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MG Midget and Sprite Technical - 1500 rocker cover vent + excessive oil consumption

I've fitted an allow rocker cover (from Moss MG) to my British 1500 midget, and have noticed a dramatic increase in oil consumption... to about a pint every 200 miles!

I also fitted an auxiliary rocker shaft oil feed (Moss again) and initially thought that this increased oil supply might be causing the problem, so I removed it, which made no difference at all.

The engine has only done about 3000 miles since a complete rebuild (including a new oil pump) and has a Stage 2 head, new valves, K&Ns etc. And it's all in very good condition.

My theory is that oil is being drawn through the breather tube that connects the rocker cover to the carbs. The inlet manifold which was nice and shiny on the inside 3000 miles ago, and now appears to be black (looking down the plug hole on the manifold.

I got hold of an original rocker cover (I'd chucked mine) to compare it with the new alloy one. The 'vent pipe' on the new one is just 4 cm from the top of the head compared with 6 on the original, and it is completely open. By contract, on the original the vent is cover by a large baffle, beneath which is a kind of wire wool (a bit like a pan scourer). Presumably the baffle and mesh is there to help prevent oil being drawn in, and only to allow vapour to be drawn off.

So two main questions:

1. Has anyone else had this problem with an allow rocker cover, and what is the solution? I like the look of the new allow cover and am considering trying to create some kind of baffle

2. Are the breather pipes necessary? Is their function compromised by the K&Ns. Could I remove them and fit a little K&N filter to the vent pipe? Or is the vacuum they are designed to create critical to the oil flow around the engine (even though i my case most the the oil seem to be going out through the carbs!)

Apologies for such a long message..

M Davies

Where it says 'allow' rocker cover it should of course read 'alloy'! I've noticed loads of other typos (I think I getting rather steamed up about it all...) so if any of it doesn't sense please don't hesitate to ask for clarification. Thanks. Michael
M Davies

Hey M Davies,

Dont worry about proper spelling and grammer here, Im the cheif resident savant for typos, poor grammer, and bad puntuation...AKA Ive got this board trained well...LOL,

I dont know alot about 1500s, But my guess the problem lies with the "Open vent" on the (allow) rocker cover, Id at least try and close that vent off so its not venting to atmosphere and see what happens

what you want to do is create a vacume inside the engine so you have negitive crank case pressure...Like a shop vac. that will hold the oil in, If you have a equal or positive crankcase pressure inside the engine it will try and force the oil out thur what ever weak spot the pressure can find...aka a An over inflated tire with a nail in the side wall..

Just a pure guess, but perhaps run a hose from the open vent on the rocker cover to a catch taank then from the catch tank to a aux port on the intake manifold.... others will be along shortly to give better advice.


Hi Michael,

I've got a 1500 with an alloy rocker cover & we certainly don't get that high an oil consumption. Not been in for a while but I'm almost certain that there's no pot scourer type filter on the breather tube. The alloy cover has been fitted since we got the car so can't compare it to a standard one.
We also tried the external rocker feed from moss but not long after that the car wouldn't start & the cause was oiled plugs. We removed the feed & everything has been fine since.
Can't really help with other diagnoses but I wouldn't have thought just the rocker cover would be to blame. I guess it could be something like slightly worn rings, in which case a compression test would be in order but I'm sure there will be more expert individuals along with ideas soon!

C Robertson

Thanks Prop and Craig,

My engine is in very good condition with high and even compression (175 from memory). The 'open vent' was referring to is the unbaffled outlet on the alloy rocker cover, but I might have misled you here as it has a breather tube attached (leading to the carbs) and is not venting to the atmosphere.

I think your 'catch tank' is a very good idea, but I'm a bit reluctant to add more 'clobber' as I like the simple, almost original look!

I'm still concerned that the increased air flow through the K&Ns is causing more vacuum effect on the rocker cover breather tubes...

Eureka: Your comments have give me an idea. I'm going to change the breather pipes for transparent PVC pipes, drive fast, get the engine and hot, and see what is being sucked through the breather pipes.

Is the simplest solution to remove the breather pipes, block the holes on the carbs, and add a little K&N filter to one end of a hose set higher than the other end of which is attached to the vent pipe on the rocker cover?

I look forward to your responses. Thanks. Michael
M Davies

Hi Michael

If you suspect that the breather rom the rocker cover is causing your hi oil usage then disconnect it and see how it goes.
You need to block the tube that runs to the carbs othewise this will affect mixture although that will be affecte anyway but for the sake of the test should be unning a tad rich.

The hole on the rocker can then simply have a pipe attached to it and run ino a plastic bottle or something to act as a temporary catch tank. do not like the small KN breathers as hey often cover under bonnect in oil mist admittedly on poor engines

Naturally you are correct the oil outle on any breathing system ought to have a oil seperator or pan scourer. :)
Robert (Bob) Midget Turbo

Hi Bob,

Thanks for your very useful message. I'm going to do as you suggest.

So what's your view on the negative crank case pressure?

Is a neutral crank case pressure OK (i.e. not venting to the carbs), or is it advisable to persevere and make the venting to the carbs work properly?

And what kind of pan scourer do you favour?

Thanks Michael
M Davies

If you have an original type rocker cover why not fit that and see if the problem goes away. If it doesn't then the problem is being caused by some thing else.

If the problem does go away, try to copy as much of the design of the old accross to the new, wire wool and all.

Generally a slight negative crank case pressure (perhaps less than 0.5 psi or 5kPa) is good to avoid oil leaks from static and crank seals. Particularly on A-Series engines with the rear crank 'scroll', although I expect you have a proper seal on the 1500?

James Eastwood

This thread was discussed between 01/02/2010 and 12/02/2010

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