Classic Motorcycle Build

Ironhead Transmission

Text and Pictures by Mark Trotta

With over 50 individual pieces, overhauling 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.

Sportster 4-Speed transmission overhaul

The four-speed Sportster unit transmission was produced for over three decades, starting with 1954-1956 Harley K-models, then 1957-1985 Ironheads, and 1986-1990 Early Evo Sportsters.

Although it's a bit complicated to assemble, there's no groundbreaking technology here. Similar to engine building, the skills needed to rebuild one are acquired as needed.

The first step is to learn how to remove it.

ironhead transmission install

Read: Remove And Install Ironhead Transmission


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


Ironhead Mainshaft Assembly

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.

Sportster 4-Speed 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 push rods 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 Assembly

The transmission 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

Both transmission shafts should be checked for straightness. The picture below shows checking mainshaft straightness on a homemade flywheel truing stand. Anything past .003" is questionable.

check mainshaft straightness

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Carefully check all gears before re-use. Once pitting occurs, gear wear is rapidly accelerated.

Ironhead transmission countershaft gear damage

The picture above shows a badly pitted countershaft third gear.

Once assembled with other gears on a shaft, every gear should roll free without any noise or binding.


Trap Door And Mainshaft Bearing

The transmission access cover on Sportsters is commonly called the trap door. It is mounted with four bolts that do not require washers.

Ironhead transmission trap door

The trap door assembly contains the mainshaft bearing, which is held in place by two retaining rings. After the rings are removed, it's a light press fit to remove the clutch gear and bearing.

press fit mainshaft bearing into trap door

Once separated from the trap door, the mainshaft bearing can be spun by hand to feel for any binding or excessive wear.

Ironhead transmission replace mainshaft bearing

Oiler Plug

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


Shifter Assembly

Sportster 4-Speed transmission pawl carrier

Shifter Forks and Spacing

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, a meticulous builder may 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.


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