Crankcase Ventilation

Being a mechanic, not an automotive engineer or lawyer, I suggest you treat the message below as something to think about, not as absolute wisdom or legal advice.

Two reasons for crankcase ventilation (a system of positive, purging air flow through the engine):
1) Purging water condensation from the crankcase. We all have seen the nasty effects of non-vented crankcases; a thick whitish paste of oil and water mixture, most commonly seen when removing the oil filler cap. In severe cases, the area on the top of the cylinder head – rocker arms, valve springs, etc. – may also be covered with white glop. Very bad – water retention in the crankcase oil ruins lubricating qualities.

2) Eliminating pressure buildup in crankcases. I’ve personally seen this on several LBCs in the last few years, especially on small British engines where part of the original ventilation system has been removed. Most often, as our cars pass from one owner to another, the emission devices are missing and many of us don’t understand, or think about, their original functions. Air pumps are almost always missing from the LBCs where they were original equipment. Environmental issues aside, that doesn’t hurt engine operation.

Worse, the original PCV (positive crankcase ventilation) valves have been removed from engines. Worse yet, the original hose plumbing has been removed that carries the crankcase air from the separator can (mounted on the timing cover on BMC/Leland ‘A’ engines) through a hose to a forked fitting and then to each carburetor. Those twin hoses are the low pressure source that causes air to flow through the crankcase. Eliminated, you’ve got serious problems. The cars I’ve mentioned had no breather opening on top and developed enough crankcase pressure to blow oil out of crankshaft seals and elsewhere. And the British system that depends on a ‘reverse scroll’ thread on the crankshaft to keep oil from passing into the bell housing is worthless with crankcase pressure. Messy!

Let’s look at some history of ventilation systems. Even in the beginning, the necessity of air purging the crankcase of water vapors was understood. In the twenties (and probably even before) through the early sixties, engine air flow depended on a simple road draft tube. A metal tube, connected to the crankcase, was brought down near the bottom of the oil pan. The rear-facing half of the tube was cut at an angle, so that forward motion of the car created a small amount of reduced pressure, compared to the atmosphere. Somewhere near the top of the engine there was a large oil filler cap, open to the atmosphere, stuffed with horsehair or something similar. Forward motion of the car caused air to enter at the inlet and purge to the atmosphere out of the low vent.

In 1963, Federal law mandated that oil fumes no longer be vented freely. The industry used PCV valves to draw oil fumes into the intake manifold, to be then consumed in the combustion process. Air entered the system in the air filter housing, on the filtered side of the air filter element. I always remember a good customer and friend who rushed out and bought a new, leftover 1962 Mustang, “PCV valves! The government is going to ruin our cars”! Well, he was about 10 years premature with that prediction.

The point of all this is that good crankcase ventilation is essential and does not hurt the engine or cause power loss. Just the opposite – not having a nice big crankcase venting system will cause big problems, especially on racing engines run at very high speed. On my modified 1275 Austin engine, I used the OEM vented style oil cap and ran a hose from the separator on the timing cover to the front of the K&N air filter – this gave acceptable air flow and was probably the very minimum acceptable. A larger air breather on the valve cover would have been better. Something as large as the lump most of us are familiar with from old pre-emission Chevy OHV rocker covers or oil fill pipes would be ideal, but don’t forget that valve cover internal baffling is essential to prevent oil from rocker arms (or cam lobes, if an OHC engine) from being thrown into a breather or PCV valve when mounted onto the valve cover. PCV valves are great, but getting American replacements for small foreign engines can be a problem.

On racing engines with radical cams, idle vacuum is iffy and at heavy loads with multiple carbs the intake manifold atmosphere is a series of wildly fluctuating pressure waves, cycling dozens of times per second. With that sort of condition, the best way to pick up a low pressure area is from the air flow from near the carburetor/s inlet, but not directly in or restricting the carb inlet/s.

Finally, a single crankcase vent outlet is not a ventilation system if the vent is pinprick small.

By Wil Wing



'Crankcase Ventilation' have 11 comments

  1. November 1, 2013 @ 5:06 pm John Borchardt

    I thought the first Ford Mustang came out in 1964. Is “a leftover 1962 Mustang” a misprint?


    • November 4, 2013 @ 11:29 am stuursmad

      If I hear back that it should be 1972 (and chances are…), I’ll make the change. Good catch John, thank you.


      • August 4, 2015 @ 3:38 pm Olmon

        Since the Mustang was introduced in 1964 as an early 1965 model, (All were titled as 1965s) & starting in 1966 all American made cars had PCV valves, I don’t know what year would be correct for your article, but 1962 definitely isn’t correct


    • April 7, 2016 @ 10:10 am Rich Timian

      1964-1/2 was all titled 1965’s


  2. November 19, 2013 @ 7:11 pm tyler irwin

    I have oil vapor blowing out the valve cover vent at high speed. What do I do about it?


  3. April 29, 2014 @ 8:41 pm steve korbay

    I disconnected the crankcase vent pipe that went from the side of the block to the rear carburetor air cleaner and rerouted it down to the side of the pan to exit below the engine.. I have a ’64 Austin Healey and did this to prevent the excess water that accumulated in the air cleaner. Now it just vents to the underside of the engine much like the overflow tubes from the two carburetor float bowls. Am I missing something? I could not find anything in the literature that indicated that this would cause problems other than minor amounts of emissions being placed into the atmosphere. I have never observed any oil or water coming out of the vent pipe.


  4. December 7, 2014 @ 4:53 pm Peter De Bever

    Crankcase ventilation is simple. Air in via the oil filler cap or a pipe on top of the valve cover. Air out via the tappet cover up to a pcv valve or to a port on the carburetor(s).


  5. January 20, 2015 @ 2:04 pm Tiaan

    Hey Guys,
    I need some advice please, my 1275 mini is blowing black smoke and oil from crankcase breather(which only has a pipe connected and runs down the back of the engine)…

    What is causing this and what is the solution to get this fixed? Some said that I should replace the rings, others not… I also lost a bit of power and the temp is up since I had this issue…

    Thanx in advance!!


  6. September 7, 2015 @ 9:18 am Mike Powell

    TRIUMPH TR4A crankcase ventilation ” Emission Control ” . All a bit confusing, the moss catalogue (UK) says on page 21 that the TR2-3A,TR4 has “open circuit breathing” then on page 47 says its a “closed circuit breather system”. Bill Piggott maintains, in his excellent book on TR’s, that the TR4A has a “closed circuit breathing system”, in another of his books he says that the air intake is via a vent hole in the oil filler cap. The original TR4A oil filler cap reference 143393 NLA had an air vent hole & its replacement GZC1400 (Rimmer Bros) has to be drilled (1/16″) by …guess who ? I agree with Peter De Bever, keep the PCV valve (its easy to maintain & parts available from Rimmer bros) & don’t discard the original filler cap with its vacuum avoiding hole….its original & it works. Nice article from Wil Wings
    Cheers Mike Powell


  7. February 25, 2016 @ 4:28 pm Chevy beaver

    Hey I need help. I built a 350, done good until 4000 miles. Now it’s blowing oil out of intake breathers. I don’t have PVC or anything on them there just little breathers. What do I need to do. My oil is a thick white color


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