Rocker cover cap

[MKT]

Can anyone tell me if I have the correct oil filler cap, I have a swan neck breather pipe fitted on the side plate, and a pipe from the rocker cover to the air filter, and a vented cap is this correct or should the cap be solid

Ron

[philthehill]

You can have either as your engine breathing system is not sealed i.e. is open to the elements via the side plate breather.

[MKT]

Many thanks

[IslipMinor]

My understanding is that with the earlier non-closed circuit, or 'open' crankcase ventilation systems, the rocker cover cap should be not be vented'? The argument being that the breather circulation is IN from the air filter, through the crankcase and OUT through the 'Draw' tube running down from the tappet cover towards the ground. The oil filler cap plays no part in this, so must be non-vented.

Equally with the later 'closed circuit' ventilation systems, the circulation is IN through the oil filler cap (i.e. vented) and OUT though either the PCV or direct to a port on the side of the carburettor.

Makes sense to me!

[edd_barker]

I just dropped my car in Friday for MOT and a look over the new engine by a mechanic who did his apprenticeship on minors, and spent the next 20 years building race engines. I have a tappet canister breather plumbed direct to HIF38 vacuum intake, with vented cap as per the 'general' consensus of this forum! He immediately spotted this and was adamant that a non-vented cap was correct! I left it with him so I will be interested to see which he has gone for, and I will be able to compare the two options. It didn't really leak much with a vented cap, but then hadn't exactly done many miles.

Edd

[philthehill]

The original metal cap (Pt No 8G612 (BMC)) fitted to the Minor rocker cover was vented as was the earlier plastic cap (Pt No: 8G612 (MOSS)) fitted to non sealed crankcase systems.
The early plastic cap was fitted with a gauze oil separator to stop oil venting to the atmosphere.
My original post was made in the context of and relative to the engine being discussed - so if your engine is breathing fumes it does not matter whether the fumes exit the engine via the rocker cover cap, to the air cleaner or out of the breather tube from the cam follower chest - you are still breathing/passing fumes to the atmosphere.
When an engine is in good condition there is very little breathing of fumes out of any of the orifices.
http://www.moss-europe.co.uk/cap-oil-fi ... 8g612.html
http://www.moss-europe.co.uk/cap-oil-fi ... e6003.html

Edd
Your man is correct in that an engine fitted with a sealed breathing system should not have a vented rocker cover cap otherwise the required depression in the crankcase is not generated.
Your system pluming is also correct.

Phil

[IslipMinor]

Phil,

I disagree!

The later crankcase ventilation system is 'closed circuit'; similar, but much simpler, to the system current car engines have.

If you fit a non-vented oil filler cap to a closed circuit breather system, where does the fresh air come from to replace the air/fumes being drawn into the inlet manifold? The vented cap has a mesh filter and is designed to restrict the amount of air allowed to flow in to create the 'partial vacuum' intended to keep the oil inside the engine - with very mixed results as we all know.

With the earlier 'open circuit' breathing system and a vented cap, oily fumes venting from the oil filler cap makes a mess of the engine very quickly. At best with a non-vented cap the fumes are drawn away underneath when the car is moving at a reasonable speed, but otherwise they find other ways to escape - into the air cleaner and clog up the element, or drip on to the floor from the vent tube. If the cap is vented, they can escape through that and coat the rocker cover, and more, with oil mist .

I don't know if the original caps were vented, but if they were it may explain why the outside of the engines became mucky quite quickly?

Our original well-modified 948 engine had the metal cap, and ~30 years ago in a vain attempt to keep the top of the engine clean I fitted the later, vented cap (on the basis that later = better?), but it didn't help. By contrast our 1380 with the standard closed circuit system venting into the side port of the HIF44 keeps the top of the engine very clean - still drips from the rear main scroll though!

