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Home Repair And Restoration

Crankcase Pressure And Breathers

Job Date 7 March 2016

By Ren Withnell

While putting the 125 back together after adjusting the tappets I recorded this short video...

What happened is I started the bike up to ensure I'd reconnected everything correctly and that all was well. As soon as it started I knew I'd forgotten something as it was blowing almost like an exhaust leak. It only took a moment to realise I'd not refitted the inspection cover where the timing marks can be seen. 

So what's going on here?

An engine is essentially a glorified air pump with a couple of additional strokes that include an explosion and that's where the power comes from. But...hang on...the air pump bit is within the cylinder, between the piston and the cylinder head. Why does the bottom bit, the crankcase, also pump air? Everything outside of the cylinder is just there to make things work, not pump air surely?.

Indeed there's no point in pumping air around the crankcase. The crankcase is a sealed unit to keep the oil in and dirt, water and other nastiness out. As the piston rises and falls the underside of the piston is also going up and down. As the underside of the piston is within the sealed crankcase the air within is compressed and relieved as the piston whizzes up and down. 

There is quite a lot of air within the crankcase, certainly compared to the cylinder itself. As such the pressures achieved are quite minimal and as you can see in the video I can easily seal up the crankcase with almost no effort using my thumb or finger. Logically if the underside of the piston rises and falls the pressure will - slightly - rise and fall within the crankcase. The overall effect should be self cancelling, piston down = pressure rise, piston up = pressure drop. So why do we need a crankcase breather?

The pressures in the top part of the cylinder, the business end, the air pump end, are immense. No matter how well a piston is sealed by it's rings within the cylinder bore some of that pressure escapes into the crankcase. As such with a piston rising and falling thousands of times per minute the cumulative escaping gases would build up within the crankcase causing high pressure leading to leaks and limiting the ability of the piston to rise and fall easily. 

To combat this the crankcase is vented usually through a small pipe that leads to the airbox. Why the airbox?

Another reason to vent the crankcase is the temperatures within the crankcase get very hot. Hot oil gives off noxious gasses that need to be vented. The gasses that escape from the cylinder contain unpleasant contaminants too. Older engines used to vent to fresh air but this causes pollution so in these modern times the excess nastiness is vented to the airbox where it will go through the engine and be burned. While this does not produce fresh air suitable for twitty birds and bumble bees it does reduce the levels of nastiness that is emitted.

Reader's Comments

Bob said :-
A nice write up.
Apparently the Hayabusa gained 10HP by having the undersides of it's pistons gas flowed, the pumping losses in the crankcase being sufficient that a small change can make a big difference. If you remember the SLR650, that had an oil catch tank attached to the front of the airbox - the crankcase breather goes to the catch tank and a series of internal baffles catches the oil blown out of the engine and allows it to drain back into the cases.
It is worth having a look at what's coming out of the breather occasionally, as you say it's directly affected by seepage past the rings and excessive blowing of gas/oil can be indicative of a top end problem.
08/03/2016 08:53:29 UTC
Ren - The Ed said :-
Cheers Bob. 10bhp just from some clever piston shaping you say! Wow, that's almost as much as my 125 actually puts out.

What would you deem as "excessive"? While the pressure from my little 125 was very low I was somewhat surprised at the level of outflow. I'm sure deep within some Honda technician's handbook there will be some kind of figure, so many litres of air per minute perhaps and an impressive tool to measure this. All I know is the engine uses only a little oil, starts very well, returns good fuel economy and doesn't rattle like a pound of nails in a tumble dryer. I have to presume everything is within acceptable parameters.
08/03/2016 14:57:21 UTC
Bob said :-
I don't think it's something you'll ever be able to measure as an indicator of engine health. It's more important to check what kind of stuff is coming out of the breather. As you say, most breathers feed into the airbox, so if you find a build up of oily gunk in the airbox and/or the filter is getting black and oily (as opposed to grey and dusty) then that may be indicative of a problem.
The best way to be sure of an engine's health is a compression check. When buying loose engines I insist on a compression check using a battery and jump leads and an inspection of the camshaft. If there's no starter motor then just a camshaft inspection is usually enough. I've very rarely seen a poorly 4 stroke engine that hasn't taken out the camshaft before anything else - I think it's because above all else it's oil that kills engines, usually lack of it or else lack of changes and in that situation the camshaft is always the first to go.
09/03/2016 10:05:42 UTC
Ian Soady said :-
The other check you can do is a leakdown test - you need an old spark plug shell fitted with a Schrader valve. Get the engine at TDC on the compression stroke for whichever cylinder you're checking and pressurise it via the valve. You can then check with a pressure gauge whether it's holding that pressure (or more correctly, how fast it's leaking).

