Posted in: Austin Healey 100 (BN1) Engine Engine Engine & Drivetrain Healey Stuff The Restoration of Healey #174

Austin Healey 100 Engine Oil Leak Solution

When I first got my very early Austin Healey 100, B.#174, on the road after its total frame up restoration I was very pleased that the engine was virtually free of oil leaks however, after a few hundred miles, it started to exhibit the “usual” oil leaks for which these engines are so notorious. Although the quantity of leakage was relatively small, oil had started to ooze from many places and in particular from the crankshaft rear main seal which on both 4 and 6 cylinder Healey engines is a “scroll’ type seal. I spent quite a bit of time checking all the fasteners and gaskets on the engine but my efforts were mostly in vain.

Once the system is installed the only part visible upon close examination is the vacuum connection at the balance pipe.

After pondering the issue for some time, and frequently cleaning the garage floor, I decided that the root cause of the leakage was crankcase pressure (blow-by) which is quite a normal thing in engines built before the advent of emission controls. Blow-by is caused by the small amount of air that leaks past the piston rings when the engine is running and the method used relieve this pressure in these Austin engines when they were idling was to vent the valve cover and crankcase to one of the air cleaners. The theory behind the air cleaner vent was that the very slight decrease in air pressure resulting from intake air passing through the filter element would be sufficient to draw the blow-by from the crankcase of an idling engine through the carburetors allowing the oil fumes contained within the vented air to be burned as they passed through the cylinders and out the exhaust pipe. To handle the larger volumes of blow-by that are produced when the engine is under load a “road pipe” was installed and this, in of itself, was the source of much of the mess.

Although engines fitted with these systems were reasonably acceptable 60 years ago, these days they would be considered to be gross emitters of pollution.

The first emission control system fitted by manufacturers around 1968 was the Positive Crankcase Ventilation system, PCV. This was achieved by routing all the crankcase blow-by directly into the intake manifold through a PCV valve. The PCV valve regulated the air flow into the intake manifold and prevented any positive pressure which may occur in the intake manifold due to a backfire from finding its way into the crankcase.

One of the unanticipated benefits of these PCV systems was that the slightly lowered pressure in the crankcase resulted in a significant decrease in engine oil leaks because, rather than engine oil oozing out through tiny holes, air would travel into the engine through such holes.

Based upon this I surmised that a PCV system may be a solution to the oil leakage issues of B.#174 however, as the engine was never designed with a PCV system in mind, devising a method of installing such a system took some thought.

After trying several of different prototypes this is the system that I finally devised and installed on the engine in February 2022.

There are hundreds of different models of PCV valves and after testing several I decided to use one which was all metal, installed in a thread and was designed to operate horizontally and more importantly produced the desired results.

The installation of this system required the removal of the carburetors, intake manifold and the engine road-pipe.

After the installation of this system, I connected a primitive manometer to the engine dip stick tube and took the car for a test drive to see how much effect it had on crankcase pressure. The results were quite satisfactory although occasionally, on overrun, very little if any negative pressure was evident in the crankcase so, in order to improve things a little, I installed a plug in the hose which runs from the valve cover to the rear air filter and this resulted in a significant improvement although may not have been necessary.

A small flange plate is fitted at the location where the “Road Pipe” was originally installed.

The result, after a few hundred miles of testing, is that the oil leakage problem was massively improved with virtually nothing leaking from the rear main seal and an engine and engine compartment which stay clean and dry.

There is no question that this has proved to be a very beneficial modification and is highly recommended.

I will be making a small batch of these systems for a few friends who have expressed an interest so, if you would like to try one, let me know.


After many inquiries I am purchasing and fabricating all the components required to assemble a small batch of PCV kits for 100 engines. These kits will be be ready for shipping in early April 2023 and will be priced and $CDN160.00 ea including North American shipping. If your 100 has an oil leakage problem caused by engine blow-by this will fix the problem and will be undetectable to all but the most discerning of concours judges.  Part # PCVKIT100.

Comments (4)

  1. Hi Mike,

    My wife has a 1953 Austin Healey, I’m in the process of rebuilding the Carbs I was interested in your PCV system and or the PVC information you used for this system.

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