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Old 06-15-2003, 10:16 AM   #16
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Default I run the XP12

Originally posted by rodneybarrett
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.
Take the thin shim out and just run the thick one. The engine will run better not hotter, and you can run 30% without having to add another shim. I should add that I ALWAYS run MC59 plugs and never have problem burning them out.
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Old 06-15-2003, 10:22 AM   #17
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Default Shim and Nitro content debate


I also have an STS MT12 in my Kids Reflex and ran that last night at our big permanent track here on 30%.
The original head clearance was .027, I have reduced it to .012 (Total) to some performance out of this engine. Runs better, but still slow to what I'm used to.
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Old 06-15-2003, 11:09 AM   #18
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There's really no right or wrong answer for this debate. Two indentical engines will run different at different altitudes, humidity, ambient temp, fuel type (manufacturer, not nitro %), etc. EVERYTHING is relative to engine performance.

This is from personal experience. I have a few .12 engines, all run O'Donnell 30%, and all run Novarossi plugs (either 6 or 7). One of my stock MT12s runs beautifully with stock shims (.3mm I think). My other 2 stock MT 12s needed an extra .1mm shim to run at a safe temp & still have good performance. Two of my Richey MT12s needed a .1mm shim, while another need a .3 shim. My 3 port Sirio did not need extra shims, but the 5 port Sirio needed a .1mm shim. All these engines run in the same climate, same fuel, same plugs, and there is that much difference in them. And we won't even talk about my .21 engines!!!

Also, remember when the engine gets really worn, you may need to take out some shims to get performance back up to a respectable level. Obviously, this isn't a solution to "renew" a worn out engine, but it can help get a little more life out of it.

Basically, what I'm saying is run the engine, if you don't like the performance, try adding a shim. You will not hurt the engine by adding shims (but you can definetly hurt by taking out too many). If you don't think it helped, take it back out. EXPERIMENT!!!!!!
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Old 06-15-2003, 03:05 PM   #19
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The problem with running 30% nitro in a small block engine is it changes the timing of the engine, This causes the fuel to ignite to quickly, The other problem is it can cause detonation (the fuel explodes rather then burns). By adding a shim you are addressing the symptom (timing) but are not addressing the problem (detonation). Many in the hobby have bent connecting rods running high nitro. A general rule of thumb is the higher the nitro the cooler the glow plug, so if you are running a high nirto fuel you should be running a cold plug and it does not require taking the engine apart. If this does not work (engine still over heats), then try adding shims. There is no such thing as pre-detonation, what is refered to as pre-detonation is actually pre-ignition, where the fuel ignites to early in the cycle.
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Old 06-30-2003, 10:08 AM   #20
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Thanks for all the information. Apparently there is no correct answer...

I contacted Serpent USA about the issue (like Accord), since I am running a Novamega SX12 3P MS RT with 25%. They claim not to use any shims when running 25 or 30%. I have been running an extra shim since I got the engine, and the performance was great. Last week, I took out the extra shim and retuned my engine. The increase in power was very noticeable. I was worried that I might ruin the engine, but I have not been having any problems with detonation or burnt glowplugs. For now, I am going to keep running without the extra shim. BTW, the climate here is moderate humidity and temps around 20-25 degrees C.

On a similar note, someone here is running 40% in an RB .15 with no shims, and the engine screams. The power almost seems uncontrollable. Seems a little dodgey to me.
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Old 06-30-2003, 10:36 AM   #21
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I have a SX12 that I have run nothing but 25% in it. Never shimmed, runs and idles like a champ. No additional shims-lots of power. BTW run 25% in my unshimmed OS12 TR too.
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