Classic Mini A-Series Engine Breathers - The Lowdown

Pressure occurs in the crankcase of an A-Series, as with any internal combustion engine, via piston ring blow-by. In other words, pressurised gas from the combustion process leaks past the piston rings and into the void below where the crankshaft rotates.



Blow-by levels are worse on worn engines, and also on competition engines where greater clearances between piston and bore are employed, in combination with higher RPM and compression levels. You often see race Minis and Sprites smoking from the exhaust while trundling around the paddock, only to clear when up to full operating temperatures.



Excess crankcase pressure can lead to oil pushing past the crankshaft seals - good for rust prevention perhaps, but less kind on clutch plates, your driveway or the environment. So the correct functioning of any classic Mini's breathing system, or crankcase ventilation system, is vital. 



The breathing requirements for any A-Series engine will obviously vary depending on the performance level. Until 1964, standard Minis vented straight to the atmosphere through an outlet pipe on one of the tappet chest inspection covers at the ‘back’ of the block. In addition, the rocker cover area to the air filter, as above.

In 1964, to reduce emissions, a Positive Crankcase Ventilation (PCV) system was introduced on the Cooper and Cooper S, which had twin SU carbs. With a PCV system, all unburned gases are returned to the induction system, sucked through the carb and burned along with the fuel/air mix.

 
An oil separator, to remove larger oil droplets from the mist, is fitted to the engine in place of the open pipe, and the separator linked by a hose to a one-way (PCV) valve, mounted on and feeding into, the inlet manifold. A few export market single-carburettor Minis were also fitted with the PCV valve and system.

In 1969 the PCV valve on the inlet manifold was deleted and all Minis were fitted with a tappet chest oil separator with a pipe leading into the SU carb body with a Y piece used to feed twin carbs (see pic 5, below).



When tuned to higher performance levels, the A-Series engine can demand a greater breathing capacity, highlighting the limitations of the standard PCV system. With these engines, a higher crankcase pressure can cause the fuel/air mix to become diluted by excess oily residue.

Add to the mix different aftermarket inlet manifolds, and/or carburettors without the provision for a breather system, and many Mini enthusiasts choose to install small crankcase filters directly to the engine breathers, or look towards a more race-orientated catch tank setup. 

These return to a 'vent to atmosphere' breather system, with any oily residue captured by either a mesh filter or a catch tank with mesh filter.



As to the question of 'How large do my A-Series engine breathers need to be?', it depends. For full competition use we tend to run 25mm/1" breathers from the transfer housing to the rocker cover and back out to an oil catch tank in the inner wing area. 

For road use, especially on more mild engines, this would likely be overkill, but some like to replicate the race-style aesthetics! 13mm diameter breathers would usually suffice on tuned road engines.   

But let's take a look first at the various engine breather systems used on the Mini over the decades, so you can decide on the best setup for your particular application...

1) An early 850 engine, more specifically 621 AOK, the first Mini. The only visible part of the breathing system in this picture is the hose connecting the outlet pipe on the top of the rocker cover to the standard air filter housing. The downside of this system is that the air filter tends to get dirty faster.

2) The other component of the early open breathing system is the front (radiator end) open vented tappet chest cover. See picture 8 to see where it fits on the block.

3) The Positive Crankcase Ventilation system, which was basically a closed breather setup, was introduced to the twin-carburettor Minis, starting with the 998 Cooper and 1275 Cooper S. On these engines an oil separator was added to the outlet pipe on the tappet cover and a rubber tube then took any fumes to the inlet manifold via a one-way PCV valve (centre of picture).

1275cc non-S twin-carburettor engines fitted to the 11/1300 range used the same valve with a flywheel housing mounted separator.

4) A late 1970s single carb 998 engine – this one is modified with a later HIF carb. The neoprene pipe running along the top of the rocker cover from the oil separator on the rear of the block and directly into the carb body is the main difference. Excuse the rust!

5) A Mk3 Cooper S with twin carburettors, both linked to the breathing system. On later model twin carb models the PCV valve was dispensed with as per the single carb engines.

6) Early open breather rocker cover caps were solid, as this one. Later closed system caps incorporated a breather, by way of a small hole and gauze filter. The later caps should theoretically be replaced every 12,000 miles.

7) Older breather pipes are often a very loose fit to the carb, which will result in a lean mixture if left unchecked, drawing in air. Be careful.

8) The engine block style fitted to the majority of pre-1990s Minis. This is a 998 block but all small bore i.e. 848, 998 and 1098cc engines have tappet chest inspection covers fitted – they are removed in this picture. The large bore Cooper S Mark 1 & 2 (all capacities) and some 1275cc Mark 3 also had tappet chest inspection blocks.

