Open Primary Motorcycle Belt Drive Conversion [Pros and Cons]

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So, I have recently started researching the pros and cons of a primary belt drive conversion for my 1972 Harley Davidson Shovelhead.  As you know, the Shovelhead project is a big deal for me and is featured a lot here on HappyWrench.

Reasons to Convert to an Open Primary Belt Drive

Why this sudden interest in a primary belt drive?  Well, upon recent inspection, I noted that my existing Shovelhead primary chain setup has seen better days in terms of overall condition.  In particular……

1) The back of the primary interior case has JB Weld sealing the primary chain oiler (not that big of a deal, but not exactly the way I like to see things done).

2) One of the primary cover bolt holes has been plugged with “something” (okay, having one bolt missing probably isn’t a big deal if the rest are fine).

3) One of the primary bolt holes has been drilled out oversized and re-tapped poorly (again, maybe not a huge deal on its own, but a bad tap makes me nervous that it will eventually cross-thread and strip).

4) One of the more critical primary cover bolt holes is currently stripped and the bolt can’t be torqued (okay now things are looking a little dicey).

5) The primary case itself is looking a little shabby.

primary belt drive

Each of these on their own isn’t a big deal and would be something that I would normally attempt to address. 

However, when you put them all together, it seems like maybe another band-aid repair might not be a fantastic idea.

Additionally, when I first acquired the Shovelhead in November of 2015 (and before I took it apart), the bike was leaking a little bit of oil along the left side.  It was pretty clear that this oil was coming from either the front or back of the primary case.

The leaking stopped after I ran the bike a few times, probably because the little bit of oil in the closed primary leaked out.  The leak was probably caused by one of (or a combination of) the items above.

So now you know why I started thinking about a primary belt drive setup (and are probably like, “jeez man, get on with it”). 

Basically, if the entire primary case needs to be replaced, I want to replace it with the best setup possible

Also, you see open belt drives all the time on custom bikes.  There has to be something to it besides a cool look, and there is no way I would spend the money on a primary belt drive without fully understanding the pros and cons first!

Below are the results of my research.

Pros and Cons of an Open Primary Motorcycle


1) Power-to-Weight Ratio Gains

These are huge. 

First, you have the fact that the belt itself weighs a fraction of what the primary chain weighs. 

Additionally, the sprockets that the chain wraps around are usually steel, while the belt pulleys are generally made of aluminum. 

An open belt setup allows the removal of at a minimum the outer primary cover and hardware. 

If you go kickstart only, you can also remove the inner primary case, the starter motor, and the solenoid (Harley Specific).

2) Better Power Conversion & Responsiveness

As noted above, the steel sprockets are getting replaced with aluminum pulleys. 

This means less power is spent on turning these heavier items and transferred to the road. 

There is no compensating sprocket and less slack in the belt, so responsiveness should be improved.

3) Less Maintenance

Whether you are running an open or closed primary, not having to worry about primary chain oil is a pleasure and it is one less place for oil to leak from. 

You also avoid major primary chain adjustments (i.e. moving the transmission, adjusting the rear drive chain, etc).

4) Ease of Inspection & Access

Everything is out in the open for you to see. 

Additionally, I think having the inner primary removed provides easier access to certain components in the middle of the bike (shift linkage, speedometer cable, oil hoses, wiring etc.). 

To me, that is a nice perk.

5) Less Drivetrain Vibration:

Harley’s vibrate like crazy, so a little less vibration is always nice. 

Also, theoretically, the shock absorbing properties of the belt itself should put less stress on the rest of the motor. 

When a belt breaks there is a lot less potential for catastrophic damage to the engine versus a primary chain.

6) Ground Clearance:

Removing the outer primary cover gains some ground clearance. 

My bike needs special wheel lifts just to get it high enough off the ground to then slide my Craftsman lift underneath the frame. 

Ground clearance is definitely a factor that isn’t always considered.

7) An Open Primary Looks Cool:

Thought I was going to skip this one? Well, I didn’t.


1) Belts Can Break

Obviously a chain is stronger than a belt and it is “possible” to break a belt if you are running a high performance setup. 

Belts come in different widths (wider = stronger), so keep your bike’s HP in mind when choosing the width of the setup. 

Also, inspect your belt regularly. 

Something to keep in mind, however, is that belts have gotten much stronger in recent years. 

You can also pack an extra belt and some tools if you are going on a long trip. 

A belt replacement can be done on the side of the road, if required.

2) Crud Buildup:

With the primary open the setup, including the clutch, can collect some crud over time. 

Keep on top of your cleaning and polishing.

3) Clutch Noise

Running an open primary means you are exposed to the noise coming off the clutch. 

That said, my Harley isn’t exactly quiet to begin with. The pipes drown out the additional noise.

4) Heat Buildup:

This is a fact, but obviously can be compensated for by running the belt drive open or with a ventilated primary cover if you wish to partially enclose.

5) Replacing Your Pants

It is possible with an open belt primary to get your sock or pant leg caught in the spinning belt. 

I hear this one all the time from the anti-belt drive naysayers. 

Ask anyone with a belt drive, however, and they say it really isn’t an issue keeping there leg away from the belt. 

To be honest it seems like there is a mile between where I put my feet on the floorboards and the primary belt, so I am not sure how I would get my pant leg caught unless I was trying.

There you have it – the pros and cons of a primary belt drive setup. 

I am pretty good at counting and by my tally the “pros” take the cake.  Also, I think the cons are less serious in nature. 

Anyways, I see a primary belt drive in the Shovelhead’s not-so-distant future (I have actually gotten as far as narrowing it down to this kit made by BDL – seems like a good bang-for-the-buck).  I will post the details of the installation and the pictures once available.

Have an opinion on primary belt drive conversions?  Give us a comment below.  Also, please please please, share this post via social media!  HappyWrench is new and needs your support!

8 thoughts on “Open Primary Motorcycle Belt Drive Conversion [Pros and Cons]”

    1. Clearly I missed this very important positive to the belt drive! Thanks for the comment and cool 😎 website. Have a good one.

    1. Christopher Pumo

      Well I have/had the same problem, multiple times, multiple bikes – so I can totally relate.

  1. Appreciate being able to read this ive been debating on switching my fat bob to belt drive pretty sure im gonna go ahead and do it thx

  2. Great article and I really like your web site. I have a 75 FX Harley with a dry primary (4 speed tranny). It has a BDL 1.5″ -11mm (4731-SK3) w/starter ring gear and tensioner. My question is: What system do I change over to, in order to make my drive open with no primary cases? I already purchased the bearing support and altinator cover from APM. Obviously I must change the belt because I will have no tensioner, and my 99 tooth belt will flop all over the [;ace, right? So I kind of recall someone saying it’s an 8mm system (like a BDL 62-40-SK3). The 8mm belt has more teeth, but is actually a shorter belt??? It would be great If I could simply cut off my ring gear and just change to a shorter 11mm belt with my current parts (4731-SK3), because I’m trying to do this as inexpensively as possible. As you can tell, I have been investigating this a lot, but just can not find this little piece of info that I need, so I don’t go on a bogus mission trying a bunch of belts on for size ha ha

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