I have to confess I've never actually done this myself.......

Good points made all round. However, you haven't told us about 2 strokes which (usually) of course use that crankcase pressure to fill the cylinders.
09/03/2016 10:26:06 UTC
Ren - The Ed said :-
Hey Bob, I am wondering whether or not to get a spare motor for the CBF 125. Buuuuuuuut... Wellllllll... Hmmmmmmmm... I've owned the bike now for 3 or 4 years and I'm happy with it but once it curls up and dies I'd be tempted to get something else for a change. The only problem would be...which 125?

Ian you are quite right, I have omitted the whole 2 stroke community whose nasty little ring-a-ding smokers require that very crankcase pressure to operate successfully. While I do understand the merits and simplicity of the 2 stroke engine I am not a fan. There's a bunch of local lads whom worship at the alter of the stroker and we spend a lot of time saying rude things to each other...all in good taste you understand.

As for a leakdown test. Yes I understand the principle but as you say yourself Ian - it's something I've never done.
09/03/2016 13:51:11 UTC
Ian Soady said :-
Many older bikes (my Sunbeam included) had a one-way valve on the breather which tried to keep a negative pressure in the crankcase to minimise (note I didn't say eliminate) oil leaks. A bit like having negative pressure in biolabs so that any nasties don't escape to the outside world.

It didn't work.

Others like Triumph twins had a timed breather, usually on the end of a camshaft (pushrod engines remember). These didn't work very well either.

The best was on my Norton Commando 850 which was just a 1/2" diameter (13mm for you lot) bore pipe from the back of the timing cover to the back end of the frame with no attempt . Amazingly no oil came out of that at all but you could hear it sucking and slurping.
09/03/2016 15:14:30 UTC
Ian Soady said :-
If we had an edit function.....

I meant to say no attempt at a valve.

And there is a thing called a Bunn breather which has a valve and is often fitted to classic race engines.
09/03/2016 15:16:00 UTC
Ren - The Ed said :-
The idea of a negative pressure crankcase to stop oil leaks has some logic to it Ian - but and it is a big but - it may become possible for the engine to actually suck in nastiness such as water.

I've seen the long tubes on some older models, such as your Norton 850. If memory serves it would follow the curve of the rear wheel arch. Due to the rise and fall I suspect that would help keep rain out?

As for the edit function - you know why I don't have one. For anyone else reading it is because I do not wish to become involved with the complexities of accidentally having your passwords stolen if ever the BAT servers are hacked. I suspect if this website every actually becomes world-wide famous and I'm making millions of pounds in advertising revenue I might hire someone who actually knows what they're doing...
09/03/2016 17:19:14 UTC
Ren - The Ed said :-
Ian - I've just read up about Bunn Breathers. So the idea is that you have a pair of one-way valves on 2...yes 2 entries to the crankcase. One valve on one entry only allows fresh filtered air in, the other only lets the gases and blowby out. The idea is a stream of fresh air coming in and nastiness coming out?

The chap who wrote the page I read was quite surprised by the amount of pumping action even from the "not really a pump" underside of the pistons. I had a thought...if the outflowing side of the Bunn breather is connected to the airbox you'd have a mini turbo! Whoosh!

