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MG Midget and Sprite Technical - Crankcase breathing - Advice please.

Sorry for returning to an oft-discussed topic but I don't think my particular query has been covered (recently!).

My rear main oil seal has become a bit dribblesome. I have been contemplating a Peter May style up-rated oil seal but I have seen mixed reviews on these, plus I don't want to take the engine out unless I really have to.

Perhaps reinstating a vacuum-based breathing system might help.

My car is a 1967 1275 Sprite Mk IV.

At the moment the hose from the oil separator goes to a Y connector where it is joined by a hose from the rocker cover to vent into a catch tank. I believe the vented rocker cover is non-original.

Which would be the better option? Fit one of the flat-topped emission control valves or go for the simpler hose to a Y connector between the carbs with a connection to each one. I can't find any detailed pics or drawings of the Y pipe arrangement - how do they connect to the carbs or inlet tract.

I would block the rocker cover vent pipe and it already has a breathing oil filler cap.

Or do I need to bite the bullet and whip the engine out?

Any helpful advice gratefully received.

Colin Mee

Lawrence...... where are you??
Graeme Williams

I did wonder about trawling the archive for contact details!

I also wonder whether the PO set up this system to get rid of a smoking problem!
Colin Mee

<<go for the simpler hose to a Y connector between the carbs with a connection to each one. I can't find any detailed pics or drawings of the Y pipe arrangement - how do they connect to the carbs or inlet tract.

I would block the rocker cover vent pipe and it already has a breathing oil filler cap.>>

This is the final version of the development of an effective crankcase pressure control / breather system on the A series. The transition from a passive breathing system which worked well in earlier (lower powered) versions of the A series engine, through progressively greater degrees of positive extraction systems matches the potential of the higher powered, higher compression and higher revving engines.

The Y branch should connect to two stubs on the upstream side of the butterfly disk valve on the two carbs. Or it can be fitted direct to a single HIF carb. The point is that upstream of the butterfly valve the suction is less on over-run and at a max on WOT, so no PCV valve is needed.

The other important element of this system is that the amount of ventilation INTO the crankcase is properly controlled and this is done by closing all openings, including use of a sealing washer on the dipstick tube, and then to use the correct ventilated filler cap. A properly set up system will then ventilate the crankcase, but also provide a degree of negative crankcase pressure which will reduce or minimise oil leakage from the rear crankshaft scroll.
Guy W

Incidentally, I believe the chronic oil smoke condition that Lawrence so thoroughly researched was principally down to an excessive build up of oil in the timing chest, rather than specifically a fault with the breather system as such.

Under certain conditions the oil level rose to such an extent that the suction on the oil breather system then took over causing the dramatic oil smoke problem. But it wasn't specifically a fault of the extraction system employed.
Guy W

Thanks Guy, that's quite reassuring.

I did read quite a bit of Lawrence's thread but I don't think it answered my question about which option to choose.

When you say 'upstream' do you mean before or after the butterfly? I would expect the tapping point would be after it, ie. 'downstream'.

There is a small spigot on the front side of my front carb which is sealed with a plastic cap. I've always assumed that is for the distributor vacuum pipe which my distributor doesn't have. There is no spigot on the rear of the rear carb.

For the Y pipe set-up should there be stubs or spigots on the black spacers between the carbs and the manifold? I haven't been able to find any in any of the online catalogues.
Colin Mee

this is a good pic as it show a plastic rather than bling oil cap on the replacement rocker cover

Nigel Atkins

below shows the plastic oil filler cap you want to use - it has better venting than a bling one and incorporates a mesh wire filter - check you have this type of plastic cap,

Nigel Atkins

Thanks Nigel.

That's the cap that I've got. The rocker cover also has a 'snorkel'.

I've also seen the picture before but I just can't see how the pipes fit on the carburettors or inlet tract as there doesn't appear to be suitable points on my car.
Colin Mee

Nigel's photo shows what you need.
By upstream, I mean on the carb (air cleaner) side of the butterfly. So no, not into spigots on the black spacers, which would be downstream in the air flow through the carbs.

