The Savage 4.1 engine has a "button" that is basically the top of the cylinder. The glow plug screws into this instead of the cooling head. The cooling head tightens down on top of this button and holds it tight in place. There's a mod posted on the HPI forum where you use a Dremel tool to cut small grooves in it to supposedly create channels for better ignition for the glow plug and fuel mix. On a Redcat you'd have to dremel the cooling head. The button is cheap to replace if you mess up or it doesn't work, a cooling head is not. There is no real proof it does anything for performance. [8D]