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Hi gentlemen,
All of you are correct in your statements about the occurrence of piston slap.
The video is showing to me some “excessive clearance” at the top of the piston and tells me that the piston had “gone round.”
It’s an old term used back in my days to describe the skirt dimension.
For me there has to be some for the reason for this, not stated in the video.
You will notice that the slop looks even around the top but he can only shove it in that direction because of the wrist pin axis. He has some play at the bottom too in order to do that, but there will more play at the top normally.
He is probably shifting the connecting rod as well as it pivots along that axis.
The rings are moving on their lands of the grooves.
The piston is cold and therefore shrunk. Piston are really shorter in today’s engines.
I have yet to rebuild a red block as engine rebuilding has not been my claim to any fame but I know of principles.
The pistons themselves are like a bearing holding the connecting rod and it is supported by an oil film within the cylinders by the skirts and those sealing rings themselves.
The piston is never supposed to run on the cylinder walls.
Clearances are setup to allow for an oil film and also some for carbon build up.
Carbon builds up around the top of the piston over the compression rings and is helpful to seal and support the piston, up to a point.
The pistons on todays cars do not have that feature so much, due to the fuels and the fuel management systems of today.
The idea of shorter piston skirts was to reduce mass and to get an engine to rev up faster. Friction difference is minimal when compared to other factors.
This compromise comes with some risk or costs in reliability.
In the long run it was done just to achieve competitive performances in several areas of manufacturing and sales.
The application of turbos and superchargers started the fascination and thinking to change lots of things.
Pistons are fitted to the cylinder by measuring over the skirt at the bottom of the piston.
This is done 90 degrees to the wrist pin because the shirt is machined in an elliptical or oval fashion.
The wrist pin has a heavier mass and the most cooling takes under the piston head and into the connecting rod itself.
The top of the piston gets hotter first and stays hotter than the rest of the piston for an oblivious reason.
One being it’s gets hit first with the heat from the burning fuel.
The cooling jacket around the cylinders runs hotter at the top as the heat dissipates into the water there during its dwell period.
Same goes at the bottom of the stroke but heat rises causing the water to be less dense at the top of the water jackets.
The heat it will stay higher up there at the top zone because it’s out of the most circulation.
Even though you think it’s circulating the heated water exhibits its boiling tendency all the time.
It’s one of the reasons that coolant recovery systems were invented to help keep air bubbles away from the cylinders walls.
A lot of experimenting goes on in R&D and with hopes that it gets into production in a timely manner.
This whole heating question comes into play.
Air cooled engines versus water cooled engines run completely different clearances. Air cooled engine will not run up into the mileages as high at a water cooled engine for that simple reason. They start out more loose.
In my youngsters days, I was instructed to start my motorcycles and run them at idle until I could feel heat coming out of the fins. Just a healthy amount of warmth, not hot hot before driving.
This meant the piston was expanded and oil was conducting heat effectively for service.
But there is a kicker in this here story.
The pistons are working with the same principles and still have to expand and that takes a “wee bit” of time.
Ford use to say, in the seventies, wait 30 seconds before driving and slightly longer in colder weather.
This is not even going to get mentioned for todays use for a whole host of reasons.
Engine design has changed in 40+ years and of course, those emission requirements push in the real big one. Idling or driving slowly at first would hurt the manufacturers EPA ratings.
The use of thinner oil has some up side here. This allows for some Small clearance changes.
Machining got more precise to do that and the oil is faster to pump and less restrictive to crankshaft motion.
The oil cheaper to distribute and manufacture because there is less of it or mass.
We are not supposed to think about that, especially, if you know what a slide rule is! I’m not just old but I can still calculate within the realities.
Yep, I see it all the time. People in a hurry-hurry and a week after getting a new vehicle, they jump into it thump the throttle and blast away, cold! For whatever mind numbing reason.
Later they scratch their heads or their wallet and barely know to ask, “Why did the vehicle start knocking?”
Also to say, they never ask, “What they did with those extra seconds in their day?”
Oh I know, they used them to cutoff someone to an exit that they almost missed or they just needed more time?
Go figure.
Forgetfulness runs deep with those short attention spans and instantaneous gratification of today.
Electric cars can make a lot of this slapping go away in our vehicle propulsion systems, but not to our faces! ((((:-))))
I have some weak sounding of noise in a few of my more recently bought cars. All nineties!
All six were second hand and you can tell which were treated better.
My oldest ones have none and with 30% more miles.
The newer ones are smoother in some comparisons of owner performance.
Overall, there are some givens with them, apparently?
Phil
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