The running engine provides suction, drawing the air/fuel mixture in from the carbs and passing through the inlet manifold. The amount of suction increases with higher revs. (which lifts the chamber pistons and the jet needle up thereby increasing the fuel and air for the faster spinning engine). On over-run, when you take your foot off the throttle the butterfly snaps shut so that no more fuel/air can be drawn in. This "strangles" the admission of air whilst the engine is still sucking, resulting in a sudden drop of pressure in the manifold.

This is exactly the wrong time to maximise suction on an oil breather system! By comparison, if the breather pipe is connected upstream of the butterfly then the suction is at its greatest when the butterfly valve is fully open and the engine running on full power. When the butterfly closes the pressure in the carb body increases and suction on the breather reduces. Exactly as is wanted!
Guy W

Ahh, Colin earlier carbs didn't have the connection points on the carbs. Yours may be of this sort, in which case the design of the extraction system is different, - and less efficient.
Earlier versions can be thought of as less effective attempts at producing a negative crankcase system and were probably not as good at combating crankshaft oil leakage.
Guy W

Oh, and make sure the hose you use to the y-piece is either vacuum standard or thick walled (the 1/4" hoses are thick enough wall as standard). The hose to the other type of breather valve assembly is a smaller size so don't get that.
Nigel Atkins

On earlier 1275 cars, without spigots on the carbs, suction was provided by a connection direct to the top of the inlet manifold. But this provides far too much suction so a PCV valve is needed between the oil breather pipe and the manifold. For the reasons explained earlier this method is less efficient as the peak suction is really provided at the wrong times, though it still works better than a passive breather system in controlling rear crankshaft oil leakage.
Guy W

Thanks for the explanation, obvious when you think about it! That also explains the slight differences between my carbs and the one in Nigel's photo, plus the need for the PCV on the older system. I guess that's the route I'll have to try. My manifold has the PCV spigot in place and capped.

Thanks too for your comments. The hose I've got is quite thick-walled so I think it will be fine - it was sold to me as engine breather hose!
Colin Mee

I think the first photo is the LS photo!

sounds like you have an earlier (?948/1098?) rocker cover that had the pipe from the rocker cover to saucepan air filter.

You can see it in this chassis plate diagram.

(BTW IIRC 10mm breather hose for the valve type(?))

Nigel Atkins

Beginning to think my Sprite is more of a mongrel than a pedigree!

I actually already had the PS photo open on another tab as a result of a bit of un-productive Googling!

I've always assumed the rocker cover was non-original - it's the same as the one on my Minor which connects to the back of the air filter casing.
Colin Mee

The best thing I did to stop leaks (after a disasterous PM rear seal try out!) was to have three breathers, or four if you count the cap. Basically I've got the black plastic cap, a pipe from a take off on the rocker cover, a pipe from a take off at the fuel pump blank and the usual one from the timing cover. All three go into a catch tank. The thing I think that made the biggest difference was cutting the top off the timing cover canister and removing the gauze (and loads of black crap!) before brazing back up.

Now I get very little oil leakage and nothing in the catch tank. Also I know there are no crankcase emissions fouling up the mixture which can be the case if connected to the carb.

When mine was a pure road car the closed system was fine but as soon as I started using more revs the three breathers seemed the best way to stop the leaks.
john payne

I thought you'd already got a relevant copy of the Drivers Handbook, that would show you what is original on the car.

Or a copy of Terry's book to cover original cars and see where the mix 'n' match bits might originate from - Original Sprite & midget The Restorer
Nigel Atkins

I only have a digital copy of the Good Book for the Mkiii Sprite. I'll be looking for the right edition at Race Retro. I don't know the Terry Horner book so I'll keep an eye out for that one too.
Colin Mee

There seem to be two distinctly different approaches to the engine breathing problem.
One way is the open "passive" breather system where the aim is to prevent the crankcase and rocker cover from building up any excessive pressure by having as many open vents which breath OUT to the atmosphere as possible, without allowing oil to splash out. Often breather pipes are taken to the air filters to minimise oil vapour engine bay and oil smells in the car!

