Cylinder Head Leak

I have two 850 lylinder heads in my shed, that leaked oil from the 3rd fin up. Has onyone ever found a cure for this?
They used to be on my Commando until they started throwing oil everwhere. On my third cylinder head now, this one seems alright.

If the head is leaking from a porous casting, no cure. Another cause that's common, but really difficult to see is the rear valve cover leak that gets drawn to the front from air flow. It looks like a blown head gasket. Look for a stream of oil from the fixing nut downward. If in doubt just use a good gasket sealer. My 750 has a real problem with this to the point where the silicon re-useable gaskets don't work. I have to use original type fiber gaskets with sealer and also seal the aluminum washer under the nut.

I'm not saying you don't have two porous heads, but it is somewhat unlikely. Unlucky, to say the least. I experienced the identical leak with my 72 Combat. Turns out oil was wicking up the 5/16" studs from the cylinder to the third fin of the head. The third fin is where the stud terminates, hence the appearance of oil there. I even pressure checked the head and found no leak due to porosity. This is easily done by using an old head gasket and bolting the head to a flat plate. I then filled the rocker boxes with a very light oil, attached the valve covers and pressurized with compressed air. This required one exhaust cover to have a Schrader valve fitted to it. It's best to heat the cylinder head. I used an industrial heat gun.

JimC said:

I'm not saying you don't have two porous heads, but it is somewhat unlikely. Unlucky, to say the least. I experienced the identical leak with my 72 Combat. Turns out oil was wicking up the 5/16" studs from the cylinder to the third fin of the head. The third fin is where the stud terminates, hence the appearance of oil there. I even pressure checked the head and found no leak due to porosity. This is easily done by using an old head gasket and bolting the head to a flat plate. I then filled the rocker boxes with a very light oil, attached the valve covers and pressurized with compressed air. This required one exhaust cover to have a Schrader valve fitted to it. It's best to heat the cylinder head. I used an industrial heat gun.

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This is exactly what leaked on my head too - i replaced the washers w/ new ones and retorqued the head - leak gone - drove me crazy for a while though....

Mike,

I think it's more common than a porous head. Unfortunately, most of us, myself included, conclude we have a hole in our head; as it were.

iI have two 850 heads that have cracked across the front of the push rod tubes starting where one of the two front studs protudes from the head possibly machined to thin when manufactured, and yes one head cracked across the left push rod tube the othere on the right. showed up when i had them magnafluxed.

Leaks from the rocker boxes can be hard to pinpoint as someone else has mentioned. The oil can work its way into the fins and reappear elsewhere, especially when you are going down the road. I switched to using the reusable silicon gaskets AND sanding the rocker covers flat on a piece of sandpaper on a flat surface like a sheet of glass. This solved rocker box leaks for me. I also had a leak around the pushrod tunnels and/or the head bolts that push through the tunnels. Very common on Commandos it seems!!! A little bit of Wellseal around the pushrod tunnels on the head gasket and sealing the threads for the two front head bolts with Loctite worked for me.

Have you guys tried baby powder or better yet spray on foot powder? You can get a pretty good trail though that.

The reason I was asking this, is because I want to refurbish one of these heads. I can't really believe that they are porous.
The thing with these heads is that any oil leaks always settle on the same level, which is the third fin up.
I have had leaks from the rear rocker cover & the oil banjo bolts, & they run down to this level. I have tried talcum powder.
These heads did leak when I used to thrash the bike nearly all the time, for instance, the last one went after a stretch of 15 miles at 95 mph, 2 up. I still ride it hard, but not to the extent I used to.
I have cleaned out the inside of the pushrod tubes with emery & looked everywhere for any cracking, but seems alright.
My thoughts are the oil may have been coming up through the front engine bolts, as some of you think.
Whenever I rebuild a motor now, I lap the head on the barrels to get a perfect match. If the head is competelty apart, I flat emery the rocker cover faces, & flatten the rocker covers.

Take it over to a friendly light aircraft shop and see if somebody will crack test it for you.

tpeever wrote:

I switched to using the reusable silicon gaskets

I switched to these also and can't get them to stop leaking. Using fiber gaskets with gasket sealer did work. It's difficult to tell how much torque to use with silicon gaskets. How tight should they be?

Hi Illf8ed
I have a leaking rear rocker cover, and suspect that oil is being drawn around to the front also. I end up with oil splattered over the front fins of the head and ends up on the leading edge of the primary drive cover.
:cry:
(I also have a head that needs work, as some of my fellow club members will testify after riding behind me this weekend.
One member made a comment about wiping the oil off his helmet before the return jouney )

I was going to use those silicon gaskets to sort out the leak, but see a comment about those not working well either.
Any experience here? I have used the blue hylomar sealant/standard gaskets in the past.