Edd,

If he fits a non-vented cap, please keep a very careful eye on the oil level, as I have heard some horror stories of oil being sucked into the inlet manifold and plumes of smoke out of the back, using the combination of closed circuit and non-vented caps. May be exaggerated, but worth keeping a good check on it! Also please come back with an update on the results.

[philthehill]

Richard
I will stick with the content of my post even though we may disagree.
I will agree with you though in that Edd should keep a watch his oil level and see if there is any increase in oil consumption with the sealed cap.
Compare the oil consumption figures and if worse revert back to the vented cap.
I do believe that particular oil caps are better suited for some engines (Horses for Courses so to speak) and both types can work successfully on vented and non vented engines.

Phil

[BLOWNMM]

Hi all
For what it's worth I have chosen to throw my hat in the ring. When I built my side valve motor I decided to attack the problem of rear crank oil leaks and came up with the solution outlined. I chose to replace the tappet cover with a machined alloy one with a fitting to accept 10 mm. oil proof hose connected to a PCV valve on the blower intake side of my motor. The original oil filler cap and tube was replaced with a stainless steel one machined to accept a later A Series oil filler cap. This filler cap is connected to a catch tank with a filter to keep rubbish from being drawn into the crankcase due to the vacuum created by this setup. Initially The hose to the catch tank had a 3.8 mm. dia. restrictor. The
maximum crankcase vacuum measured at the dipstick tube with a non vented cap was 4.8 inches water gauge. With the 3.8 mm. dia. restrictor the crankcase vacuum was 1.8” WG leaving the rest of the max. 4.8” WG to purge the crankcase of water vapour and other combustion by-products. This setup worked well with minimum oil leakage on the drip tray which is always under my car. Recently when the motor was removed to fit a modified gearbox a check revealed no ‘mayonaise’ build up so I decided to decrease the dia. of the restrictor to 3.2 mm. This resulted in the crankcase vac. Increasing to about 2.1” WG. The car has since been driven on four occasions and there has been no oil dropped onto the drip tray after the car is parked. I feel confident in claiming this a success. Have attached pics of the manometer used to measure vac. and one of the A Series oil filler cap connected to the catch tank. I leave it to readers to determine the best solution to a long term problem.
Bob

[Declan_Burns]

Bob,
I did exactly the same experiment on my MG TD XPAG engine and came to exactly the same results. I measured -121mm water gauge =(1.21mbar=~4.8") on the U-tube with a non vented cap and everything else blocked off. At the moment I have two 1.7mm restrictors between the inlet manifold (tapped at both ends) and a Ford Pinto PCV valve with two inlets. It does have an adverse effect the idle and I am still experimenting with the restrictor sizing. It could be that my throttle spindles are worn so that's the next job.
The reduction in oil leaking from the rear scroll was significant so it is clear to me that fitting this closed loop system forces air to be drawn in through the rear scroll. Where air flows in oil cannot flow out!

Regards
Declan

[BLOWNMM]

Declan
Assuming the manifold vacuum for both our motors is the same at a max. of about 29" WG (except that possiblity your worn buterfly spindles may have a bearing), it would appear that considering the sealed crankcase vac. is 4.8" WG for both, I guess we can assume the cross sectional areas of the annulus at the crank scroll would be similar. I took great care to ensure my radial clearence was 0.002" even to the extent of making replacements for the 'die cast' part which attaches to the block. I machined replacements from 6061 alloy.
Regards
Bob

[Budgie]

I'm a bit confused as I have just put my engine back in the car after a gearbox rebuild and general tidy up and paint of the engine I have replaced or cleaned up the ancillaries and bits and bobs on the engine. I bought a new oil filler cap which was the exact swap of what was on the car pre tidy up which is a plastic long reach cap on a 1969 minor, 1098cc, so is this the correct cap, if not would it cause oil the drip down the back of the sump? I changed all the gaskets and as per my posting else where, I still have a slight drip in this area but is a lot less than pre clean up .