Of course I do understand the levels of airflow and pressures would be minimal but it does get me thinking and when I think like this I get stupid hair-brained ideas. DO NOT allow me to try this Sharon.
09/03/2016 18:01:13 UTC
Bob said :-
A crankcase breather powered turbo? Great idea, but you'd find the volume of air available is far too low to make an impact.
Did you see the article about the bloke who turned one of the cylinders of his Ducati V twin into a supercharger? The engine went from 70HP (twin) to 100HP (blown single)!
Don't discount the simple pleasure of a good two stroke. Note I said "good", because many of them are not. I have a KE100 which I love like a pet, it comes out to play in the snow and the occasional sunny evening bimble in the hills. The KE is wonderfully simple and due to its disc valve induction remarkable tractable and non-peaky. It's quite like an MZ in that respect. The primary compression ratio on a two stroke is very low, but then the cylinder compression is also very low due to the holes in the cylinder bore!
Dry sumps are an attempt to manage oil pumping out of the breather, most big singles are dry sump but interestingly Suzuki never went down that route and all their thumpers are wet sump, that said in my experience the Suzuki thumpers are not the most oil tight...
I've had a few bikes with the old garden hose under the rear mudguard breather, actually I had a couple set up as a chain oiler. This worked very well but it's effectiveness as a chain oiler was in inverse proportion to the health of the engine.

Spare engine - do it. If you like the bike then you can ensure it's continued service. I guess a lump for yours would be £200ish?
I have a spare engine for my X-Country, the bike was ~£3K and the engine £400.
A spare engine for £500 KE100 was £100 and for the £750 KLE500 the spare engines were £200 and £120 (I have two for this).
The trick is not to spend too much on the engine, when I was into the Honda FX/SLR650 the engines were £400ish a piece, which is too much given that the bikes are only £1K or so.
The KLE is a breath of fresh air, cheap to buy and because they sold loads there are loads of cheap parts available. GPZ500 engines also fit straight in too (which a change of alternator rotor).

Since I'm discussing my mechanical paranoia I'll also mention that I consider the minimum essential spares for any bike to be an engine, carbs/throttle body, CDI/ECU.
All other parts can be swapped / bodged but these are the unique parts for each bike which cannot be substituted.
10/03/2016 13:32:59 UTC
Henrik said :-
Good to see a fellow KLE500 owner, agree about collecting spares and making a long term strategy when being absolutely sure it's a keeper. Guess I will use the next two seasons to decide, unfortunately there is a problem with to
high consumption that I am fighting, at constant speed at 55 mph I only do
14-15 km/l where it was supposed to do 18-20, if I got this fixed I keep the bike, order custom seat, and add an auxiliary tank, for 400 km action radius.

I would definitely get a hole bike for spares, the later body-parts are much
smarter than my 1993-ones, and extra set of wheels would be nice for off/road

10/03/2016 20:37:05 UTC
Ren - The Ed said :-
I hadn't heard of the Ducati you mentioned Bob, but I have now! I'll add a link. Now...again you've got me thinking and that's not a good idea. CD200 Benly, reduced to 100cc but supercharged from the spare cylinder? Hehe!! The most efficient compressor though would need to just "suck-blow", 2 stroke, as opposed to "suck squash bang blow", 4 stroke. To achieve this the camshaft would need to be altered. As parallel twins typically have common camshafts this would be more difficult than a V-Twin. I could weld on some extra lobes...if I could weld that is.

That made me laugh regarding the chain oiler, "... it's effectiveness as a chain oiler was in inverse proportion to the health of the engine."

If I were to purchase a spare CBF 125 for parts it is quite likely the "donor" bike would be in better condition than the current one. As such it would be simpler to use the current bike as the donor! I am on the look out for a crash damaged one, that would make more sense financially.