This works well enough but as power outputs and revs increased then the amount of blow-by at the pistons increased with a tendency to create positive pressure in the crankcase,forcing oil out of the rear scroll seal.

The later designs attempted to positively extract fumes from the crankcase and to attempt a degree of negative crankcase pressure to combat leakage. To achieve this the principle breather flow is reversed, whereby air is drawn IN via a controlled oil filler cap breather and sucked OUT by the induction system vacuum.

Both systems work, but cannot be mixed as they distinctly differ in the principle of the method used.
Guy W

a paper copy is best but you can get the MK4 in digital format - at least you can on the clunky DVD that covers a wide range of factory Spridget publications.

The Midget handbook (Ref: 0057 here) will cover all that's in the Sprite Mk4 -

Or if you are particular about having the Sprite version I can look up the number for you on my clunky DVD.
Nigel Atkins

<<One way is the open "passive" breather system where the aim is to prevent the crankcase and rocker cover from building up any excessive pressure by having as many open vents which breath OUT to the atmosphere as possible>>

Indeed, that is what John is doing and what many racers do.

With a race car, you don't want to dilute the fuel charge with oil fumes, so you vent to a catch tank.

The photo below shows the breathers on my racer - a very large hose from the timing cover and a smaller one from the rocker cover. Some people use more vents and larger hoses.

Dave O'Neill 2

Here's a pair of HS6s showing the brass tubes that the breather pipes connect to. On a Midget, they would be inboard.

Dave O'Neill 2

This is a chrome plated version of 1275 rocker cover, showing that there is no breather, only a boss where it would otherwise be.

Dave O'Neill 2

If you do fit a seal kit, this is the best we have ever used, it requires some machining and tapping but is very professional.
Peter Burgess Tuning

Thanks for the link Peter. I'm going to try the PCV route first but that solution looks really interesting.

The machining would be beyond me but is it something any good machine shop could do or would it need to go to a specialist?

Would the engine need dismantling or could the drilling, tapping and 'smoothing' be done with it assembled?

Is it the sort of work that you undertake?

Colin Mee

Hi Colin
The rear main bearing cap has to come off, ideally the machining is done with the engine fully disassembled.
Peter Burgess Tuning

I noticed that no one has mentioned, that with both types of positive ventilation systems, carb ventilation, and PCV type, that the oil filler cap is a service item and should be replaced every 12,000 miles, and that goes for all BL vehicles with this type of oil cap.
Andy Tilney

Oh, Andy, there was a verrry long discussion about that on a previous thread, note my photo shows the end cap lifted to give access to the wire mesh filter.

I (now) think if you fit a new plastic, vented and filtered oil filler cap and you're engine is in reasonable condition and running then you wouldn't often need to clean the filter, or if you can't be bothered with cleaning replace it.

Be careful where you buy the cap from, I've just checked again and MGOC Spares have them at 3.10 (which I think is less than the last time I bought one from them) and B&G at 12.07 and those two at least were the same item as I've seen both.
Nigel Atkins

There are at least 2 versions of the vented plastic filler cap. The correct one, and there has been a cheap copy one on sale which has a smaller section ventilation hole.
Guy W


I decided to try the PCV route first and managed to get hold of a second-hand one with a new spring and diaphragm. I finally got round to fitting it this weekend.

First up, the brass bolt that a PO had used to block the adaptor on the manifold would not undo, so off it all came. Of course it wouldn't unscrew - it had been braised in place!

Fortunately the replacement came with a used but perfectly usable adaptor. After putting it all together a few times due to realising that I had forgotten something several times along the way, it went together-ish! The hose to the PCV is fouled by the rocker cover snorkel! There was just enough flexibility in the hose (very thick brake servo hose Nigel!) and, for the time being, a hose from the snorkel to my catch tank.

It started it up beautifully once there was enough charge in the battery and it warmed up nicely.

Lo and behold - there was blue smoke where there's never been blue smoke before! I guess the PO hit Lawrence's oil sucking problem when the engine was re-built at 86,000 miles, hence the bizarre plumbing that it came with. I have a relatively new vented oil filler cap in place but, with the open-ended hose on the snorkel I wonder if it is breathing just a bit too freely.