Thanks, Stu.

My 1974 750 Commando has suffered a leaky cylinder head over the ten years I've had it. Originally is was a slight oil mist on the front fins of the head by the pushrod tunnel, I vapour blasted the head to clean it up and the leak got worse. I decided the head must have been porous so I sent it off to Pete Lovell in the UK, he triple impregnated the head to seal it, this made the leak 80% better but I still get a fine mist of oil that turns into mess after a long run. In the September 2009 edition of Roadholder (UK Norton owners club magazine) an interesting article appeared on how to cure this problem, the article was submitted by a knowledgable source so I've copied it for your information.

A few years ago I had a Mk2a, it too sufferred with the leaking cylinder head. Oil mist would somehow escape from about 2 or 3 fins up at the front, blow along the outside edges and spray all over the back of the bike, looking a complete mess, despite starting off immaculately.

I removed the head and with the help of an engineer pal measured the thickness of the pushrod tunnel wall at the front, which was little more than 1/8". We cleaned the head throughly and then crack tested it. If I remember correctly, it was immersed in some red dye like stuff for a while, then washed off and coated in a white powdery substance which showed up any cracks in red (called a dye penetrant test). Sure enough there was a fine red vein inside one of the fins. Not enough to leak but enough to open sufficiently when hot and allow out a light mist.

The answer was simple, thicken out the material in front of the pushrod tube. I purchased a tim of Devcon, a grey putty like material which, when mixed with its hardener turns to a treacle-like substance and gives off heat. With the head on its edge in the vice, we poured the Devcon into the thin areas between the fins and brought them up to around a quarter of an inch thick or so. They found their own level and the bubbling solution eventually settled and cooled, being quite unnoticeable (it was the same colour as the head alloy anyway). Problem solved from thereon in.

I am going to try this, but Devcon make a number of products, the persom who submitted the article wasn't specific about the type of Devcon used. I'm led to believe that the cylinder head temperature can be in the region of 160 degree C so that limits the choice (I think he probably used a lower temperature rated product that did the job anyway!), I'll try the Devcon Titanium H.P. putty that is rated for 177 degrees C, this doesn't pour and will need to be applied with a screwdriver between the fins.

A proper PCV valve will eliminate most of the nasty oil leaks in the head. Getting rid of the pressure in the crankcase is the key.

hi andyl,i used devcon on a dommie head some years ago when i repositioned the inlet ports,as you say they do different types for different metals,i used the one specified for use with alloy,not sure what temp range it has but it worked

Turns out oil was wicking up the 5/16" studs from the cylinder to the third fin of the head

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While at RGM I was shown a Norton factory drawing noting a change to the lower thread on these two studs, they'd tightened the fit. Norton must have known about the leaking stud problem towards the end of manufacture.

I've been lucky enough not to have worked on a porous head, every time it's been those two darn studs. I always Loctite them in and fit the head gasket bone dry. So far never a leak, 100% oil tight.

I've used Aluminum Devcon on a porous set of crank cases with good results, it should work on a head.


Cash

A proper PCV valve will eliminate most of the nasty oil leaks in the head. Getting rid of the pressure in the crankcase is the key.

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I agree with Jim C. I had that same oil mist on the front of the cylinder head but after fitting the XS650 reed valve, the oil mist has not re-appeared, touch wood. Also, since the PCV was fitted, there is no more dampness around the rocker covers or the tacho drive.

That's very interesting, I've seen Devcon but never dared to use it on something that heated and cooled like a head.

I had a similar prob with my 850, until a guy told me that the pourus castings were the cause for the leak! just inside where the push rod tunnels are.
The advice I was given was as follows:

a) Remove all internal rockers , valves etc... everything!!!!!
b) Clean the internal push rod tunnels of all oil .
c) Very carefully mask and blank the head so these tunnels can be bead blasted clean give a surface for the loctite to bite in too.
(Caution make sure all gasket surfaces are blanked and protected from any abbrasion damage!!!!!!!d) Then very, very "throughly" clean the head Internally to remove any grit! from blasting.
e) Clean pushrod tunnels with brake clean!
f)Warm the head in an oven to about 140 degrees C.
g) Carefully coat the pushrod tunnels in the suspect area with loctite Wick-In. 590, I think!, (Apply the wick-in while the head is still warm to allow it to fill the expanded pores that are in question!
h) Install all valve, rockers etc.....
Allow the recommended cure time before putting the head back in service.

I did this "procedure" on my 850 head, and It cured my problems with the oil leak that I had in the area of about 3 fins from the top!

Brewer.