Piston reset.

Here is an image of my used .049 anvil from DDD. There is only a load/contact in the very center, i.e. right below the conrod connection.

(the tiny depression in the very center is from a machining point of view, as it is very hard to make completely flat center on a lathe)

On another note, does one really need to reset the pistons frequently?

I never did it when I was a kid and at modest/regular revs I cannot imagine the piston and conrod being pulled apart. Surely as long as the engine is running and firing on every stroke, there is always a positive load on the conrod?

Setting the ball joint really tight could in principle hinder the oil from getting into the joint. Regular conrods on large engines are the same, you don't want them to be too tight, as you then risk getting problems with binding.

There is actually a tolerance of .002 or something like that of play in the rod to socket fit. I suppose if you really tightened it up hard you would have some issues. Read the Gibeault race program notes or head over to the .049 Yahoo collectors group as there is some really good reading on how to set the play.

Mostly a sloppy fit will effect compression/overall performance.

If you already knew that, sorry for the repeat info.

On sport props I haven't see any difference in performance, that's why I raised the issue.
I also cannot logically understand how it would affect the compression ratio, with the conrod under constant compression...

Slop means the piston travels higher up the bore hence more compression as the piston's upward travel is arrested later.

That sure would take some high revs to have the piston pull on the conrod at TDC and increase the compression, 22000rpm or more?

My point is that unless you are in Venom rpm territory, I can't see how the slop would have any influence on the performance.

I've found that it affects how easily the engine starts most of all. I can also see how too much slop could cause the joint to eventually fail. I would expect this to happen sooner as the engine is revved higher. How ever, this may take longer in terms of minutes of run time but it would be interesting to see how many revolutions it takes to fail and if the higher the RPM the lower the revolutions (tach vs hobbs time in full size).

When I got home earlier the Missus said "Why are you so late and where's the car?"
I said "Piston broke."
Said said "I know you are but what happened"

..............................

haha, nice!

Thanks for the replies regarding my mention of my resetting troubles. I already closed up the shop tonight, but I'll get my tool out tomorrow, get a picture and we'll take a look. Resetting is new to me in the past two of my 45 years of flying. I can't recall any problems in the past, that I look back and think must've been caused by a loose piston.

RknRusty wrote:Thanks for the replies regarding my mention of my resetting troubles. I already closed up the shop tonight, but I'll get my tool out tomorrow, get a picture and we'll take a look. Resetting is new to me in the past two of my 45 years of flying. I can't recall any problems in the past, that I look back and think must've been caused by a loose piston.

Yeah, I hear ya! play in the ball socket never even crossed my mind until I joined this forum. After reading posts here and literature I started looking at my engines and noticed this "play" so being the OCD guy that I am with IC engines I bought a tool.

I haven't noticed a difference yet although it's hard to hear what a few hundred rpm's sounds like! So I guess I see a tach in my future.

Me too, I never thought of a tach until I started hanging around this bunch. Funny, my planes always flew great before too. I must have one. Soon!

John Goddard wrote:Slop means the piston travels higher up the bore hence more compression as the piston's upward travel is arrested later.

John, a year or so ago a fellow on another forum published the results of a series of tests that he had run. His results would indicate just the opposite, i.e. the increase in wear would decrease compression.
He was investigating the effect of various lube mixtures (synthetic/castor) on the rod/piston ball joint wear. His data showed that the increase in ‘slop’ was due entirely to wear on the top side and NOT on the cupped (lower) side. In his worse case the rod actually wore itself all the way thru the piston and broke out thru the top. But there was NO measurable wear on the bottom side. This would suggest that (at least under his test conditions) the piston always has a downward force against the rod and there is probably never any contact between the rod and the cupped (lower) side. Thus the wear would actually lower the piston and decrease compression. It would also imply that the ‘slop’ (up to a point) probably has little or no effect on performance. His tests were conducted under simulated ‘sport’ conditions and as I recall he acknowledged that the results may not apply for high nitro/high RPM conditions.
Al

Big Al wrote:

John Goddard wrote:Slop means the piston travels higher up the bore hence more compression as the piston's upward travel is arrested later.

John, a year or so ago a fellow on another forum published the results of a series of tests that he had run. His results would indicate just the opposite, i.e. the increase in wear would decrease compression.
He was investigating the effect of various lube mixtures (synthetic/castor) on the rod/piston ball joint wear. His data showed that the increase in ‘slop’ was due entirely to wear on the top side and NOT on the cupped (lower) side. In his worse case the rod actually wore itself all the way thru the piston and broke out thru the top. But there was NO measurable wear on the bottom side. This would suggest that (at least under his test conditions) the piston always has a downward force against the rod and there is probably never any contact between the rod and the cupped (lower) side. Thus the wear would actually lower the piston and decrease compression. It would also imply that the ‘slop’ (up to a point) probably has little or no effect on performance. His tests were conducted under simulated ‘sport’ conditions and as I recall he acknowledged that the results may not apply for high nitro/high RPM conditions.
Al

Now that you mention it when some of my big offenders had a flat spot on the top of the rod that I could physically feel after the reset. I read in the performace doc over in 049 CG that you should take rubbing compound and pack it into the socket to smooth out the top of the ball and then flush it out. I was wary of that just in case I couldn't get it all out.

cribbs74 wrote:
Now that you mention it when some of my big offenders had a flat spot on the top of the rod that I could physically feel after the reset. I read in the performace doc over in 049 CG that you should take rubbing compound and pack it into the socket to smooth out the top of the ball and then flush it out. I was wary of that just in case I couldn't get it all out.

The piston is softer than the rod, so I bet what you did was take more metal off the under-face of the piston. Maybe better left alone???