Originally Posted by spurgear
After reading that the tuned version is shimmed differently from the non tuned version because of the sleeve shim etc.. What is the correct shims required for 25% fuel for a tuned version?
The best way for any engine, if you are in doubt, is to do the actual head clearance measurement, and from there determine what shim sizes to put, according to the fuel you will use.
Position the piston at top dead center. You will have to keep the cylinder firmly seated as you rotate the crankshaft to the TDC (top dead center) position. ABC engines will usually push the cylinder out of the crankcase due to the tapered fit. An easy way is to turn the engine up side down and press it firmly down onto a table top while you rotate the crankshaft.
The piston must be at TDC and the cylinder liner must be firmly seated into the crankcase to obtain the correct measurements. If it is not the final result will be incorrect.
Measure the distance from the top of the cylinder to the top of the piston using a good caliper (Dial or digital better). Be careful not to allow the measuring instrument to push the piston off TDC.
Now letís suppose it measures 0.20mm
Now you must measure the portion of the head button that protrudes into the cylinder.
Here letís suppose we have a 0.20mm measurement on our sample.
Now do the math...0.20 -.0.20 = 0.00
This means that whatever shims you put on the engine will be your actual head chamber clearance.
For Picco .12 engines use this table.
Real Head Clearance For Picco based engines = Thickness of head shims + 0.05mm (costruction tolerance)
Nitro content: 16% 20% 25% 30%
Real head clearance: 0.45mm / 0.45mm / 0.55mm / 0.65mm
Head shims: 0.40mm / 0.40mm / 0.50mm / 0.60mm