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MG Midget and Sprite Technical - Breathing Again 2 - Does it Matter?

1967 Sprite MkIV HAN 9

Another crankcase breathing question. Could this be a PO's answer to Lawrence's suction problem?

This was basically the crankcase breathing arrangement that was on my Sprite when I got it. I have just renewed the parts and made it vent into a catch bottle rather than to atmosphere.

As I mentioned in Breathing Again 1, I think my car originally had an AGR valve. If it did, then I don't think it would have had the rocker cover with the periscope take off. Rather than going to the hassle of finding an AGR valve I decided to leave things as they were as it seems to run fine. It has a ventilated filler cap and, with the crankcase breather connected directly to the rocker cover together with the open vent pipe, the air inside the engine can pretty much do what it wants!

After running it like this for two years there is still nothing more than a bit of oil staining in the bottle. When I run the car after it has been sitting for a couple of weeks there will be a bit of steam and a few ml of water in the bottle but nothing else. I have what I consider to be a normal amount of oil drippage on the garage floor and I hardly ever have to top up the oil so it's not getting blown out through the rear main seal. I can account for a lot of the drippage as being from other sources yet to be tackled!

The Sprite has been through a couple of MOTs like this and has been scrutineered for track use loads of times with not comments.

My question is: Does it matter and is this arrangement likely to throw up any other problems?

Any thoughts?


PS - I usually run with a radiator but that's out while I try to fit an electric fan. Currently trying to free off the seized thermostat housing!

Colin Mee

Ah ha I see now (see previous post) Its not got a sealed system so the risk is there is there could be a rear scroll leak. You mention a few drips on the floor but if its not excessive and you are ok with it then I would just carry on.
Bob Beaumont

Similar system to mine. Vented top, middle and bottom into a catch tank. Only very slight leak from scroll and more importantly none of the crankcase crap venting into the carb and upsetting the mixture.
john payne


Regarding scrutineering, I thought it was required to have a means of viewing the catch tank contents level, if any. I used a similar arrangement but used a translucent milk bottle.
David Billington

Hi David

So far I've only done hillclimbs at Loton Park and none of the scrutineers have made any comments - yet. I considered a plastic bottle but then found this colour coded aluminium one for 2 in a local sale.

For interest sake only, to see if anything's being blown out, I also have a catch tank for the radiator - that one's a Sprite bottle :)

Colin Mee

DB, 'I thought it was required to have a means of viewing the catch tank contents level' - not at all; in fact if running under FIA rules the catch tank has to be metal.
David Smith

The catchtank isn't strictly legal but falls within the adequate-ish category. Speed Events needs a 1L tank, racing needs 2L unless the engine size is over 2L in which case it needs 3L. This tank looks more like 650-750 ml.

Yes there is meant to be a means of establishing the level Q19.9.3 (page 282) for MSA events (there is a nice example in a book about tuning Sprites and midgets etc). However, I've seen E-types with a very nice aluminium oil catch/breather tank that have a little dip stick which seems to meet the spirit of the regs rather than the letter of them in providing a means of checking the tank isn't full.

Good point about FIA regs requiring the catch tank to be metal (old oil can but not modern oil plastic 'can) - I'll check that out of interest.

LMES (P1 and P2 I guess) cars have a sensor in the oil catch tank and the data from the car acquired at a pitstop (not sure if its full live telemetry) will report if it fills up past a certain level. If it is fill to a pre-determined point the team have to empty it etc before the car is let back out on the track.
Daniel Stapleton

Well... its set up incorrectly, unless there is something else going on in the photo im not seeing

The correct path would be the hose from the timing chain cover to a catch tank then a 2nd hose from the catch tank to the carbs and it all needs to be sealed.tight, with no air passing by the joints...which appears you have at the red bottle

At some point you need to have a vac introduced for the system to work properly

Currently its still being vented to atmosphere by way of the red bottle


Prop and the Blackhole Midget

Thanks for the comments.

Daniel, I made sure the bottle is 1l capacity before installing it so that it complied with that bit of the regs. I stand to be corrected but, for Speed events section S of the MSA Blue Book 'trumps' section J.

