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Old Jun 17, 2014, 03:37 AM
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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, 05:37 AM
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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 06:53 AM. Reason: Accuracy...
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Old Jun 17, 2014, 08:10 AM
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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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Old Oct 31, 2015, 08:35 AM
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Arjan
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Hi,
After the wrist pin bearing in my DLE30 failed I decided to replace the piston at the same time. I received the piston with the two holes above the wrist pin but I noticed this piston to be slightly higher. After fitting it turned out the crown of the new piston is hitting the top of the combustion chamber. In other words, these two pistons are really different and can not be interchanged. Anyone else noticed this?
I wonder if it needs a different cilinder or a different crankshaft.
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Old Oct 31, 2015, 09:01 AM
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It looks to me like you've received the wrong piston. Not too concerned about the different design, but the height of the piston above the wrist pin appears to be way different. They have to be the same!
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Old Oct 31, 2015, 09:16 AM
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Arjan
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hi ahicks,
You are absolutely right about the piston being the wrong one - but it is an original DLE30 piston, and if you look at the opening post of this thread I am not the first one to get one. That is what puzzles me. Also, if you look at the DLE engines website they have a picture of the piston with the holes, not the one that is in my engine. Yet if you look for replacement parts at dealers you will always find pictures of the piston without the holes.
That one is pictured in the manual too.
I wonder if there are different cranks and cilinders as well.
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Old Oct 31, 2015, 09:32 AM
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Perhaps there are different generation designations we don't know about with the DLE engines. Those two pistons are obviously very different and the new piston is obviously not correct. I'd be contacting the vendor you got the replacement piston from. I'd ensure you're ordering from a reputable dealer and not a place selling knock off DLE parts. (How ironic is that? There is a clone of a DLE which is a clone of something else..)
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Old Oct 31, 2015, 09:37 AM
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Arjan
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Thanks for the reply.
The got the piston from an official dealer, I am sure things will be sorted but I was wondering if anyone else has noticed this. Surely I can not be the first (or only) one....?
I contacted DLE Engines too, I'll let you know what they have to say about this.
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Old Nov 03, 2015, 02:56 PM
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Arjan
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okay, problem solved. It turned out there are two generations DLE30; mine is of the first generation.
The piston DOES fit alright, there was a very small ring of carbon built up in my cilinder and the new piston is ever so slightly higher. After removing the carbon all is well.

Answer from Hobbico:
Dear Valued Customer:

Thank you for your Email.

Your original piston was for the original DLE-30 cc. engine, The piston that is
currently available is for the one with the rear intake, which has
completely-different internal components. The parts for your engine are no
longer available, and have not been available for over two years. The only way
you may be able to locate parts would be to locate a engine you could get parts
from, or to find some old unused parts on the internet. Unfortunately, we'd now
know where you might be able to locate them.


I will run it and I expect it to be okay. If it's not I will have to replace the engine anyway.
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