Classic Motorcycle Build

Ironhead Transmission

Text and Pictures by Mark Trotta

With over 50 big and small individual pieces, rebuilding an Ironhead transmission requires attention to detail and a lot of checking and rechecking. A well-intentioned DIYer can easily cause damage by forgetting a retaining ring or installing a wrong-size thrust washer.

install ironhead transmission

Although the Sportster transmission is complicated to assemble, there is no groundbreaking technology. Similar to engine building, the skills needed tend to be acquired as needed. The first step is to learn how to remove and install it.

ironhead transmission install

Read: Remove And Install Ironhead Transmission

An Ironhead transmission is easier to understand when broken down into categories:

Ironhead Mainshaft

The mainshaft sits above the countershaft and is longer. It rides between the clutch gear bearing in the trap door and 23 loose needle bearings in the right-side engine case.

Ironhead mainshaft

From inside to outside, the four main gears are 1st, 3rd, 2nd, and 4th. This is done for fitment reasons.

Mainshaft Differences

Throughout it's production years, the Ironhead Sportster was fitted with three different mainshafts. There is an early dry-clutch mainshaft (1957-1970), the later dry-clutch mainshaft (1967-1970) for electric start bikes. Dry clutch mainshafts are hollow to allow pushrods to activate the clutch.

Later Sportsters with wet-clutch setups (1971 up) have a solid mainshaft.

ironhead transmission mainshaft differences

Pictured is an early dry clutch mainshaft with third gear attached.

Ironhead Countershaft

The countershaft rides between an open needle bearing in the trap door and a closed needle bearing in the right engine case.

Ironhead transmission countershaft differences

There are two different countershafts for the four-speed Sportster transmission. The early style was used from 1958 up until early 1984. The later countershaft is used on both Ironheads (1984-1985) and early Evo Sportsters (1986-1990) until the last year of the four-speed.

Ironhead transmission countershaft differences

Oiler Plug

The oiler plug sits outside the open needle bearing and press-fits into the trap door. Before installing the oiler plug, check the final countershaft end play. Once installed, the oiler plug should stick out of the case about 7/16" with the hole facing up.

Trap Door And Mainshaft Bearing

The transmission access cover on Sportsters is commonly called the trap door. It contains the mainshaft bearing, which is held in place by two retaining rings.

ironhead transmission install

Once separated from the transmission, the mainshaft clutch gear can be spun by hand to feel for any binding or excessive wear in the bearing.

Shifter Assembly

ironhead transmission pawl carrier

Shifter Fork Spacing

All transmission gears need to roll free without any binding. The two slider gears need to be spaced apart as equally as possible. The most common way to adjust gear spacing is to replace the shift forks with different sizes until proper clearances are obtained.

Ironhead shifter forks

Although only two are needed, the meticulous builder will often remove and install half a dozen shifter forks before the correct gear clearance is obtained.

Ironhead shifter fork spacing

Mainshaft and countershaft end play need to be established and adjusted before final assembly.

Read: Remove And Install Ironhead Transmission