9) If you are upgrading or changing your engine, the capacity will have an influence on the breathing system required. This is a large bore 1275cc solid wall block with no tappet chest inspection covers. All non-S blocks are of this type, as are some Mark 3 S blocks - of which incidentally, this is one.



10) Small bore flywheel/idler gear housings are non-vented. The mounting area for a breather canister is still cast into the housing but is un-drilled.

11) Most 1275 non-S engines were fitted with a breather on the transfer gear casing. The casing was drilled and tapped and there was a hole in the middle of the separator mounting point.

Non-breather casings all have the mounting cast into them and can be modified by drilling a hole and drilling and tapping two mounting holes where necessary – many casings are already drilled and tapped. Do not attempt this in situ, as the swarf will drop down into the engine oil.

12) This was originally a solid 998 A-plus casing which has been drilled and tapped. It is fitted with the more common type of oil separator, with a Pipercross crankcase filter on top. A ‘lay flat’ design of the same canister was fitted to 1990s 1275cc Minis to clear the brake servo.

13) Some 1275 non-S engines were fitted with a breather on the timing chain cover. If you are fitting an A-series non-S 1275 to a smaller-engined Mini, it will probably have a non-breather timing cover and no tappet chest inspection covers.

14) To fit a breather here you will need an A plus timing engine plate with all the A plus timing gears and breather cover. Alternatively, why not upgrade to the MED Alloy Timing Cover kit, with built-in 1/2" breather outlet.

15) The standard MG Metro timing cover breather fitted to a Mini, and the breather heading back to the inlet manifold/carb.

16) On standard Minis, the breathing system will have to be tailored to the carb(s) fitted to the engine. Early SU carbs had no provision for connection to the breather system. This continued after the PCV valve system was introduced.

17) Later non-breather pipe SUs were clearly designed with the option of fitting a breather tube in the future.

18) This developed into the carb body being bored to take the tube but then blanked off. Carbs such as this can be modified to take a tube which is ideal if twin SUs are being fitted to a 1970s-on carb Mini and a period original look is required.

19) This is an original PCV valve from 1968. These valves are a nice period touch on a matching-numbers show car. They do require periodic maintenance but the parts are available. 

20) PCV valves are easy to dismantle: just one spring clip. Internal parts are easily cleaned. A new diaphragm is shown here. 

21) If you have the type of carb shown in picture 17 it is quite easy to remove the blanking plug and insert a breather tube. The plug should be drilled with a small diameter drill initially. We recommend that this is done before the carb has been restored! 

22) The drill size should be increased a couple of times. It is important not to damage the carb body. The remains of the plug can be eased out with a screwdriver. 

23) These are the breather tubes needed, OE part number AUB502. 

24) With everything cleaned up, the pipes can be tapped gently into position using a piece of wood to avoid any damage to the tube. 

25) A pair of modified 1 ¼ inch SU HS2s - ideal for a standard 1275 or a fast road 998.  

26) If originality is less important to you, and where legislation allows, the simplest way to overcome most engine breathing dilemmas is to have a breather system venting direct to the atmosphere.

This avoids diluting the fuel/air mixture of the engine on a more modified setup - if applicable. 


27) The downside of simply attaching a catch tank filter on your breather tubes, is that you can find an oily residue misting around the engine bay. This can vary from mild to severe depending on where you attach the filters, the health of your engine and how hard it is driven.

28) We keep various breathers for the A-Series engine, including the fuel pump location on the back/side of the block if you have converted to an electric fuel system. We would attach an oil line here with stainless steel over-braid and feed to a suitable catch tank, or back to the rocker cover - stainless steel as this fits close to the exhaust manifold.

We would not advise fitting a breather from here straight to the carbs, if planning a closed breather type system, as there will be a fair amount of oily mist here.

29) For the top of the transfer gear housing, we keep this neat billet alloy pipe adapter with a 25mm diameter. Some choose to fit a small crank case filter directly to this outlet, but again we would not recommend doing this, as the gears below tend to spray up a fine oily mist.

29) Instead, we prefer to feed this back to a specially-modified rocker cover via a 25mm oil hose and 90-degree fitting. This recirculates any oil residue back into the engine.

30) With this race type setup, feeding all of the breathers back to the rocker cover is a rather fruitless exercise in itself - the cover should, in-turn, feed back to an auxiliary catch tank with filter. For competition Minis, in the UK at least, you must feed any open engine breathers to a two-litre oil catch tank, to avoid spreading oil around the track.

31) The MED race breather kit comes with a tank that's specially shaped to contour the inner wing, freeing up engine bay space. Granted this will not suit all Minis, but it's a neat solution that's ideal for more competition-orientated cars. 

Another benefit of the catch tank system over the standard PCV, is that you'll have a measurable indication of how much heavily your A-Series engine is breathing by how much oil is accumulating in the tank. Full of oil? There is certainly a problem!