[BLOWNMM]

Budgie
I am not that familiar with A Series donks however Richard (IslipMinor), Philthehill and Declan have opinions which may be of help. Assuming the cap is correct there are other reasons for a leak ie. worn bores and/or rings leading to excessive crankcase pressure, excessive gap at the crank scroll and blockage of the pipework.
Bob

[smithskids]

I used to work for Mirrlees Blackstone Diesels and all our K Major engines and most of the later E Type engines had crankcase extractors fitted. They used to run at 1/2 " water gauge on full load and about 2 " on idle. One reason was to stop oil leaks the other was to discourage crankcase explosions.

[IslipMinor]

Budgie,

1968 should have the PCV in the inlet manifold, I would think? Does yours, is it connected and is it working?

[BLOWNMM]

Declan
Having given thought to the similarity of our closed crankcase vacuum, I recalled having sourced info from the MGA Guru site re radial clearance for the scroll for MGA and Magnette motors. I have attached a link to the MGA Guru site. If you go to MG/212 it quotes that the radial clearance should be between 0.003” and 0.006”. In another place MG/327 it quotes that on 30 August 1960 this clearance was increased by 0 0015”. It was then that I realised that providing your clearances were similar and if my radial clearance was 0.0.002” the area of my annulus would be much less than yours and the similarity of the vacuum readings should be considerably different. I did some thinking and remembered that I had recorded my findings after having machined the scroll clearance. Rather than repeating I copied my entry and have pasted it below:
Wed.16 Jan. 2013 Machining scroll seal in bearing cap and aluminium housing. On NOS
crank the scroll diameter is 49.655 mm. Was aiming for a bore of 49.855, but due to
difficulty in measuring it turned out to finally be 50.00 mm. This gives a radial clearance
of 0.0068”. Having read the MGA Guru site I decided to settle for this. As it turns out it is entirely satisfactory.
Unfortunately I relied on my somewhat aging memory instead of reading recorded notes.
If your radial clearance is as outlined for the MGA/Magtette then it would more satisfactorily explain
the similar vacuum readings. Hope this clears things up a bit. The only thing I did not say was that my reading was at idle speed 950 RPM.
Bob
http://mgaguru.com/mgtech/engine/en1.htm

[Declan_Burns]

Bob,
The clearance of 0.003" to 0.006" for the MGA is correct. The XPAG factory clearance is given as 0.0053" to 0.0088".
There is a good website explaining this
http://www.mg-cars.org.uk/imgytr/oilleaks.shtml
It shows the dimensions of the XPAG scroll.
I have never measured the clearance on my engine-just tried the PCV valve to see if it helps.

Regards
Declan

[Budgie]

So what exactly am I looking for with regards to "pcv" in the inlet manifold. It's only a small drip now since I renewed all the relevant gaskets so when I read this post my thoughts immediately went to whether or not I had the right oil cap on mine.

[IslipMinor]

This is an extract from the MM Workshop Manual:

Closed Circuit Breathing.jpg (122.55 KiB) Viewed 3684 times

In the picture, it refers to the 'Breather Control Valve', which is the PCV Valve (Positive Crankcase Ventilation Valve).

It is a mushroom headed valve that is connected into the top of the inlet manifold by a short hose, and has a hose connected to the side of it running up from the oil separator on the front tappet chest cover. The description refers to the 'filtered' vented oil filler cap, and to the 'closed circuit crankcase breathing' system that was fitted to the 1098 engine cars (all of them??).

Assuming that the PCV is fitted and connected, what happens if you remove the oil filler cap while the engine is running at tickover speed? It should increase speed slightly, or maybe run slightly differently, but does it?

[MKT]

I am a bit confused, I have only owned the car for a few weeks, as I said in my original post the car is opened vented, it is a 1968 saloon, the logbook says 1098 cc engine, reading the other posts it sounds to me as if it should be vented through the inlet, could this mean I have an earlier engine fitted, is there a way of telling

Ron