MSA Blue Book, Section S 10.3.5: "Engine Oil Systems - Unless equipped with a closed loop system, a Catch tank of at least a one litre capacity to be incorporated in the oil breather system."
There's no mention of sight glasses or measuring systems etc. and no mention of oil systems in the Roadgoing Production car regs.

Prop. I knew from the start that the system is not correct and presumably a PO has changed the rocker cover to give even more ventilation. As far as I am aware, the system as it is does not fall foul of any regulations as my 1968 Morris Minor has always ventilated to atmosphere and never had an AGR valve, although I think some of the later ones did.

Would there be any benefit it introducing a vacuum? I'm inclined to follow John's thinking in preferring to have "none of the crankcase crap venting into the carb and upsetting the mixture."

Having read some of the epic Sucking threads I feel that, so long as I'm not causing any undue pollution, I'd rather stick with a freely venting system than risk creating a smoke screen.

Incidentally, the engine was rebuilt at 86K miles, about 20K miles ago - so I wonder if these breathing mods were introduced then to overcome a sucking problem.
Colin Mee

Prop, the whole point of that system is that there ISN'T a vacuum as the the only way of achieving that (without an external pump) is to use the intake system. It works. I've not read the oil sucking thread as it sounds like a load of hogwash to me!

Colin, I've had the catch bottle 'sight glass' mentioned by scruitineers in the past so changed it to a plastic 1l gear oil container that has a clear section to show how much is left in it.

In my experience some scruitineers can be quite misinformed about the rules and regs with one being happy with something but then the next wanting it changed. The catch tank is one example, I knew it was ok but you just have to nod and say I'll change it. One chap at Cadwell was about to refuse me entry to the event because my harnesses were out of date, it was only when another scruitineer was called over that he accepted that as it was a sprint and not a race the date was irrelevant!

john payne

If your conserned about lawerwnces sucking oil problem

Its super easy to remedy ... just let the engine warm up to or near normal operating temp... about 5 minutes before driving and you wont have an oil suck issue

You can tap the intake manifold for vacume but it draws alot of vacume you would need.some kind of regulator to turn down the vac

Prop and the Blackhole Midget

Yes you are correct, for speed events you only need to worry about your reg for 1L as the 2L and 3L reg and the sight gauge etc etc are circuit racing only regs. I forget what the reg is for rally cars - I think they are on loose and slippery surfaces and don't care where the oil goes! There are of course some regs which are across the board but even then there are some age related exemptions which for us is Frogeye Sprite era. The other thing to watch for regs is where the supplementary regs impose stuff over and above the blue book reg, usually by applying a higher blue book standard - but nearly always from the same discipline section.

My excuse is that it's been several years since I've been to a speed meeting and haven't looked at Speed regs in years. For the competitor something a bit like the reserve of that happens when they switch from speed events to racing while using the same car and discover a bunch of stuff they now need to watch out for.
Daniel Stapleton

Daniel, the blue book doesn't help with all the cross referencing you need to do!! It's been improved in the last few years though.
john payne

John Payne said "I've not read the oil sucking thread as it sounds like a load of hogwash to me!" Many idiots make pronouncements about things they haven't read. ;)

I don't race, so won't comment other to say that some race engine builders go to great lengths to achieve negative CC pressure. Read Vizard for just one source, but there are hundreds of references to PCV and power loss on the web.

Colin asked. "Would there be any benefit it introducing a vacuum?" And said, "so long as I'm not causing any undue pollution, --- "

Yes there are benefits, and yes you are causing undue polution, because you aren't burning off the gasses, they are escaping from the neck of your catch can -- unless it's sealed, in which case your CC is pressurised too. The sump shouldn't leak, nor should the front oil seal or the rear scroll(there's no rear main seal unless you have the converter).

"When I run the car after it has been sitting for a couple of weeks there will be a bit of steam and a few ml of water in the bottle but nothing else." That'll be from the water in the oil that's been sitting in the engine, because it wasn't being drawn out.

The Spridget PCV system is old, basic, and can cause problems. But irrespective of polluting the environment, -- which isn't my major concern -- I personally prefer to have it working.

Up to you what you do, nobody really cares what others do. But you might as well know how it actually works.

Here's a couple of references. Not Spridgets, but it applies just the same.

Lawrence Slater

This thread was discussed between 26/01/2015 and 27/01/2015

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