An Easy Method to Adjust a Harley Primary Chain

Big twin powertrain, BIG HARLEY

Big twin powertrain, BIG HARLEY—ronsaunders47 (

The design of a Big Twin’s Powertrain is non-unitized. Meaning the gearbox is attached separately from the engine and the two should work side by side to transfer power. That’s where the primary drive, found in that huge metal case near the rider’s left foot, takes action.

As the primary drive transfers power from your engine to the back wheel. The reciprocating power that’s generated through the engine’s crankshaft is changed into rotating energy from the moving major chain and sprockets that spin the transmission’s input shaft. Once the energy regroups within the transmission it’s distributed via the gears and into the secondary drive, which is the belt or chain drive that turns the bike’s back wheel. This is often a simple explanation of a Harley Davidson powertrain, and basically, each part plays a vital part in running you and your bagger around the road.

Given that the powertrain consists of a set of shifting parts, you will have to make regular adjustments to them so they work efficiently. Over time, dynamic parts such as the main drive’s sprockets and chain, require replacing. As things wear, they demand tuning to help keep correct tolerance. And it’s the main driver’s multi-row roller chain that needs to be checked out and adjusted at certain times. It’s a fairly clear-cut process too, one that you can complete in your own garage area or shop.

Most of the action happens inside the inspection window that’s reached by taking off the plate placed on by four mounting bolts. You’ll need a ruler to make the measurements and a set of essential tools to perform the corrections. To adjust, release the adjusting shoe’s 9/16″ bolt a couple of turns and move the shoe upwards to tighten the chain or down to loosen.

An excellent time to inspect and adjust the main chain is when you drain and change the primary case oil. Harley highly suggests its multipurpose manufactured oil, however, if you do have a desired brand that’s been designed for the main drive, by all means, use it. Just make sure the lubricant you use is designed for this purpose.

Once the inspection plate is off, you’ve got a decent look at the chain’s parts, so makes use at this time to visually inspect them for corrosion. Take a look at the chain’s rollers to see if they are getting a total oiling, and check the adjuster shoe for cracks and the like. Inspecting and adjusting the main chain is quick and easy, when you’re done you can have one less component to worry about on your next ride. Be sure you change the inspection plate gasket.

Things You’ll Need:

Pipe Sealant, Pan for Draining, T27 Torx, T40 Torx, 9/16″ deep socket Ratchet, Extended Ratchet, Ruler

1. Standard maintenance like adjusting the primary chain can be carried out within your own garage area or shop when it’s time to change the primary oil. Ensure the motorcycle is secured in an upright stance once you start.

2. One thing to do is to drain the primary case of its used fluid. Once a drain pan is placed underneath the primary, have a T40 Torx to remove the drain plug which is found underneath the derby cover.

3. Allow the used primary fluid to empty in the pan. Once emptied, you’ll be able to check the primary chain to see if it requires tuning.

4. Use a T27 Torx socket to get rid of the few bolts that keep the primary inspection plate in place. It’s recommended that you change the inspection plate gasket just before you mount the plate.

5. With the inspection plate removed, you may check out the chain’s rollers for signs of wear. Also, check them for signs of damage. With the transmission in neutral, check the rest of the chain by spinning the engine a few turns.

6. Always note down the readings when measuring the chain. First is with the chain’s top run at slack. Second, raise the ruler against the top run to get rid of the slack and to measure once again. The difference in the two on the chain’s tightest point is the chain’s slack.

7. Use a 9/16″ deep socket to adjust the chain’s free play. With the chain cold, set the free play from 1/2″ and 7/8″. Right after you have re-tightened the nut again, make measurements of the slack to ensure that you have set the proper amount.

8. Since the inspection plate is open, also examine the adjuster shoe that is pushing against the chain’s under run. On the upper right corner, you’ll find the adjustment nut.

9. Compared with the engine and transmission drain plugs, you’ll notice it doesn’t have an O-ring. To aid in sealing it, rub a dab of sealant (for pipes) along the bottom edge of the bolt head.

10. Soon after you’re done clearing off all the oil out of the primary cover, reinstall the primary case drain plug. When already set, slightly twist the bolt’s from the inspection plate as well as the derby cover bolts to 108 in-lbs.

11. Now you can stock up the primary case with fresh fluid. Most shops use lubes from a common source instead of single-quart cans.

12. Fill with 32 ounces of fluid. Put in the proper amount for your bike’s model and year.

Keep in mind that it’s very important to make certain you travel with your motorcycle safely. Make certain you put on authentic carbon fiber helmets.

You might also want to read these Harley blogs:

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