[NSRCA-discussion] 120 AC issues UPDATE

Tue Jul 15 05:42:41 AKDT 2008

Stu,
   You have to measure the piston with an inside micrometer and the sleeve with a bore gauge. Ideally the sleeve should be installed in the case and if you're really anal, something called a "torque plate" should be bolted to the deck. The torque plate is nothing more than a thick piece of aluminum or cast iron with a hole slightly larger than the bore and a thickness sufficient to handle the head bolt torque. It simulates the head being installed. This is important. The cylinder will change shape as the head bolts are tightened. You must measure the cylinder / sleeve at several points: top, middle and bottom. At each point you should measure both ways: in line with the thrust and in line with the crank axis. The piston should be measured at it's widest point. This is usually close to the bottom (skirt area) in the the thrust axis. I'm pretty sure YS pistons aren't cam ground but measure it in a few places just to make sure it's not horribly distorted.

  Take everything to a good machine shop - they'll have the tools to do this. This stuff is small so an automotive machine shop probably won't have a bore gauge small enough to measure the sleeve. You might get lucky though. A motorcycle shop might be a better choice.

  You can't stop now - we all want to know what's going on! :)

  BTW - If you really want to stop fiddling with this stuff, you could buy your son a nice, new OS 120Ax. <LOL> 

  John Pavlick

Stuart Chale <schale at optonline.net> wrote:
  Interesting email.
I guess in the betterment of science I should do one more test with a different piston/ring in the offending engine before swapping it out totally for another motor.
I am getting tired of pulling apart motors though.
What I really want to know is how to measure those gaps :)
I tried but the best I good get was somewhere between 0 and 1 mm with my rulers :)

Stuart

Dave Harmon wrote:                   Stuart
..here is an old email from Mike McCormick that is interesting.

  >>>>Because the YS piston compresses in both the up and down stroke, the ring is 
  serving double duty.  It must not only seal the mixture being compressed, but 
  it must also separate the two compressed mixtures.  Once the piston begins to 
  rock and the ring loses its seal, then surge begins.  If you think about it, 
  the YS engines with the highest compression are the most problematic (SC and 
  the L; the L is really just an SC version of the FZ).  Why do the pistons 
  rock?  No side wall.  Plus (as Dick stated) the pistons are made of soft 2017 
  aluminum and will wear quickly (especially if the engine is run lean or fuel 
  with low oil content is used).  This piston design has been in use since the 
  AC and always been a pain.  But in the 1.2's we could alway install an FS 
  Piston and cure the ill's.  FYI, I once did a test on an AC just to see how 
  much more boost the AC piston created vs the FS piston; it was less than 1/4 
  psi!  I also noticed that any engine in which I installed a FS piston ran 
  better than it did with the compound type piston.  I figured this occured 
  because the FS piston ran "truer" in the bore; it created less drag because 
  it was less prone to rock or cock in the bore (more side wall).  But along 
  comes the 1.4 and we no longer have the FS piston to fall back on,  Plus the 
  1.4 piston was shorter and bigger in diameter,i.e, more rocking force 
  (around) the wrist pin is created and there is less length to stabilize the 
  force.  Things didn't get too bad until YS increased the compression of the 
  engine (L version) and then the piston really started to get beat up.  
  Because of the higher compression, the ring seal is even more important and 
  the piston/ring fit has to be right for the engine to perform optimally.  And 
  the optimal fit is within a narrow range.  I set the piston/sleeve clearance 
  at .003 to .0035".  Max clearance is .005"   .0035" is tight, and engines I 
  set up must be run rich for a long time to properly break in.  I also set the 
  ring end gap as tight as I can get it, usually at .0055", though I like it 
  best if I can get it at .003".  But when set up at these clearances and run 
  rich, they will last 500 plus flights.
  So what is boils down to is the old piston is soft and has very little side 
  wall. The forces on the piston are high and attack it from both ends.  This 
  creates high side forces on the piston.  The minimum side wall works hard to 
  keep the piston straight, but even in the best of conditions (running the 
  engine rich and using lots of oil) it is a losing proposition for the piston. 
   The new piston has 4 times as much side wall area under the ring groove 
  (where the old style piston experienced the greatest forces and therefore the 
  most wear).  It runs very smoothly and I believe this is because it too tends 
  to run very true in the bore.  I am sure it will last longer because of the 
  increased side wall area.  
  Sorry for the length of this reply, and I hope it answers your question.  But 
  heck while I am at it I might quickly address one other issue I see a lot of 
  problems with.  Many flyers tend to set the mixtures too lean on 1.4's.  
  Because of the different plumbing in these engines, the regulators do not 
  respond as quickly as did those on 1.2's.  To set these, you must do it 
  slowly and wait a few seconds for the regulator to equalize with the change.  
  If you move the needle quickly, and not wait it is very possible to get a 
  false reading on the mixture and usually it ends up too lean.  Kills the 
  engine quickly.  

  Mike McCormick<<<<<

    Regards

Dave Harmon
NSRCA 586
K6XYZ[at]sbcglobal[dot]net
Sperry, Ok. 

  -----Original Message-----
From: nsrca-discussion-bounces at lists.nsrca.org [mailto:nsrca-discussion-bounces at lists.nsrca.org] On Behalf Of Stuart Chale
Sent: Monday, July 14, 2008 10:07 PM
To: adriancwong at earthlink.net; General pattern discussion
Subject: Re: [NSRCA-discussion] 120 AC issues UPDATE

  I have been mostly using Cool Power 15%, but tried 30% heli today with the same results.  No special YS blends available locally.
This plane is for my son to fly.  Sure makes my electric Beryll seem real easy :)
Stuart

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