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MG Midget and Sprite Technical - Piston Rings or Valve Guides/Seals?

Having been very good for the first six years of ownership and barely putting anything in the catch tank, my Sprite has started to blow oil out through the breather system and I have just emptied around half a litre of oil out of the catch tank.

I have a hose coming off the separator on the timing cover. The hose goes into an 'Illy Coffee Can Catch Tank' and then via another hose into the PCV.

The Sprite gets driven very hard when hill-climbing and, when warm, has a clean exhaust, with no smoke. On its last outing, there was a very long wait for the start line and it sat ticking over for 10 - 15 minutes before I switched off. On starting up, the car following me disappeared behind a cloud of blue smoke!

After some outings there's hardly any oil in the can and the rate of oil loss seems variable. After a good run the plugs are quite clean but after short runs a couple of the plugs get oily, these have the highest scores when I do a compression test.

I'm definitely going to have the engine out over the winter and I'm pretty sure I know what I'll find. I just wondered what folk think is the most likely cause of the oil blowing - valve guides and seals or piston rings.

Colin

C Mee

Colin, do you do compression tests?
If your oil problem is rings/bores, then there will be a difference in the compression reading for that cylinder between the "dry" test and the "wet " test.

Do a standard dry test and then for the wet test, add a couple of cc's of oil - no more - to the cylinder and then do the test the exact same way, If the rings are suspect, then the oil will help seal them and the compression reading will increase. If the fault is guides/ valves then adding the oil won't have any impact on the reading.
GuyW

Thanks Guy.

So far I've only done dry tests and it's been the cylinders with oily plugs that have had the highest scores, so I suspect I know what results I'll get.
C Mee

You could have worn rings/bores and valve guide problems. If your cylinders with oily plugs are giving higher readings, it could be due to oil helping to seal the worn rings.

Do you have historic compression test figures? Are the lower readings lower than they have been in the past? Are the higher readings higher?
Dave O'Neill 2

Colin,
in the Archives there's plenty on the subject that includes clouds of smoke that come and go, catch tank, oil sucking.
Nigel Atkins

I think you can expect both problems.
Oil in the catch tank means a pressurised sump coming from blow by past the compression rings
The blue smoke after idling an stopping indicates worn valve guides or seals.

No big issues but I would calculate on a set of guides and rings
O K

First of all I would check your ignition timing - detonation can be inaudible and cause the rings to flutter which leads both to blow by and oil consumption.
Chris at Octarine Services

Onno - The PCV, which was missing when I bought the car, and catch tank went in to try to eliminate / reduce rear main seal drippage as I thought that might be due to crank case pressurisation from blow-by.

For two years it seemed to work - slightly less drippage with next to nothing in the catch tank. This year it's been different, so it's time to act.

Chris
That's interesting. The timing, dwell angle etc were as good as I could get them the last time I serviced them about 400 miles ago but I will check them again before I start dismantling to see whether things have changed.
C Mee

If it has started to burn oil, also check on the condition of the rocker shaft. If worn, then it will pump more oil out onto the cylinder head and this can increase seapage down the valve stems.
GuyW

This thread was discussed between 02/10/2019 and 05/10/2019

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