Refilling the primary chaincase lubricant on your M8 Softail is relatively straightforward however, it is a good idea to determine whether your Harley Davidson has a wide or narrow outer primary cover prior to replenishing with oil.
The service manual does not specify how to determine what is considered a wide or narrow primary so Mike grabbed a couple of straight edge rulers to figure it out for us. To determine wide or narrow, simply measure from the gasket surface area to the outermost edge of the derby cover sealing surface. If the measurement is 4 inches, you have a wide primary. If the measurement is 3 inches, you have a narrow primary.
Regardless of wide or narrow, you will need more than 1 quart of primary chaincase lubricant to achieve the correct oil level. With the motorcycle level, Mike suggests pouring in 1 full quart of primary chaincase lubricant. Then, slowly pour in more oil while incrementally checking until the oil rises to the bottom edge of the clutch plate.
A wide primary case will hold approximately 42 ounces (wet) of primary chaincase lubricant. A narrow primary case will hold approximately 36 ounces (wet).
A chaincase that has been completely disassembled, cleaned, and dried, is considered “dry” and will take a little more primary chaincase lubricant to fill than an already “wet” compartment.
Reinstall the derby cover with a new o-ring. Then, reference your factory service manual to torque to spec and in correct sequence.