I have been living with a big oil loss for some time, about a litre every 200 miles. I suspect the rear crankshaft seal and am aware of the size of job to fix this. It has been suggested to me that I should first make sure that there is no excessive pressure forcing the oil out caused by a blocked breather. Has anyone an opinion on this and any advice?

Keith I take it we are considering a V8 engine here.

The SP250 and the saloon engine can lose oil from several different places, it depends if you are saying it burns away and shows as blue smoke in the exhaust or if it is dripping away and the evidence will be on the garage floor.

My SP250 has always lost some oil from the rear crank oil seal and it drips from the gearbox housing.

When I bought the car it was also losing oil from the rocker covers, dripping down over the timing chain casing onto the floor, it looked like it was leaking from the front crank seal but was not. Nothing like 1 litre per 200 miles though.

As far as I am aware the rear crank seal is an engine out job, although there are some very clever people on this forum who may have figured out a way to do it in situ. The best cure is the Russ Carpenter lip seal treatment, but that is an engine strip and crank out and specialist job. But my Russ Carpenter saloon engine is completely dry and has been for the seven years I have had the car.

The rocker cover problem can be easy to solve if you have the type of cover which has grooves around the outer rim. In which case you need some 3 mm "O" ring material, 1 meter per cover and some super glue. Replace the cork gasket with the "O" ring material glued in place with super glue, make sure you start and end at the high side of the cover and glue the ends together.

It is a certain cure which works for a very long time. If you don't have grooves you can get a machine shop to grind them in or new cork gaskets will last a good while.

Other than that oil can leak into the spark plug tubes, it can be cured by removing them and softening the copper ring gasket under them by heating the rings to red hot and letting them cool then refitting the tubes with a drop of Mylar blue sealant. Mine do leak but over a very long period, from time to time I just remove the spark plug and drain them into the cylinder.

If it is blue smoke then we all suffer some, the engine will run for ever in that condition, It could be a stuck ring, tip a good slug of Redex into the cylinder and leave it 24 hours, that could cure it, otherwise it is again an engine out and overhaul job.
Hope that helps Syd

Hi, If it is a Dart engine you will be o.k. with anything up to 60p.s.i. oil pressure.

Dave.

There is a scroll on the front end of the crankshaft returns the oil to the sump that drips down the inside of the timing case, it can be swamped by braking hard at low rpm, also the short tube welded in to the timing case can be damaged and oil leaking past the tube, if the timing case is adjusted incorrectly the scroll can cut through the tube, depending on your mileage and driving style a lot of oil can be lost through the scroll but only show a small amount after the car has stopped.
Regarding the spark plug tubes, the copper washers if they are not too flattened can be softened as Syd says but dropped into cold water instead of letting to cool slowly, copper rings can be purchased from decent plummers suppliers or ironmongers, it is only the outside diameter that is important, a thicker ring is not a problem, the only breathers are from the rocker covers to the air filters, ensure they are not blocked at either end.
Mods. Can we change the title to Crankcase Breathers as it was probably a predicted text problem.
[EDIT - DONE]

Thanks for the help and yes the title can be changed to breathers.I get the impression that my instincts are probably correct and that a blocked breather is unlikely to be my problem. I didn’t want to go to the expense of an engine out if the problem was more simple.

Oil loss is rarely due to ONE issue such as leaking rear main seal..think 'cause and effect' and ask WHY it is leaking from rear main seal.. commonly the actual problem is wear in the engine.. the symptom of that wear is oil loss.. rear main seal usually leaks for 2 reasons:- main bearing wear and wear related to pistons/rings/cylinder bore adding extra oil mist due to 'blow by' ..