I clean the outside of the speedometer housing using a rag dampened with alcohol. Coat the O-ring lightly with RTV sealant as recommended in the Advance Adapters instructions. This unit fits the new tailhousing/retainer perfectly. My original alignment marks will also work here, as the clamp bolt locates just like the original '99 XJ housing. Make sure that the clamp indexes for the correct tooth count and engagement of the speedometer gears. Here, I install the mode rail bore's end plug and supplied gasket.