I have still yet to break in my 2105 as I have been busy with a couple other platforms . It’s sad OS released a V2 version like 2 months after the first one to try and rectify the issue some are having with the piston . Since I have the first version do you think I can take a rubber wheel and carefully polish the upper edges of the exhaust ports in an attempt to keep the piston burn mark free ?? To really add salt to the wound I preordered the Ielasi Gp23r then next thing you know there is a V2 but before that happened I ordered a Part set from ASM , needles to say I wish I waited some as I fear I will mark up each piston in time .
Thank you for the intel on the R2105 case capacity vs previous models , do you think using the r2105 case for the GT2 sleeve will be beneficial for a Gt engine ? With less capacity it may/ should increase response and torque.

I suggest you just use the R2105 as is, you can do some carfull polishing at the exhaust side of the sleeve but be carefull.

Yes, you could try and use the R2105 crankcase with the GT2 sleeve, just as a precaution mesure the sleeve of both engines, just to be sure they are the same.

Thanks Rody . When I get the partset broken in I’ll test both and see how it feels with the 2105 case .

You are welcome and keep us posted about your findings.

Digging up this topic because I now have 2x an R2105 in my hands.

In the past I had many doubts of his (Kings Racing) numbers, he had blowdown numbers I have never seen that large. All seems to be based on some calculator sheet he got from RW mods. There is another guy using that same sheet, his name TastyB
On Kings Racing his Youtube page you can also find a video of how he mods a Picco also based on RW mod instructions. I know, you may not speak bad of the dead but there is a lot wrong with that knowledge.

With both I had huge discussions about their blowdown because in their information that number makes the RPM range. As example I provided how come when raising the sleeve with a shim I basically do not change the blowdown but do get a better RPM range. Both could not tell but stick with the blowdown numbers. It seems Kings Racing did listen to me about doubling the numbers giving the total opening and so the blowdown was also doubled which should not. TastyB is still using the doubled blowdown numbers.

I did compare my timings with those of Kings Racing. He did not give the actual transfer and exhaust port timing so those I can not compare. He did gave a blowdown of 29 degrees, that is 3 degree more than I did measure. For who do not know, the blowdown is the moment that the exhaust opens until the moment the transfer opens, so the difference of both timings. Also he measured the opening and closing of the crankshaft 1 degree more than it is. And believe me, the timings I measure is used on almost all stock R210x engines I have seen so I doubt his numbers although they are close.

But OK, here a list of all different OS (based) engines I have seen:

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.

I am guilty as well for doubling the blow down numbers monkey see monkey do - guilty as charged