Exclusively manufactured for us, our bronze valve guides have more material on the inside diameter allowing for more freedom at arriving at the desired valve guide clearance with various valves.  Our HP bronze valve guide and Viton seal kit for GT6, TR250 and TR6 models allows up to 0.500" of valve lift.  Additionally, the HP bronze intake guides have been bulleted allowing for increased airflow at higher lifts.  Our HP exhaust guide has been sized so it does not protrude into the interior of a port.
We only use metal jacketed Viton rubber seals, they are the most durable and conformable valve seal available, as well as being easy to install.
When using our bronze guides we recommend 0.0015-0.0017" clearance on intake valves, and 0.0018-0.0020" clearance on the exhausts.  Turbo applications and other cars where the valves will run extremely hot, add 0.0002" to the recommended clearances.