OS 55hzr shimming

[bmclaurin]

I'm using CP 30% with a 55 HZR. I've always used the standard head gasket that comes stock with the engine. Engine runs well with decent power, although it has always been a bit on the rich side for a given needle setting. In order to optimize the mixture, I usually need to go farther in on the needles than most others seem to need. I'm just curious if adding an extra shim might help equalize things a bit.

I'm still a nitro novice, but I thought I read somewhere that the engine comes more or less configured for 20% nitro, and that 30% could potentially benefit from an extra head shim. I'm not sure I completely understand the concept, but I gather that, with the extra nitro content, the combustion chamber doesn't need quite as much compression, so the additional shim gives some extra "breathing room," so to speak. I've also read that adding an extra shim usually necessitates richening your mixture. Again, I'm not sure I totally understand the concept, but I suppose that's because there is slightly more displacement and, therefore, slightly less fuel (relatively speaking) in the combustion chamber, thus requiring an slight tweak on the needles to compensate. I might very well have it backward, but this is what got me thinking that an extra shim might possibly help the relatively rich tendencies of my engine.

Anyway, "fixing" this isn't a big deal for me (as the engine runs pretty well as is), so this is more of an educational question at this point. Can anyone out there enlighten me on this topic?

Barry

[Helinut]

BM,

I am certainly not an expert, but your on the right path.  Typically the higher the nitro (10%, 15%, 30%) content the thicker the shim.  Thicker shims--lower the compression and retard the timing (supporting for the higher nitro).  Other variables certainly play a role like the environment/weather/elevation, and your flying style.  My past experience has shown that the HZR regulated motor requires smaller adjustments then the non-regulated version 1-2 clicks vice 2-3.  If the motor is running well maybe try changing the glow plug temp instead of re-shimming.  What plug are you using?  As a plug change can make a difference too!   A rule of thumb I've used--warmer weather=cold plug and vice-versa--cold weather=hot plug.  Again, I'm not an expert just trying to pass-on my two-cents.  I've not had to re-shim lately as my HZ with an OS-8(medium heat) plug/stock shim have performed well here in Florida.  If you try a different plug though, let me know how it goes.

http://www.osengines.com/glowplugs/index.html

[Jon Mills]

I recommend trying a shim (extra head gasket) more nitro produces a fire at lower compression.  If there is too much compression you will start to predetonate, when the piston is still on the up stroke the mixture fires putting extreme pressure on all the rotating part and robbing power.  With a shim the mixture is not compressed quite as much so it only fires on the down stroke. Mixture will ignite when compressed enough no matter what diesel engines are a great example. If your motor is hard to start and the idle is higher than you expect try the shim.  I shimmed my 55 and the difference is night and day.  I am at 5000' asl.

[bmclaurin]

Thanks, guys. I went ahead and pulled the engine tonight and added a shim. I'm using an OS #8 plug, and I'm located in fabulous Las Vegas--about 2,200 feet ASL, very low humidity, can get very hot in the summer but temperatures are pretty mild this time of year (December). Hoping to get out to the field tomorrow to test it out. I'll follow-up with an update when I bet back.

[Jon Mills]

Great Looking forward to the update.

[bmclaurin]

One other quick [semi-unrelated] question: what collective pitch are you guys running? I previously had mine set at +/- 13, but I had to dial it back to about +/- 11.5 due to bogging. I certainly acknowledge it could be mostly due to my thumbs, but even though I don't have the smoothest collective in the world, I'm not the worst either. At 13, the 55 had trouble in full climb outs. At 11.5, it's definitely better, but it's not like the machine is just jumping right up either. I'm hoping the extra shim is going to be the difference. Again, I'll post an update when I get back from the field. Weather looks great out there again today here in fabulous Las Vegas.

[Helinut]

I fly 13 collective pos/neg--and about 8 on cyclic. OS-55HZ Hatori/OS#8

[bmclaurin]

Good grief!!!!! It's a new heli! I absolutely cannot believe how much difference a shim can make. It's night and day.

However, it didn't look too promising at first. I had the hardest time getting it cranked, and once I did, it was just sloppy rich (spitting gobs of fuel out the exhaust), and I could not get a steady, reliable idle. Once I got the idle somewhat stabilized, I decided to put her up in the air. It flamed out on me on a very mild rainbow. Fortunately, I had just enough head speed to touch down without incident. Then, it finally occurred to me that maybe the "extra" shim was simply too thick. I popped off the head cover and removed both gaskets (the stock one and the extra that I installed the night before). And sure enough, the new gasket, based on visual inspection, appeared roughly twice the thickness of the original. So, I tossed the original aside and reinstalled the new, thicker shim (this time by itself). I knew immediately upon cranking it that I had finally found paydirt.

This thing pulls like a beast now. I spent the next few flights dialing in the needles. And it is just amazing how much difference this simple adjustment has made. Now, when I adjust my needles a couple/few clicks, I can actually tell the difference. Whereas before, it was rich (seemingly) no matter what, until bam! then too lean (i.e., razor thin line between rich and lean).

For any of you guys out there that are running 30% nitro (particularly those of you that are still relatively new to nitro like me), if you're suspect of the engine's power output or if it is seemingly resistant to needle adjustments, then I highly, highly recommend taking a look at your shim thickness. If you're running the stock head gasket with 30%, you really might benefit from a shim. Takes about 10 minutes to switch it out, so it's easy to experiment.

Thanks, guys, for the help. 

[flyalan]

Glad you are now happy :-)
I have never needed a shim in my OS55 though. Runs great! The folks I fly with also run 30% with no problem.
Unless someone seems to be having issues with the 55 running 30% I would leave well enough alone. The stock OS55 was designed to run on 30%.

Your particular environment may necessitate this but not everyone's will.

Cheers

Alan

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[Rodney Kirstine]

Mine blew a great big hole in the piston before I shimmed it.

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[Jon Mills]

Moves out right! I agree with Mr Alan too shouldn't need a shim except on a few rare occasions.  Great to hear!!!

[flyalan]

Mine blew a great big hole in the piston before I shimmed it.

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Really?  Never heard of that happening running 30% nitro unless the it was running too lean.  Head shims won't fix that...

Cheers

Alan

[Rodney Kirstine]

Now you have. .

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[flyalan]

Regardless I would not shim the OS55 based on another persons experience (good or bad).  I know lots of folks running  it stock.  Of course your fuel choice, muffler selection and plug choice can affect preignition.  In addition to over leaning the motor..  MTC 

Cheers

Alan

[Rodney Kirstine]

Oh yeah, for sure. I was just sharing my awesome luck.  I have run two different brands of fuel and one of them runs quite a bit differently than the other.  It's definitely an art that I'm still learning.

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