The weather was awful so I haven't had a chance to try it on the road to see if it has cured the oil dribbling habit but time will tell.

If the PCV solves the original problem I'll be looking for a rocker cover. I suppose I could always turn it round and block the snorkel if I get desperate. I was surprised to find that the alloy one that I looked at recently was heavier than the steel one, is that the case for all cast alloy rocker covers?

In the meantime, I have a couple of ideas still to try and I'll report back in due course.

Colin Mee

Colin, can you determine if the rocker cover "snorkle" is breathing out, or sucking air in?If its drawing air in, then together with the inflow of air through the breather oil cap, it may simply being pulling too much air through the crankcase and collecting excessive oil fumes on route.

I think - 'though I am not sure of this - that with a PCV valve you wouldn't normally have a breather from the rocker at all. As a test you could just plug it with a cork and see what happens. (you may find you have to drink a bottle of wine to achieve this).

Colin, for the system to work properly the 'snorkel' needs to either be sealed or part of the ventilation system. Also, don't underestimate the dipstick hole. That needs to be sealed also.

The only hole that should admit air is the hole in the filler cap. This controls the level of vacuum inside the engine but only when all other ways for air to get in are sealed.

Of course, you can't seal the crank scroll seal but that is the general idea. Create a vacuum in the engine and ONLY the scroll seal is open hence any oil will (should) be sucked back in.

If you are getting blue smoke it's from oil vapour. The oil sucking problem will throw clouds of white smoke out of the exhaust.

If your catch tank is sealed but able to vent pressure to the atmosphere, why not make it part of the breather system by sucking the air out of it instead of directly from the engine. This should condense any oil vapour which will fall into the catch tank.

MG Moneypit

My DIY sealed catch tank, plumbed into the hose between the chaincase breather and the carb.

As Rob's suggestion


Guy, that's exactly what I have in mind - I've just been rooting through the shed and garage to see if I have any suitable containers to adapt rather than splashing out on a super-duper Demon Tweeks jobbie! I just might have to treat myself to a tin of decent coffee! Is there any correlation between the price of the coffee and the effectiveness of the catch tank?

Do you know of any 'high street' suppliers of stainless steel wool? I'll probably try ordinary steel wool or a kitchen scourer to help with the oil condensation for starters.

Rob. It was as I was typing my first piece today that I began to think that, with the snorkel hose being effectively open to the atmosphere, ending as it does in an open-topped catch bottle, perhaps there was too much air flowing. The aim of the exercise is to generate suction in the crank-case and, as you point out, reducing the intake to just the oil filler cap vent should significantly increase the suction - the dip stick has a good rubber grommet on it but I will inspect it more closely to make sure.

Thanks for all the advice.

Colin Mee

Colin, I used to rebuild the breathers on Montego Turbo engines using pot scourers, stainless steel ones, like this:



Inside, the extracting nozzle has a long tube down to below the scourer which is suported off the bottom of the catch tank, so that the oil fumes are drawn down through it and it helps to catch oil droplets. I empty it every couple of thousand miles but it never has more than a 100ml or so in it.


I've used stainless steel and nylon pot scourers to catch oil and both work fine in my experience.
David Billington

That was a typo! Meant to say never more than 10 ml or so (per 2,000 miles).

I was surprised how little it actually was, except of course the oil fumes have already been through one separator on the chaincase.

I had the same idea, but didn't have access to the great looking coffee can (I use coffee pods!). But, I could have bought the coffee and dumped it out, would have been cheaper. I opted for the "super duper Demon Tweeks" version. Model 52206 from Its very small and sits inline between timing cover and carbs. It came with a very small filter to separate the oil from the vapor, but I was afraid it would be too restrictive. I think its designed for big American V-8s. So, I use the SS scrubbers, too. It does not have a drain valve on it, so you have to unscrew it to dump it, but, like Guy mentions, it doesn't accumulate a large amount of fluid.

Jack Orkin

Oops, here is the picture of the installed unit.

Jack Orkin

Ah, so the Demon Tweeks one comes filled with coffee too! Excellent!

