Sorry about the tardy responce guys.
The button on the Precirotate engines gives a clearance of .3 without shims, and comes with a .1 and a .2 shim installed from factory plus an extra .1mm shim in the box for run in to reduce the compression further.
If you are going to run 30% nitro fuel, put the extra shim in(.1mm) during run in and pull it out after its broken in properly.
If you are going 25%, run the motor in as it comes out of the box, once it has broken in properly, pull the .1mm shim that is in it from factory and your good to go hard.
If you do this, you will notice a significant increase in performance and I notice no detrimental effects to fuel economy (I've got 10 mins with stock shimming and with the .1mm removed)
The net effect of removing a shim is to put a little bit more heat in the head and little less in the pipe, it also means you can run a slightly cooler plug and be a little more aggressive on the tune. The extra heat makes the combustion process run a bit cleaner and gives more bang for your buck. I have gone as far as a total of .4mm clearance (.3 in the button plus the .1mm shim) with no detrimental effects. If you are running in cold or humid (high moisture content in the air) conditions, it keeps that little bit more heat in the head as apposed to the pipe which will be a little bit of a guard against flame outs when the car is upside down.
If you are concerned about doing this (this is why I said pull only the .1mm shim from factory, not the .2mm one), the thing to watch out for is distortion in the shape of the coil in your glow plugs. To higher compression will distort the shape or pull the coil out of the plug. .5mm (button step of .3 and the .2mm shim) is safe for 25%.
I run a Go FEMCA 0801 pipe on my motor which is far as I am aware, illegal for Euro events however is fine for ROAR and FEMCA and AARCMCC events (AARCMCC is the governing body here in Australia which is part of the FEMCA block). Apparently the EFRA pipe have noise restrictions as well as the 3 chamber design rule which eliminates some of the more aggressive 3 chamber pipes like the 0801. All good in the states though, the Aplus 0801 is great, I haven't tried the 2041 yet, I don't recommend the Go/Alpha 2072 pipe on a .21 engine as I feel they are too big on the chamber and cause a rich condition down low thus less initial punch/mid (truely brilliant on a .25 though). The Sirio 2013 is a ripper pipe for the PR21BS. I also run the HN 2027 pipe if I want better fuel economy and smoother power. Max would be the man to speak to when it comes to pipes.
Me and a good friend of mine who got PR's when they where first released have been experimenting a little bit to see what we can get out of them. I have to say appart from a little issue with carbs initially (those problem needles no longer come in the carbs) it has been a truely epic little motor. I hear along the grape vine there is new carbs for the PR as well. For those of you out there that own Go GX or MG series motors, you can also pull the .1mm shim out of them for extra oomph.
What I have written here is my experience, it is absolutely up to you should you choose to play with the shims and its not for the inexperienced. Don't go less that than stock shimming for 30% as the opportunity for it to pull a coil from the plug and damage the piston & sleeve is too great. I will also mention that I ONLY run OS plugs as I have had too many other brands elements come out. Also, I'm running australian fuels, I don't know how the euro or American fuels behave in this setup as the rules for ROAR, EFRA and FEMCA are slightly different. Nitro grades allowed are the variance from memory.
My engine setup on buggy... PR21BS (black headed PR), .5mm clearance on head, OS P4 plugs. Give it try if you would like, its your choice. I'm sure as heck not twisting any arms.
Have fun guys and girls
