These vented primaries are designed specifically for those who are using an up to 13/4" wide belt drive. The vented slots provide an improved airflow to keep belt running cool thus improves belt life. Fits 4 Speed Big Twins 1970 thru 1984 and 5 Speed Softail models thru 1988. The cover is ready to install on all 8-hole primaries and also features a flat boss that can be drilled when the ninth hole is needed. Installation on floor board equipped FL models requires the drilling and tapping of the mount tab on the provided flat boss in front of the inspection cover. Starter shaft needle bearing must be ordered separately.