I think you're jumping ahead, as put before why not first try the PCV set up as it should be, close off the rocker snorkel, oil cap and dipstick tube (if not already done). All this will be very quick and simple to try and easily reversible (except dipstick seal) if it doesn't work.

If you get an alloy rocker cover the bling chrome oil caps are (often?) vented (but insufficiently) so bear that in mind too.

For a plastic non-vented cap (13H2296) should you need it try here, it comes complete with the seal (12A403) -

(seal) -
Nigel Atkins

Just a confirmation on the standard crankcase ventilation systems - competition engines are excluded!!

Early - 948 and early 1098. Non-vented oil filler cap, hose from rocker cover to air filter housing, tube from tappet cover to atmosphere. The end of angled 'draught' tube is designed to create small a 'pull' when moving, to draw fumes through the crankcase with fresh air being drawn FROM air filter. Usually just managed to clog the air filler with oily deposits!

Later - Vented/filtered oil filler cap, no rocker cover tube and either a hose from the tappet cover to the inlet manifold PCV or from the timing cover to either the PCV or to the port on the side of the carburettor(s). This system pulls fresh filtered air THROUGH the rocker cover and draws the fumes into the inlet manifold or carburettor.

Both the later systems are designed to create a partial vacuum in the crankcase to prevent/restrict leakage past the rear scroll. I have never managed to get our 1380 drip free, but the oil consumption is only ~1,000 miles per litre, so very manageable.

There was never a combination of vented oil filler cap and rocker cover vent tube, or vice-versa.

Richard Wale

Colin, Richard,
apologies I got myself confused there (doesn't take much) - don't block the vented cap - non-vented cap info was from an earlier search and for if you try going to an earlier or different system.
Nigel Atkins

Thanks Richard

That's pretty much as I understood it - I was pretty sure there had never been a combination of snorkel rocker cover and PCV or crank case vapour recirculation.

You can be pretty sure I'll be trying the no cost option first - it's just tipping it down with rain here at the moment so I'm putting it off for tonight! I was a bit surprised at the suggestion of blocking all intake routes, but, seeing as the original set-up would have included a vented/filtering cap I was going to leave it at that.
Colin Mee

At one point with a new pcv valve and connection to the timing cover breather I had so much oil sucked up into the manifold that the engine was dieseling on the oil. Which it all disconnected and a catch pot on the timing cover breather no oil was ejected and collected.
Just saying 'cos no-one believes it! In the end I just accepted the relatively small drip at the back end.
Graeme Williams

Terry Horler's book would let you know the various breathing combinations, a very good book, but not as great as the relevant 'good book' of course. :)
Nigel Atkins

Reporting back now that the Sprite has a few miles under its belt with my version of Guy's coffee tin oil catch tank (the main difference between the two being that I think Guy's was unleaded (de-caf) while mine was four star!)

I've had several good 15+ mile runs out on the local roads and a weekend of hill-climbing.

After all that, I've just opened the can to find it full of nothing - not a drop! And there wasn't a hint of smoke coming out of the back end.

If it carries on like that I'll think about doing away with the catch tank and just pipe it direct to the PCV.

At the hill climb the car was brilliant. There were a few changes to the car: crank case breathing, rear end lowered 1.25" and a lighter, motorbike battery. It all resulted in a new PB by well over 1 second. :)

While the lowering definitely improved the handling and, the engine seems to really like the new breathing system, I think the biggest factor was the weather! It was brilliant sunshine, giving a warm track but a cold gentle breeze, giving a denser air, helping a number of people improve their PBs. I also made my fastest ever start - 2.82 secs for the first 64 ft (64 ft in 2 secs = 1G acceleration). I think that was largely due to the warm track an a good layer of other people's rubber meaning my tyres just gripped for a change instead of spinning one wheel.

Overall, pretty pleased with things. :D

Thanks for all the advice - I've quite enjoyed tackling this one and achieving a successful outcome.

Nearly forgot: - No telling off for dropping oil this time (the main object of the exercise) but an annoying little drip when left over night (in addition to the annoying drip behind the wheel!)
Colin Mee

This thread was discussed between 29/12/2016 and 29/03/2017

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