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Old Jun 16, 2014, 04:13 AM
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DLE-30 piston replacement

I just ordered a spare piston from towehobbies and it looks different than original. One thing that struck me right away was that the wrist pin was very loose inside the piston, the original is still tight. The whole set with needle bearing is also about 2 grams lighter, which can't be good in balance point of view, I have no idea if 2 grams is significant difference though.

Anyone else had similar experience and how does the new piston work, is it usable at all?
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Old Jun 16, 2014, 04:28 AM
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Just my 2 cents:

Return it. That is not an original piston, and the different specs (weight, wristpin fit) raise suspicion about the quality.
A wristpin should fit tight in a piston, since steel and aluminium have different expansion: a loose fit when cold means a very loose fit when running. THat should not happen, the small end bearing should perform the task of allowing movement, not the piston/wristpin fit.

There are of course always non-original aftermarket pistons, but they should at least meet the specs of the original, or surpass them in order to be useful.

Only if it is labeled with an original DLE label and somewhere it is stated that it is an original replacement, I would consider using it, but even then I would think twice before doing so....

Brgds, Bert
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Old Jun 16, 2014, 05:07 AM
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Tarts,


Since any and all single-piston engines are never balanced, I believe the weight difference will not even be perceptible when running.
The crankshaft counterweight only counterbalances the con-rod top-end+the wrist-pin+the retainers+the ring+ ~50% of the weight of the piston.

Making the counterweight heavier, to fully balance the reciprocating mass, results in the engine having stronger side-to-side vibration. Having no counterbalance at all results in strong up-down vibrations.
Manufacturers have found the balancing 'formula' I described in the above paragraph, gives an imbalance most can live with.
Only a reciprocating mass of 0 can truly be fully balanced....


Regarding the fit of the wrist-pin; maybe the manufacturer made it more loosely-fitting to allow more oil-clearance.
After all, this wrist-pin is supposed to be fully-floating; no?
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Old Jun 16, 2014, 07:00 AM
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I haven't replaced mine yet, but if my replacement says DLE on the packing and/or piston, I would have no trouble running it. DLE has a history of rolling changes made from one production run to the next. Non of which has resulted in a poorer running engine. The newer part has always been improved, without a downside.

If it did not say DLE on it somewhere, then I would be asking questions.....
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Old Jun 16, 2014, 09:05 AM
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I would send it back If I could get a better replacement somewhere. Also valleyRC told they source their parts from same place as tower (from DLE). I'll send a mail to tower and find out if it is a defective or all pistons have that loose wrist pn.
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Old Jun 16, 2014, 02:07 PM
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the extra holes above the wrist pin do look like an improvement but any kind of perceptible radial slop in the wrist pin fit is not a good thing at all.....the pin can and will beat the holes to death and possibly dislodge a clip....then you get to see what kind of improvements they've made to their replacement cylinders.
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Old Jun 16, 2014, 02:18 PM
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Brgds, Bert
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Old Jun 16, 2014, 02:41 PM
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Quote:
The extra holes above the wrist pin do look like an improvement but any kind of perceptible radial slop in the wrist pin fit is not a good thing at all.....the pin can and will beat the holes to death and possibly dislodge a clip....then you get to see what kind of improvements they've made to their replacement cylinders.
Dave,


In a perfectly adjusted two-stroke engine, the con-rod is always under compression. In a four-stroke engine it can also be under tension.
So, the slightly larger oil-clearance (I am not expecting a 10 mm wrist-pin to run properly in 11 mm i/d bosses!...) should not escalate... or should I say; will not compound the pounding...

This set from Aussie seems more like the O/P's original item...
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Old Jun 16, 2014, 03:03 PM
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Dave,


In a perfectly adjusted two-stroke engine, the con-rod is always under compression. In a four-stroke engine it can also be under tension.
So, the slightly larger oil-clearance (I am not expecting a 10 mm wrist-pin to run properly in 11 mm i/d bosses!...) should not escalate... or should I say; will not compound the pounding...

This is ABSOLUTELY not true, and one of the bigger misunderstanding in 2-stroke technology....

Every time you close the throttle, not only combustion pressure is absent, but compression pressure virtually dissapears too.
Back in the 60's and 70's, when the cheap Japanese motorcycles were very popular, lots of folks stretched their condrods (literally) by pumping the throttle when waiting for the traffic light, because a two-stroke on the overrun makes you feel you're on the starting grid....

Under load, yes, in most engines it is, but a high unloaded RPM (half closed throttle: low compression and low combustion pressures) is allready sufficient to have a pulling force on the rod, believe it or not....
These pulling forces are the reason why big diesels have a very strickt overspeed-redline and redundant systems to prevent them...

For most people it is very hard to understand, that a piston, pulling on the rod, can still transfer labour to the crank, but it can: the deceleration in the top of the upstroke and in the bottom of the downstroke transfer the labour, and the small pushes and tugs during these decelerations of the piston, are slightly bigger than push during compression and the slight pull in the top of the descent. The net result of all these forses still delivers enough energy to the crank to keep the thing going.
The burning and expanding gasses do their labour on the pistontop just the same like in any other running condition, but the max pressure is just not sufficient to prevent the piston from puling on the rod. The rotating crankshaft keeps the piston movement under control, so it won't hit the head, same like a fourstroke in the scavenging cycle.