Henrik - it seems the KLE is the machine to have.
15 km/l = 42 mpg.
20 km/l = 56 mpg.
While I can admire the merits of the KLE I would have to insist on 80 mpg (29 km/l). When Honda's CB500X and NC750X return 80 these days I'll be tempted to wait until the second hand ones drop in price. I do also like the CRF250L at 90 mpg BUT - and it is a massive BUT - it has a stupid silly nonsensical 7, yes SEVEN litre tank! What use is THAT!

I must admit I was at my local Honda dealers last weekend. They offered me an amazing deal on a new NC650S (the road version) and I was tempted, so very very tempted. I just can't imagine me on a new bike, I'd ruin it.
10/03/2016 22:32:08 UTC
Ren - The Ed said :-
That'll be the NC750S...Ian...maybe you're right we need an edit button.
10/03/2016 22:33:59 UTC
Bob said :-
Fuel economy is definately better with FI, my G650X goes like merry stink and returns 75MPG.
The KLE is not so good, Henrik the economy of your bike is similar to mine, or it was until I replaced the air filter, balanced the carbs, adjusted the tappets and fitted new needles and needle jets. Now it does ~50MPG and considering the stick it gets I'm happy enough at that.
10/03/2016 22:37:34 UTC
Henrik said :-
Thanks to you both, for my part I will keep the bike in case I reach around 60 mpg, and I will do a very héavy attempt, playing all strings there exist.

Also I am willing to accept a little decrease in power, 38T rear-sprocket will be tested soon, etc, etc,.. jets and needels will likely be replaced with the ones from later generation KLE's

As soon as I know its a keeper I will get a second ones like told, the idea having a spare engine is also that you can swift them quick if one goes down not having your touring plans spoild

Yet another good thing about a extra donor bike is that it would allow me to split the engines in pieces, and rebuild, something I still miss on my CV :-)

I just found this video on youtube, its easy, only 9 minutes :-)
10/03/2016 23:06:51 UTC
Bob said :-
I've watched that video a couple of times - very entertaining. I have always replaced camchains using a soft link, no need to split the engine, I could change the camchain in my KLE in two hours, start to finish, with the engine still in the frame. This is the standard method used at most motorcycle dealers. You can check for yourself - ask your dealer how much they'd charge to change the camchain, I'd be very surprised if any of them quoted the 8-10hours labour to do it the way shown in the video (they would have to remove and refit the engine don't forget), most will quote 2-3 hours.
Buying a spare bike is often the way to go. I bought a second KLE and for what I paid it was worth it just for the engine, carbs and CDI.
11/03/2016 09:12:27 UTC
Ian Soady said :-
Another blast from the past is the split single concept (sorry Ren they're 2-strokes).


A variation on the theme was the Wulf Norton double-diameter piston:


which in turn was an update of the 20s Dunelt.

Bob - although I hugely respect what you say, dry sumps were originally devised not to stop oil coming out of the breather, but to reduce the drag on the flywheel inherent in a wet sump design unless carefully designed. The historical sequence is roughly: splash lubrication (essentially wet sump with no pump), total-loss (where the rider pumps a bit of oil into the engine and it leaks out everywhere) the dry sump. There were few wet-sump designs in the 20s and 30s although my Sunbeam is an example.

Ren: Are there some tags I could wrap around links to make them clickable as otherwise I need to post > 1 time to include > 1 link?
11/03/2016 11:26:53 UTC
Ren - The Ed said :-
That guy in Henrik's video works a hell of a lot faster than I do! If I ever opened a bike shop I'd want him as my mechanic. He does make it all seem rather easy, the power of editing eh.

Ian, it seems there's little new under the sun. I *think* I have a handle on how the ideas work, using the pumping action of pistons to force feed the other. I still need to study these further though. Clever stuf but it just never seems to push through into production.

