Tech Tip: Triumph Crankcase Breather Mod For Racing

Tech Tip: Triumph Crankcase Breather Mod For Racing

I've been making some pretty steady progress on my Triumph 350 road racer build these past couple weeks. However, with what the little time that I do have I have been making some modifications to the crankcase to help the integrity, longevity and performance of the engine.

Today I wanted to share with you a quick little modification trick that many triumph 500 and 650 racers did. This also includes the factory prior to 1970 that also did modification to there race bikes. 

Lets dive in...


If you're familiar with Triumph twin cylinder models, then some of you might already know that starting in 1970 (and later) Triumph 500 and 650 engines featured a large gray breather tube attached to the back side of the primary cover / case. This breather tube allowed pressure within the crankcase to escape into the primary chain case then working its way out the tube.

The way it works is very simple. There is no crankshaft seal so it allows the engine to breathe freely into the primary. There are (3) small holes that also allow the engine oil to be shared with the primary and "level out" automatically.

Although this design was new on 1970 and later production models (including 750 twins) this design is actually nothing more than just a production change from what racers and modifiers used to do to their engine cases prior to 1970.

If you have read Stan Shentons "Triumph Tuning" book then this should sound familiar


Triumph T100R Daytona Breather Pipe Off Primary

Above is a photo of a Triumph 500 race bike. Notice the large primary breather pipe coming from behind the fairing?


Based off documents that I have read and factory racing photos, I've come to the conclusion that it took the factory many years to add this type of breathing system to the production engines.

With the need for more power and higher compression pistons (on and off the track) there had to be a better way to allow the engine to breathe. This method from 1970 and on was the best way and simplest way in my opinion.


Prior to 1970 triumph unit construction and pre-unit construction twin cylinder engines featured the breathing system that was attached to the back end of the inlet camshaft. Most of you may know this type of breathing as the timed breathing system or "camshaft breather".

This layout featured a crankshaft seal to prevent oil from entering the engine and getting into the primary side and vise-versa since it was basically breathing from the engine and camshaft.

This type of breathing system works for stock applications and should be left alone if you have a "stock" motorcycle.

The Mod

In a nutshell we are taking the 1970 and later design and adapting it to our pre-1970 engine cases. If you have 1970 and later Triumph twin engine then this mod is not for you as your engine already features this type of breathing system from the factory.

The benefits of performing this mod to pre-1970 cases is that you are allowing the engine to breathe freely instead of breathing at timed segments. This is ideal for racing purposes as pressure builds up faster then it is released.

The (3) small holes we talked about earlier allows your primary oil (shared with engine oil) to automatically "self level" it's self which means there is no real need to check your primary oil levels.

Now that you have an idea of how the breathing system works, now its time to perform the mod.

It should take no more than 5 minutes


Before we begin I wanted to express that this mod is suited for those who are racing or prefer this type of breather. I do not recommend in any way doing this on a standard road bike as there is no need to. Only attempt this if your engine is completely apart and you will be using your bike for "racing" or "competition". 

Tools / Supplies You Need

  • Reference point for hole drilling orientation
  • 1/16 drill bit
  • Hand drill or drill press
  • Center punch

Getting to work

The first thing that you need to do is to find out where the three holes should be drilled on your case. What I did for my crankcase was, I went on to eBay and I found a used crankcase and I found a close photo of the three holes and I mimicked it on my case.

*350 and 500 cases have the (3) small holes in different locations from the 650 and 750's due to the engine design

Guide Line On Crankcase


Once you have the correct orientation go ahead and draw a straight guide line to ensure that your three holes will be parallel


Crankcase Punch Marks



Get out your center punch and punch 3 starting holes where you want to drill. This makes drilling easier and also centers the drill bit. Again, try to make sure that they are parallel and equal distance... 


Crankcase Hole Drilling


Once you have everything set up you can proceed with drilling the case.

If you are happy the way that it turned out, you can consider the drilling portion done. Make sure there is no debris or swarf left behind

Additional steps

 Tri-Cor Breather Vent Stub

Next you will need to vent the primary. We stock a custom (Tri-Cor) vent stub that threads into the old inspection filler plug on the top of the case for unit construction models.

Using this alloy spigot will allow you to run a large 18mm I.D. hose.

You can also choose to make or customize your inspection filler plug as a breather if you choose to.

Should you need a vent stub, click here to purchase one.


Additional steps


Triumph Camshaft Inlet Breather Stub


The next few steps we do not have to get into detail because they're very straightforward, the inlet cam breather stub you can go ahead and plug it either will silicone or using a fuel line with a bolt on one end to prevent any air from going in or escaping.

*Do not attempt to remove the stub, leave the stub in the case - thanks


Triumph Crankshaft Oil Seal Removed


Don't forget that once you drill the three holes you will have to remove the crank shaft seal behind the engine sprocket then you done.

Side Note

If you choose to revert back to the stock "timed" camshaft breather after this mod, you can always undo your work.

Use silicone or JB Weld to fill the (3) small holes that you drilled, install the crank seal back on the case, and unplug the breather vent stub.

Thanks for reading

I hope you enjoyed today's Tech Tip blog post. If you would like to read more blog posts like this one, please check out our site blog section today.

If you have any questions please feel free to comment in the section below.

Thanks for reading and ride safe!


  • Russell Turner

    Hi Guys,
    for an accurate positioning of the three "level " holes Stan Shenton’s " Triumph Tuning " has a good description as follows;
    “level with the top of the lower run of the duplex primary chain”.
    Depending on the amount of chain wear can significantly alter this position so measure using a new or low wear chain.

  • Ray Trenter

    I have been running this mod on my 1963 TR6SS with ET ignition for more then 40 years and at this time for over 100,000 Miles. Street & trail. It has been ready to run anywere with in any 24 hour day. It is a great mod for any twin. Also a 1978 or later 5 speed trans.
    The 2 things that I do with this mod, use a car PVC ,it can blow out but not suck in and run that into carb side air cleaner. This helps keep trash from sucking into engine at low rpm. This also pulls a vacuum on engine and helps stop oil leaks.
    I worked for triumph dealers for more than 16 years. 62 to 78

  • Classic British Spares

    @D.C. Ryan – “For racing” – anyone racing an old Triumph, BSA or Norton is already putting there engine at risk… there is plenty of bikes to go around

  • D.C. Ryan

    This is a great idea if you are racing and change oil after every race.

    For the street, however, it eliminates the ability to use ATF or other clutch-friendly oil in the primary chain case.

    As one of the reasons for this venting system was to eliminate the old “road draft tube” under the crankcase for USA “ecology” reasons, I don’t recommend making the modification on bikes this old (and now, valuable.)

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