Choose the top flow hole diameter from the sales drop down list. This size is an actual mm measurement of the diameter of the hole itself and usually is stamped on the end of the seat. If converting from gravity feed fuel to pumped, down sizing this hole size eliminates the risk of seat pop off and flooding. For methanol go as big as you can.
The 8mm measurement between the seating shoulder on the seat and the top of the pin is the important one if the thread is also correct. If the 8mm is too short, threaded seats can be set at correct heights using shiming washers.
Some seats may not be found in our listings and these will only be available from the OEM manufacturer for whom they will have been specially made. If unsure call or email.