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Oil Leak On New Build - Head Flatness/gasket?

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#1 mark205gti


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Posted 02 November 2019 - 08:48 AM



I've just finished a rebuild of my block (+40 7cc dished, 544 cam, lightened flywheel, ARP, bearings, etc) and gearbox. I used a second hand head from ebay that looked in good condition.


It all fired up nicely and felt good and I have good compression of 170psi on all cylinders. However...


1. Oil pressure was low at 20psi anywhere from 1000-3000psi

2. Large gushing oil leak from what I am pretty sure is from the head gasket on the corner of the radiator/wipermotor (right rear looking from the front of the car)

3. I have a cheap minisport copper gasket (no fancy oring over the oil feed hole)... I didn't realise these were cheaper alternatives :(


So, I have pulled the head off and inspected the head more thoroughly where with a straight rule I can see a gap around the oil feed hole (pictured) of about 3-5 thou. I also double checked my pressure relief spring and oil gauge connection and checked the torque on the headstuds.


1. Does this make sense that this could result in the oil leak and low oil pressure? i.e. would this oil feed hole from the block to head result in low oil pressure if it was leaking here?


2. I was planning on buying a good quality head gasket (Payen stamped AF470) and maybe getting the head skimmed. Is this a good idea? Or should this gap be able to be sealed by a good head gasket?


3. Is 5-10 thou going to be ok to skim the head by? I don't know if this will have a bad effect on compression ratio?


4. Is there anything else I should check. My block was bored out by good engine rebuilders and I replaced the core/gallery plugs after a dip. I'm pretty sure it's not the half moon seal or timing cover as these are lower oil pressure areas? and I even have a new timing plate to make sure it wasn't warped.



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#2 ACDodd


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Posted 02 November 2019 - 11:01 AM

Yes skim it flat (this should have been done at first build. 0.010" skim will reduce the chamber capacity by almost 1cc upping the compression. If you are already too high this will make matters worse.


#3 ChrisRuffell


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Posted 02 November 2019 - 11:50 AM

I replaced my head gasket with a cheap copper minispares one and had a coolant leak/weeping between the head and the block which I couldn’t stop.
Replaced it with a composite head gasket and it’s been perfect ever since.

I may be wrong but I think copper gasket only really work on perfect and freshly skimmed mating surfaces, whereas the composite ones have a bit more tolerance.

Edited by ChrisRuffell, 02 November 2019 - 02:35 PM.

#4 sledgehammer


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Posted 02 November 2019 - 02:32 PM

check the blanking plug on the side of the head

what the oil gallery's are drilled from


some are at the top of the head , some are at the bottom

if it is leaking , it will leak a fair bit & will look like head gasket leaking

also have a look at the gasket - the quality is getting worse on a lot of these gaskets


and as above surfaces


IMO the oil leak in that area should be low volume - due to pulse feed from cam bearing , so shouldn't affect the main oil pressure


(the rocker gear oil feed is reduced , as it don't require as much oil as the other bearings) (all my opinion)

Edited by sledgehammer, 02 November 2019 - 02:39 PM.

#5 Moke Spider

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Posted 02 November 2019 - 04:03 PM

You can drop a O ring in the Gasket if the deck is otherwise flat. Check that it isn't too thick in section, only needs to be about 0.010 - 0.015" thicker than the gasket and no more. Check too on both the head and block that it won't cover the Oil Feed Hole. While it is possible the head could be warped or out of flat by 0.003 - 0.005" at that location, I doubt it would be that much without having coolant leaks as well.


To check the head to flatness, I'd suggest using an Oil Stone. Give it about 5 wipes over and have a look at the deck. You'll well see the high spots and you can gauge from the witness marks how far out it is.


Looking at your photo, it appears the Brass Plugs are also on the move. Check with your new gasket that the Firing Rings don't come over these and if they do, then you'll need to have them replaced. I fit threaded Cast Iron Plugs on the earlier heads (which yours is) or machine a step in to these drillings so that new plugs can't walk. I've always found the Plain Brass Plugs will move.


Your low oil pressure won't be related to this issue.

#6 mark205gti


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Posted 02 November 2019 - 11:48 PM

Thanks all, I might take both of my heads in for a dip in the tank and see how much they need to machine off. Everything in the engine is pretty much new (bearings). I checked the crank with a vernier and observed the correct OD and no scoring. so I don't know why the oil pressure is low. I think my compression with the +40 6/7cc dished pistons using the default cylinder volume is 10-10.5. So if I use the head that needs to be skimmed then I may need them to do some optimisation to the head?


I checkd the oil gallery blanking plug/rivet. It's definitely dry. The cork rocker gasket was leaking a lot but I think that was a separate issue. I plan to replace that with a silicon one.


It was quite a stream of oil coming out while running so are you sure this would not limit the pressure to 20psi flat. At 1000psi it was 20. At 2000psi it stayed at 20psi. It could be my gauge is buggered. But it's the same gauge from before the rebuild and my oil pressure was 90psi.


I hadn't noticed the brass plugs. I'll get the machine shop to see what they can do there as well.

#7 whistler


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Posted 03 November 2019 - 09:31 AM

I would check that the oil pressure relief valve is not stuck (big nut by the oil pressure gauge takeoff).

#8 mark205gti


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Posted 04 November 2019 - 01:07 AM

Thanks. Is there a measurement depth to the back of the 'pressure relief bullet' when it is inserted? I'd like to make sure it's going in the full distance and not getting stuck half way open.

#9 mark205gti


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Posted 06 November 2019 - 08:10 AM

Phew. My other head back on and all leaks sorted. Still has the no more than 20psi oil pressure. So I borrowed another guage that a friend had confirmed accurate after comparing to another guage and it all looks good. His gauge read about 75psi when first started from cold at about 3k rpm.

So I think I’m happy with that. Time to bleed the brakes and finish running in the new rings!

Thanks for everyone’s help!

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