Owning a Twin Cam Harley can be a real joy, but it can also come with its fair share of problems. For example, one of the most annoying issues with Twin Cam engines is oil leaking from the air filter.
Now, while this problem is very annoying, it’s actually easy and cheap to fix. In this article, we’ll take a look at why this problem is so common, the possible causes, how to fix it, and how to prevent it from happening in the future.
Most Common Reason: Failed Rocker Box Breather Strainers
The most common reason why Twin Cam engines leak oil from the air filter is due to failed rocker box breathers. The only other reason why you might see oil dripping from your air filter is if you soaked it in too much oil, thus we will only focus on the rocker box breathers in this article.
Now, all Harley Twin Cam engines have a closed-loop breather system, which routes crankcase pressure and oil mist through the head and straight into the air intake. This design choice isn’t the greatest and it does increase carbon buildup on the piston (which is why many riders use an external breather setup), but Harley also decided to use a weird oil strainer setup.
Between the rockers, there is a small breather box, which includes an oil strainer element (similar material to a scotch-brite sponge – no bueno) and a PCV valve. Over time, due to high heat and from constant contact with oil, the oil strainer element becomes brittle and pretty much disintegrates. Once this happens, oil droplets are free to pass straight through the breather box and into the intake.
Now, when you’re riding at highway speeds, the number of oil droplets that are forced into the intake increases, and they start to accumulate inside the air filter. The filter gets soaked with oil and then starts dripping down on your engine and makes a mess. If your Twin Cam is a bit tired and is producing more blowby than normal, you will probably get oil dripping even at lower speeds.
How Do You Diagnose a Bad Rocker Box Breather?
Luckily, diagnosing a bad rocker box breather is very simple. You can either remove the head covers and check the rocker box breathers or you can drain the oil and watch whether the oil pours out slowly or fast.
Now, we recommend you remove the head covers and inspect the rocker box breathers yourself – it’s the best way to do it and the only way to know for sure. However, sometimes you have to remove the gas tank in order to get enough access, which can be a pain.
If you don’t want to remove the head covers, then you can drain the engine oil without removing the filler cap and watch whether the oil will dump out quickly or drip slowly. If you see oil dumping out quick, then it’s a sign that your breathers are shot and need servicing. Rocker box breathers that are in good condition should create a vacuum and thus make the oil drain slowly.
How to Fix Oil Leaking From the Air Filter
The only way to fix this issue is to service the breather strainer system, which can get expensive if you go for premium aftermarket parts.
The only parts that wear out and need to be replaced are the spongy oil strainer elements, the PCV valves, and the breather gaskets. If you want to keep the repair nice and cheap, then that’s all you need to fix the issue. All in all, it should set you back by $50 or so.
However, if money is not an issue, then there are a lot of aftermarket breather kits that come with fancy painted billet housing, though they use the same spongy oil strainer element. Expect to pay $200 or more for those.
Removing the head covers and the breathers is extremely simple, so it’s a great DIY project, but if you’re unsure – get your mechanic to do the job for you. It should cost around $100 for 1 hour of labor.
How to Prevent Oil Leaking From the Air Filter on a Twin Cam
We’ve talked about why this issue happens and what to do in order to fix it, but what can you do to prevent it from happening in the first place?
Well, apart from staying on top of the maintenance schedule and servicing the breather system regularly, the best way to prevent oil in your bike’s air filter is by installing a breather bypass mod (also known as an external breather). By doing that, you will pretty much fix the original breather design and vent crankcase pressure and oil mist into a catch can or back into the oil tank.
There are dozens of breather bypass kits available for purchase and we highly recommend them. Not only will you prevent oil from accumulating inside the oil filter, but you will also reduce carbon buildup, which will extend the life of the engine and slightly increase performance.
We have a dedicated article on the breather bypass mod – be sure to check it out if you’re interested in keeping your Twin Cam healthy.
Conclusion
Oil leaking from the air filter of your Harley Twin Cam can be a real annoyance. It can not only make your engine look dirty, but it can also lead to other engine issues, such as increased carbon buildup on the pistons.
Luckily, diagnosing the issue is very easy and so is fixing it. Aftermarket rocker box breather kits can set you back by $200, but you can also find the strainer element and everything else separately for under $50. There’s really no point in buying a whole new kit with billet housing, as the OEM one works fine – it’s just the strainer and the PCV valve that wear out.
Thanks for reading! If you have any questions, feel free to contact us or leave a comment below.
Ride safe and be free!
Nick Kowalski is a diesel mechanic by trade, but his passion lies in motorcycles.
He’s been riding them since he was just a wee lad, and he loves restoring old Japanese street bikes and ATVs in his spare time.
Nick is an adrenaline junkie at heart, and he loves nothing more than hitting the open road on his motorcycle.