Originally Posted by DK9mm
I have an OFNA red dot JL .12
I purchased it from a fellow member. I finally ran it today and when I got home and stripped it down to clean it and noticed it has 4 shims. Two appear to be the same thickness, one is thinner and the last one is even thinner.
Question is how many should be on this motor since I am running 20% nitro.
This is my first "good" motor and I want to get the most out of it while not pushing it to far so it will last.
Any help would be much appreciated.
We recommend a real head clearance between piston and burn room of about 0.45mm (0.018inch.
Or more, when the piston is at Top Dead Center. The real head clearance can be calculated as follows:
Real head clearance = Thickness of head shims + 0.05mm*
* (that‘s due to the internal construction of the engine and the compression of the head shims, when the cooling head is mounted)
You may need to adjust the shims depending on nitro-content (see table below).
Also, depending on track layout you may wan´t to adjust shims.
Large tracks may require an extra 0.10mm (0.004“) shim to reach higher top speed and improved fuel consumption, but this will decrease acceleration slightly.
16% Real head clearance 0.45mm(0.018") Head shims 0.40mm(0.016")
20% Real head clearance 0.45mm(0.018") Head shims 0.40mm(0.016") 25% Real head clearance 0.45mm(0.022") Head shims 0.50mm(0.020")
30% Real head clearance 0.65mm(0.026") Head shims 0.60mm(0.024")
hope this helps