As for the tags Ian - when I get a moment to sit down I'll see what I can do.
11/03/2016 23:30:46 UTC
Bob said :-
Oh right, well all in all I find dry sump a royal pain in the rear, so for whatever reason they invented it I wish it would go away!
The simple luxury of being able to pick the KLE off it's sidestand and SEE the oil in the sight glass - bliss!
I love those old total loss bikes, wouldn't work for me though - I'd just ride along pumping oil all the time "if some oil is good, then more is better!" - I realise that isn't true but my mechanical paranoia would win over.
14/03/2016 12:44:55 UTC
Ren - The Ed said :-
Total loss oil system! I am aware of the concept but I can't imagine the reality. Like you say Bob my mind would say more is better therefore I would easily be found by simply following the massive oil slick along the road.

It is hard for me to imagine. While I'm 44 years young for all my life all I have known is that engines aren't supposed to leak. The idea of willfully throwing it on then watch it fly off the engine seems absurd now. But then...think of our chains. Daily I lubricate it and daily the lube ends up all over my back wheel and crankcase. The next day I repeat the process. Will some future biker one day scoff at my inefficiency and ineffectiveness as he prepares to ride his hermetically sealed eco powered 98% efficient velocipede? I imagine they will.

14/03/2016 15:17:39 UTC
Ren - The Ed said :-
Mind you when I was despatching my 1977 CB25T Dream leaked and burned around 200ml of oil a day. I stopped changing the oil after I realised it was getting fresh oil every couple of weeks or so.
14/03/2016 15:19:35 UTC
Ian Soady said :-
Actually, most of the oil is burned and you don't actually put that much in anyway (although I have no personal experience). I have an idea it would be around 5cc per 10 miles or so with a bit extra for hill climbing etc.

I had the opportunity to ride some real classic stuff on Saturday at the National Motorcycle Museum - including a Vincent Black Shadow, a 1928 Brough Superior that is estimated to be worth more than my house, and a 1914 New Comet - single speed, belt drive, no clutch (or effectively, brakes), lever throttle.....

The Brough and the Vincent were actually far too much of a handful on the short tight course we were limited to but it was a superb experience nevertheless.

Shots on me on some here: http://www.garydchapman.co.uk/soady#3
15/03/2016 15:07:35 UTC
Ren - The Ed said :-
Cheers Ian, fabulous pics! It's great to put a face to the name as well - you look like you ought to be in an episode of Inspector Morse as a Professor of the classics.

How on EARTH did you manage to wrangle a ride on such exotic and valuable classic machinery? I would think those bikes would remain in a climate controlled environment under lock and key, not taken out for a spin by random bikers, no matter how distinguished they might look. I assume you have friends in low places.

While I would never want to actually own those motorcycles I would relish the chance to experience them. To perhaps just get a tiny sense of how my own grandad would have ridden.

He told me once about a motorcycle with thumb lever throttle. The screw on top could be tightened such that the lever remained in position, much like a cheap cruise control. He would ride past a bobby directing traffic in Manchester town centre with his arms folded, mocking the PC. Until the day his narrow tyres got caught in the tram tracks that is.
15/03/2016 18:18:27 UTC
Ian Soady said :-
"How on EARTH did you manage to wrangle a ride on such exotic and valuable classic machinery?"

Actually it was very easy. I became a "friend" of the National Motorcycle Museum http://www.nationalmotorcyclemuseum.co.uk/ (costs around £30 / year). This allows unlimited free entry to the museum, plus access to various events of which Saturday's was one. Mind you it cost me an extra £5!

I must admit I was astonished that we were allowed to ride these bikes - there was no check on ability or attitude. In fact one of the participants was extracting the urine to some extent, scraping the silencers on the Triumph Grand Prix. He was warned......

Some of the bikes like the Velo and BSA twin were easy enough to ride but the earlier ones like the Brough (which apparently originally belonged to George Brough, the proprietor of the company) more difficult. The single speed New Comet was just weird. It had the lever throttle you mention although once on the move it was quite easy to control.

