I think he means 0.025 inches which is about .6mm..the GO engines are running about .8mm head clearance with the alloy and 2 copper shims plus the .3 from the head button.0.004" is 0.1mm..You can run these with that little head clearance but the tuning window gets progressively smaller as you reduce shims stack.with 0.1mm it'll eat plugs and the idle will be near impossible to keep rock steady.Besides even with no shims you still have the .3mm built into the head button.