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Hey all,

Have a weird oil leak after putting my new engine in, replaced rear main seal, valve cover gasket, resealed sump using new rubber half moons, CAS o-ring, etc...

It has developed an oil leak at the back of the engine, removed the engine and did a thorough inspection. Valve cover, CAS, sump and rear main seal are clean and not leaking a thing. The flywheel is dry as well as the inside of bell housing.

It is 100% engine oil and not transmission oil.

I've taken a photo and made some markings to explain, the red areas are dry and oil free. The blue areas are where oil was found.

The plate between the engine and bell housing also had oil all around the edges but nothing in the centre.

Could it be headgasket/head warpage? Its the only place I can think of where oil would be coming from

Not by any chance, leaking Brake/Clutch Master cylinder?
I know you said 100% motor oil. Just throwing ideas.
I only say it as once before it has been a WTF moment for me

Otherwise it has got me stumped, unless its the head gasket.

Thanks for the reply guys.

I am thinking it could be the head gasket because there are two oil ports at the back of the block I think. Could also be that brass oil galley plug next to the starter motor.

It's definitely not brake/clutch fluid.

Head gasket leak is possible. I can also imagine oil from the CAS or the back of the cam cover gasket being carried almost invisibly by capilliary action around the seam of the rear coolant connector and then down onto the engine/bellhousing join area to where you have found it all.

Not sure about B6 but on the BP there are also a couple of oil gallery plugs at the back of the head.

manga_blue wrote:Head gasket leak is possible. I can also imagine oil from the CAS or the back of the cam cover gasket being carried almost invisibly by capilliary action around the seam of the rear coolant connector and then down onto the engine/bellhousing join area to where you have found it all.

Not sure about B6 but on the BP there are also a couple of oil gallery plugs at the back of the head.

With the amount of oil leaking, there is no sign (visual or touch) of oil leaking down from the valve cover or CAS. Completely dry, made sure of that before I removed the engine.

Did some research on miata.net and seems like a headgasket oil leak is not uncommon.

Yeah the B6 head has some threaded bungs for the oil galleries, but these are clean and leak free too. Just the brass plug in the block could be leaking.

Fair enough.

Do you think it could be the rear cover? The thing that holds the rear main seal.

It can't be the rear cover if there's oil leaking above it. Take gravity into account, if a leak springs it will always go down not up. If hits something spinning then it'll get throw all over the place and will hard to spot/oil spray.

If inside the bell cover is dry then there's no leaks there. What it looks like is leaking from the head and dribbling downwards. Now at the back of the motor there can be a couple places it could leak from. First, it could be the valve cover gasket/CAS but if above the headgasket line is dry and clean then it can't be it. And the only other place I can see is the headgasket.

Its not uncommon for the head gasket to leak on the MX5s. I remember a mate that went through all sorts of troubles trying to figure out where there was oil dribbling down the side of his engine. He replaced all seals he could have replaced with the motor still in the car. He even pulled off the sump and sealed that up. Then he got fed up and decided to pull the motor out, and from there, it was very obviously that it was coming between the head and the block.

Just a quick update....

Replaced the head gasket over the weekend and Monday, no more oil leak