Both the owner of the museum (son of the late founder) and the director were there and obviously having a great time (but not as much as the riders).
16/03/2016 11:02:05 UTC
Bruno said :-
Just a comment on the "explosion"... please, don't call it that. Explosions harm the engines. Under pressure it's a combustion, which burns at a much lower speed. ;-)
26/06/2019 22:51:56 UTC
Ren - The Ed said :-
You are indeed quite correct Bruno. It looks and feels like an explosion to the untrained eye. However it is a very rapid burn under pressure.

Now, would you like to explain the technical difference between combustion and explosion?
27/06/2019 07:45:21 UTC
Ian Soady said :-
Combustion has a flame front that travels across the combustion chamber at a (n admittedly very rapid) finite rate. Explosion occurs when the whole of the mixture ignites at more or less the same time.

Pinking or detonation is an example of explosion......
27/06/2019 10:04:48 UTC
Bill said :-
Crankcase pressure, piston blow by,is measured with a manometer and is used on commercial diesel engines such as cummins to check the health.
On my 1952 AJS oil leaked past the pump and into the crankcase, if left for a length of time without starting you had to drain the crankcase via a plug before starting. It is a dry sump engine and its pumped back to the oil tank.

27/06/2019 17:48:07 UTC
Ren - The Ed said :-
As I have understand...

As burn is where heat from one molecule's chemical reaction causes the surrounding molecules to start their own chemical reaction.

An explosion is due to a shock wake of energy (pressure usually) causing the chemical reaction to happen in each molecule.

Cheers Bill. I figure there'll be specifications for crankcase pressure at certain revs for certain engines.

I've heard that dry sump motors can leak down the oil into the usually only a bit oily sump. What I don't understand is why this is a problem. I would have thought as soon as the engine starts the pump would drain the sump back into the tank where it belongs.

I await the collective wisdom.
28/06/2019 07:18:05 UTC
Ian Soady said :-
A small amount of oil in the crankcase isn't a problem. However, if it is excessive, wet sumping as it's known, one or more of several things can happen, including:

- the excess oil will be carried up into the combustion chamber laying down a smoke screen;
- the pressure will force oil out through the breather;
- in extreme cases you could get a hydraulic lock preventing the piston from travelling down the bore.

AMC singles like Bill's aren't noted for wet sumping as the reciprocating pump piston only allows oil from the tank to the crankcase in a small amount of its travel. I actually replaced the end plate with a perspex window on my Matchless G80 to check this but of course couldn't see what was going on through the mucky oil. Another pointless mod...

Nortons on the other hand with double gear type pumps are notorious. I have a tap fitted in the feed of my ES2 but also a lurid orange flag which I apply to the advance / retard lever to remind me it's switched off (only necessary when the bike's left for a few days). Roller bearing engines will in fact usually survive a few miles with no oil feed (he said hopefully).
28/06/2019 10:05:50 UTC
Upt'North said :-
Honda 750 sohc's were dry sump, can't ever remember them being a problem, my sohc f2 was fine and dandy. It was a pain to change the oil though, having to drain the tank and sump.
28/06/2019 10:33:50 UTC
Maurizio said :-
Isn't that hole in the flywheel cover? how cranckshaft pressure gets there?
Does an excessive pressure mean some gasket wear?
09/09/2022 13:23:02 UTC
Ren - The Ed¹ said :-
Hi Maurizio. Blimey! You had me questioning myself there. The alternator/flywheel on the CBF is wet, or at least semi-wet. There's no seal between the inner crankcase and the alternator. As such while the alternator does not sit in oil it does catch some mist etc from the engine. If you look at the image below you can see a little oil in the allen sockets.

In a perfectly sealed engine in good order there will still be pressure pulses. As the piston rises the pressure will drop, as it falls it will rise. In the real world the pressure will increase all the time with blow-by. This is what the crankcase breather is for. An engine in good order will only have a slight amount of blow-by, a worn out set of piston rings will cause a lot of blow-by.
Posted Image
09/09/2022 14:19:28 UTC
Maurizio said :-
Thank you, Ren!
13/09/2022 14:18:31 UTC

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