It is Obvious true, that a fourstroke can enjoy these pulling forces a at ful strength, and in a 2 stroke they are (if occurring) a lot smaller, but it is absolutely untrue that the conrod of a 2-stroke is always under pushing force, well adjusted or not....

Brgds, Bert
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Old Jun 16, 2014, 03:58 PM
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i've seen way too much evidence to the contrary to believe the "always under compression" theory.... stretched rods, piston crowns impacting the head, bottom half of the big end sheared clean off, ovalized wristpin holes in pistons....even the little ball and socket in a cox engine will destroy itself if not kept tight.

no such thing as a perfectly tuned two stroke engine...
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Old Jun 16, 2014, 07:35 PM
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<<<One thing that struck me right away was that the wrist pin was very loose inside the piston, the original is still tight.>>>

After further pondering, wondering how this (very loose) was established? Were you expecting a press fit? Something that might need some type of tool to get it centered?

Can you see movement when the pin is installed?

Please expand on why you believe it's loose.
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Old Jun 16, 2014, 10:41 PM
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Bert, Dave,


Suppose that 'excessive' play between the new wrist-pin and the new piston is 0.08 mm, instead of the previous value of 0.04 mm... and that the con-rod is not always under compression (i.e. force upon it alternates between compression and tension...

With the flame propagating normally in the engine's combustion chamber, theoretical piston acceleration at TDC (end of compression), between when rod is 'stretched' and when it is compressed (beginning of the power stroke), cannot bring the relative speed between the wrist-pin and the piston-bosses, to a value that would cause the bores in the aluminium piston to become pounded out!

If there is anything that will cause these holes to grow, it is borderline lubrication (and thus wear), which is much more probable if the oil-clearance there is too small!
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Old Jun 17, 2014, 12:17 AM
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Originally Posted by DarZeelon View Post


If there is anything that will cause these holes to grow, it is borderline lubrication (and thus wear), which is much more probable if the oil-clearance there is too small!
no lube or clearance for lube is needed because the pin is not supposed to rotate in the piston at all, if it does then the rod bearing is dead and needs replaced before it does something nasty like spit the cage or needles out and kill the whole engine...unfortunately you usually can't tell right when it goes bad so it and the pin are always changed along with the piston and rings whenever rebuilding the top end.
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Old Jun 17, 2014, 01:18 AM
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No lube or clearance for lube is needed because the pin is not supposed to rotate in the piston at all, if it does then the rod bearing is dead and needs replaced before it does something nasty like spit the cage or needles out and kill the whole engine...unfortunately you usually can't tell right when it goes bad so it and the pin are always changed along with the piston and rings whenever rebuilding the top end.
Dave and Bert,


I agree that in engines that have a needle bearing con-rod top-end, it is only the rod that should rotate on the wrist-pin; not the wrist-pin in the piston.

My explanation was in regards to normal glow engines with fully floating wrist-pins; and not to gas engines like this one.

In the Chevy Small-Block V8, for example, the the wrist-pin in the con-rod top-end is pressed into a high-force interference-fit. The only rotation is between the pin and the piston - but this is clearly not the case here.


So, I concur that excessive clearance between the wrist-pin and the piston is bad and that these parts must not be used and be returned to the vendor ASAP.
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Old Jun 17, 2014, 03:12 AM
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Originally Posted by DarZeelon View Post
Bert, Dave,


Suppose that 'excessive' play between the new wrist-pin and the new piston is 0.08 mm, instead of the previous value of 0.04 mm... and that the con-rod is not always under compression (i.e. force upon it alternates between compression and tension...

With the flame propagating normally in the engine's combustion chamber, theoretical piston acceleration at TDC (end of compression), between when rod is 'stretched' and when it is compressed (beginning of the power stroke), cannot bring the relative speed between the wrist-pin and the piston-bosses, to a value that would cause the bores in the aluminium piston to become pounded out!

If there is anything that will cause these holes to grow, it is borderline lubrication (and thus wear), which is much more probable if the oil-clearance there is too small!
It can and it will... you are underestimating what can and cannot do, and you are also assuming that compression of the rod allready happens at the start of the powerstroke (TDC) but it does not. In extreme cases like severe overrun, compression on the rod can start as late as 90 degrees after TDC, since the pressure in the combustion chamber drops way more fast during the power stroke than the acceleration of the piston drops....
Typically, at 30 degrees after TDC, pressure is allready halved (combustion chamber has doubled its volume), and the downwards acceleration of the piston is only halved later than 60 degrees after TDS. (Cosinus 60 is 0.5, and that is without correction for the sideways displacement of the conrod, which adds to the acceleration)
Don't forget that acceleration of the piston does not follow sinusoid graph. The accelerations and decellerations around top are way more violent than those around bottom, due to the sideways movement of the conrod.

Every time a Cox runs out of fuel, the rod gets a bad kick in the ball(s)....

I manually reamed the small end bearings of my Ducati 750 (which has much bigger pistons, but also pressurized oil feed) and that bushing must be brought to an installation clearance of 0.02 mm and should be rejected at 0.04 in the small end.
The 0.02 fit feels tighter than what you would feel when for example checking valve clearances with a feeler gauge, and the rejection clearance of 0.04 is when the wrist pin is just sliding out of the rod under its own weight when the rod is held with the pin vertical.
And that is the rotating point, not the pin/bosses fit....

A piston of the size of that DLE, of which the wrist pin feels loose in the pistonbosses, is definitely a no-because it has at least 0.05 clearance, which is way too much for any engine of that size.

Brgds, Bert
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