[Graham]

Grahams rules have max 7mm deep and 20mm long.
I asked as I believe 7mm will be too deep for SCX/Scalextric Classic track.
Should be fine if its Scalex Sport, I am unsure but believe Carrera, Ninco and Policar should also be fine with 7mm.
As an aside, my proxy builds that are in testing, are currently fitted with the Slotting Plus guides Kevan mentions.
These are trimmable to 21mm retaining the RKS.
I cant see 21mm being an issue, but without all the plastic tracks to test, I can't be sure.

I would specify 6mm deep guides if we use SCX or classic Scalex. The guide in theory should be ok for Scalex Sport as it is deeper than the classic Scalex. But again I emphasise that we still get issues with too thick or too long and sometimes too pointed or sharp at the front which can catch in the track joints.