-film present between the parts.
So, it is slower than that, even.
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Last edited by DarZeelon; Jun 17, 2014 at 07:53 AM. Reason: Accuracy...
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Old Jun 17, 2014, 09:10 AM
Complete RC Idiot Savant
The Netherlands
Joined Nov 2009
3,027 Posts
First: Where are you getting the 80 bar from? Most automotive diesels do not even reach that kind of combustion pressure, let alone a teeny tiny DLE 30, 2 stroke, gasser, no turbo....

It will most likely have a compression end pressure of about 12 to 15 bar, and top out at 40, maybe 45 bar. and then you have a really good one.

Now maybe that piston will hit at 5 m/s... than lets see how much that really is:
18 km/h, which is a fair running speed
Try hitting your finger with a 50 grammes hammer at 5 m/s and youll notice, that this is not something to be ignored. Try hitting that piston with that hammer, and you will most definitely see a dent, and I can guarantee you, that the actual force of impact the hammer excerts, is lots less than that of the combustion pressure in itself, let alone the combustion pressure + the impact of the piston on the pin.

Even if I assume you are completely wrong about the pressure, and halve it, and even if I assume that the projected load bearing surface of the wrist pin in the bosses is 2 sq cm (it is likely much smaller), that comes down to a pressure of over 200 kg/cm2
Imagine that happening over 150 times per second. A hardened steel wristpin hammering at plain aluminium with 5 m/s and that driving force behind it....

You really think this is nothing to worry about?
Metal fatigue happens due to repetition of forces that the metal would normally withstand with ease, any constructor knows that.
The piston will undoubtedly withstand one of those blows on its bosses, maybe even ten thousand... but that is only one minute of running....

Trust me, any play in that area is lethal, even if you don't want to believe it

Brgds, Bert
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