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Old 06-10-2003, 05:57 PM   #1
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Default Shim and Nitro content debate

What are your opinions about the need to add shims for higher nitro content (25%-30%)in .12 engines?

I have heard that it is a must to prevent detonation and related engine damage. Others claim that you can run high end .12 engines with 25%-30% without any extra head shims.

Last edited by chachi; 06-30-2003 at 09:59 AM.
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Old 06-10-2003, 06:01 PM   #2
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If the motor has regular p/s ( not domed) then the simple rule of thumb applies .006 per 10% of nitro. you must remember if you have to fudge it is better to do so on the larger side not smaller like for instance if you want .012 but the clossest you can get is .015 that is fine but don't go .010.

hope this helps.
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Old 06-11-2003, 10:21 AM   #3
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Default Shim and Nitro content debate

Guys;

I run about .018-.019 with 30% in my Picco .12 and it runs great.

You can use a piece of small O.D. solder inside the glow-plug hole to measure. Just bend the end into a "L" shape about 1/4" long, put it in the combustion chamber and turn the Motor over by hand 1 revolution to smash the solder. Remove it and measure with Calipers or a Mic. Then remove/add Head Shims to get you where you want to be.
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Old 06-11-2003, 10:50 AM   #4
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In my experience the shim is a MUST. Most of the engines were designed for 16% (ifmar) fuel, so even 20% is pushing it. I switched to 30% this year and when I ran the engines with just the stock .3 mm shim they ran hot, were cranky(idle) and burned plugs. Put the shim in and everythings back to normal. BTW RBX12 & XP12, NTC3, OD 30% and OD77 plugs.
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Old 06-11-2003, 08:33 PM   #5
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OB42TC3: With the extra shim in how much is your clearance now? .6 mm ?

Also, I run the XP12 as well but I only use 20% nitro. Do you think the original shim is enough for it or should I put another in.


This is a good topic. Let's keep it going. I can learn a lot of good stuff here.
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Old 06-13-2003, 08:10 PM   #6
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i run a Rody 5 port with 25% nitro. Added one 0.1mm copper shim to the stock (0.4mm), the motor runs stable and no sign of overheating in 30 min main. Run plug #6 on colder temp and #7 on hotter temp.
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Old 06-13-2003, 08:55 PM   #7
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No, by one shim I meant .1 for a total of .3+.1=.4; and this is not THE head clearance just the adjustment for it. I tried the solder method and adjusted to it and it was too much compression the solder is springing back it's not the right way to do it.
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Old 06-14-2003, 05:30 PM   #8
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I spoke with Serpent USA a few days ago about my NovaMega SX12 engine because I am running 30% and they told me that you do NOT have to run an extra shim, they told me that all that will happen is you will lose performance and they said there is no need to add extra shims unless you are running 40% or more.
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Old 06-14-2003, 06:53 PM   #9
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Thats what I'm saying, whats the point of running 30% fuel when the extra power gained from it will be taken away from the compression being decreased by the shims. Even in the mt-12 manual it says to run 20-30% with no additional shims.
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Old 06-14-2003, 07:47 PM   #10
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If you don't use an extra shim, do you change to a colder glow plug?

Do you find that you burn more glow plugs running higher % with no extra shims?
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Old 06-14-2003, 08:10 PM   #11
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Dennis explains this in the Richey Modified forum, the shim needs to be added.

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Old 06-14-2003, 08:12 PM   #12
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Old 06-14-2003, 09:01 PM   #13
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Some people are saying use an extra shim, others are saying not to. Serpent USA directly told that an extra shim should NOT be used in NovaMega SX12 engines running 30% fuel, unless I hear something different from some engine expert or something i'm not going to use an extra shim and I guess i'll just have to hope for the best, lol.
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Old 06-14-2003, 09:34 PM   #14
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Accord: What model SX12 are you running? I am in the same boat as you i.e. trying to figure out the shim thing. i am running an SX12 3-port MS Race Turbo with 25%. I currently have a shim in it and it seems to be running well. However, I have been told, by what I consider realiable sources, that it is not necessary.

What exactly did Serpent SA say? Did they say that you never have to add a shim for 25-30%?

Can you let me know how your engine runs without the shim?
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Old 06-14-2003, 10:26 PM   #15
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At the WinterNats this year, Art C. gave me a shim to put under the head of my SX12, but it was blowing plugs. I usually run what comes in the engine, there I added an .010.
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