About R2105 issues.
Using a heatcam on my engine heater project I did notice it did not measure the right temperature of plain aluminium. That is true if you look into the emission table of the better IR temperature meters. While the emission factor probably is set to something in the range of 0.9~0.95 which is better for anodized/colored metals you need to adjust the emission to about 0.1 for plain aluminium.
I had the theory of bad pistons with the R2105 that when tuning based on the temperature you know from previous OS models the plain shiny aluminium head could give an different reading with measuring the temp.
I placed the 2105 head and a blue color anodized OS head on the same heating source and let it warm up until the temp is stabilized. The heatcam is just an IR device as many temperature meters we use. On the pics the one with 67 degrees is the colored one, the one with the 43 degrees is the shiny R2105 one, both measured on the upper fins.
If you measure on the glowplug you also measure the surface around it and so you do get an average value of the whole spot.
I am pretty sure those people who tune their engines based on the temperatures they know from other models with a colored head they go too lean on the R2105.
I do not say it is THE problem but I do say it is something to keep in mind.