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Primary plug

The primary plug screws into the primary cover. You can remove it to add oil to the primary and transmission system. You can also reach your finger in to check the primary chain tension. You should rotate the engine to several positions when you do this in case the sprockets are not perfect. They chain tension will change at various points in the engine rotation. Do not kick the bike over with your finger in the hole, you might get it crushed by the sprocket.

34742-52


 Sportster_Engine_Primary_Primary-cover_34742-52_Primary-plug_1952-1956 Sportster_Engine_Primary_Primary-cover_34742-52_Primary-plug_1952-1956 Sportster_Engine_Primary_Primary-cover_34742-52_Primary-plug_1952-1956 CAD
Blueprint
The primary plug screws into the primary cover. It seals with an o-ring at the base of the threads. You remove the plug to add oil to the primary/transmission systems. You also can check the primary chain tension through the hole. I am not sure if this plug was polished or plain, please advise. Problems: Vent hole lets water in if you pressure-wash the bike.

34742-52A

 Sportster_Engine_Primary_Primary-cover_34742-52A_Primary-plug_1957-1970 Sportster_Engine_Primary_Primary-cover_34742-52A_Primary-plug_1957-1970 Sportster_Engine_Primary_Primary-cover_34742-52A_Primary-plug_1957-1970
Harley introduced this primary plug with the 1957 Sportster. The "A" suffix implies it will interchange with the earlier plug. It may be polished instead of plain, or it may have a fiber filter crimped into teh back as the above picture shows. Please advise. Problems: Vent hole lets water in if you pressure-wash the bike.

60569-29

 Sportster_Engine_Primary_Primary-cover_60569-29_Primary-plug_1958-1963 Sportster_Engine_Primary_Primary-cover_60569-29_Primary-plug_1958-1963 Sportster_Engine_Primary_Primary-cover_60569-29_Primary-plug_1958-1963
The steel primary takes this stamped steel plug. It is a carryover part used on the 1929 models, perhaps the 45CID flathead. It is painted gloss black. Problems: No venting for the primary area provided by this plug.

60570-64

 Sportster_Engine_Primary_Primary-cover_60570-64_Primary-plug_1964-1969 Sportster_Engine_Primary_Primary-cover_60570-64_Primary-plug_1964-1969 Sportster_Engine_Primary_Primary-cover_60570-64_Primary-plug_1964-1969
This is identical to the 60569-29 part but Harley engineers chrome-plated it. Problems: No venting for the primary area provided by this plug.

34742-71

 Sportster_Engine_Primary_Primary-cover_34742-71_Primary-plug_1971-1976 Sportster_Engine_Primary_Primary-cover_34742-71_Primary-plug_1971-1976 Sportster_Engine_Primary_Primary-cover_34742-71_Primary-plug_1971-1976
Polished aluminum. Harley introduced this plug with the redesigned primary used on wet clutch models. It installs flush to the primary cover. It is vented and has a fiber filter crimped into the back. The 37880-71 clutch adjustment plug looks very similar but has no vent hole. Problems: Hard to remove without damaging the slot. A 1/2-inch Craftsman wrench on its side works well. There are special tools with the precise arc that can remove the plug with minimum damage.

34742-77B

 Sportster_Engine_Primary_Primary-cover_34742-77B_Primary-plug_1977-1984 Sportster_Engine_Primary_Primary-cover_34742-77B_Primary-plug_1977-1984 Sportster_Engine_Primary_Primary-cover_34742-77B_Primary-plug_1977-1984
Polished aluminum. Used with new cases in 1977. Same as 34742-71 but no vent hole. Vent is provided by vented bolt in top of rear motor mount. Harley introduced this plug with the redesigned primary used on wet clutch models. It installs flush to the primary cover.  also used as the clutch adjustment plug in the primary cover. Problems: Hard to remove without damaging the slot. A 1/2-inch Craftsman wrench on its side works well. There are special tools with the precise arc that can remove the plug with minimum damage.