I know this is in an onroad forum, and I'm talking about offroad stuff, but I thought this would be a good place to ask anyway.

I've been running O'Donnell 20% in all of my engines since I got into nitro, and I'm thinking about switching up to O'Donnell 30%. I'm mainly switching because I want to run it in my MBX5, but I figure it would make more sense to just switch both of my cars. I've got a WS7 (first version) in my MBX5, and an MT12 in my XXXNT. I know the tuning differences between 20% and 30%, but I'm not sure about head clearances. Should I leave both engines alone, or should I add a shim to one or both of them? If so, what thickness shim should I use? Also, I've been running an MC59 plug in the MT12 and an MC9 in the WS7 (and considering trying O'Donnell 77's in the MT12 and 99's in the WS7, but I'll probably stick with the MC's). Will these plugs be fine for 30%? Thanks.