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MG MGB GT V8 Factory Originals Technical - Soot around the spark-plug hole???

(This is embarrassing ... I forget the cylinder numbering!) ... on the rearmost cylinder on the starboard-side cyl. head, there is quite a bit of black soot sort of around the spark plug hole, which is also very near a header bolt. The car runs fine, but what could be wrong for this black soot to be building up? The spark plugs are tight, I believe I torqued them at 15#; I don't know what the right torque setting for the header bolts is (anybody know BTW?) but they seem tight. Is the motor about to blow sky-high, or what's going on here? Why all the soot (it's a very clean engine and the soot looks like a black eye!)
Harry

Sounds most like a blown exhaust manifold gasket, Harry
Marc

Harry,

The rearmost on right appears to be a problem and front left on mine. I have lockwired rear right but a real pain for RHD cars because of steering column.

Torque is 13lb according to my book.

I never had a problem with manifolds on the 3.5 but with the 4.6 its a regular problem - I'm wondering if there is not enough slack in the exhaust mountings to allow sufficient movement of exhaust manifold.

Paul

Paul
Paul

Harry-
What does the insulator on the spark plug look like? Is the soot greasy or dry?
Steve S.

From experience it is possible to have a plug that will torque down nicely and still have poor enough threads to not seal the compression chamber. Spray some soapy water around the plug and crank with the coil disconnected to see if the plug is the problem.

Mark
Mark Marchbanks

It's possible that the exhaust gasket for that port is installed incorrectly. It looks pretty symetrical but if it goes in upside down it won't seal well, if at all.
Jim Blackwood

It must've been the plug not sealing ... today I pulled it with more light and the insulator had an even coating of soot below the rubber boot ... must be that it didn't seal down properly into the head. I will try just a tad more torque, and liberally coat the threads with anti-seize and see if that helps seal better. Do they make any kind of goop that should be used here? ... we're not the only folks with alu heads who have to make do with only low torque values on the plugs, so somebody may have run into this problem before.
Harry

Harry,

The consensus among several shops in my area was to Helicoil the plug hole. Although my project is not on the road yet, the seal of my #8 plug is now fine.

Mark
Mark Marchbanks

Mark, sounds like a capital idea. But I'd better wait until the heads are off someday (although if you were lucky, a bit of aluminum swarf probably wouldn't screw things up too badly and it would just get chucked out the exhaust). 'Til then I'll just put a couple more pounds of torque on it when no one's watching. Do you happen to know what the threading is called (e.g. 1/4-20, etc.)? ... that way one of these days I'll get a helicoil kit and stick it in the drawer.

But why only the #8 cylinder? ... I don't get it.
Harry

When you have your threads repaired make sure that the seating surface for the washer is in good shape.If not find a shop that has Wurth Time-Serts, their tooling machines a new seat perpendicular to the threads that guarantees a good seal.A heli-coil can be installed slightly off and you will never get it to stop leaking.
Dale

Harry,
#8 only due to some previous user (these are old Buick 300 heads) overtorqueing or not using anti-seize.

Dale,
Good point, the seat could be part of the problem as well. Unfortunately, aluminum is not very forgiving.

Mark
Mark Marchbanks

A proper repair is not an easy proposition. Helicoils are unsuitable because the helicoil tap enlarges the hole, leaving less seat for the washer and because they cannot be locked in place and may move later. Because stainless is a poorer heat conductor than aluminum the plug may also stay hotter. Solid inserts may be ok, but again the temperature may be shifted. Also, the inserts designed to be self tapping will loosen over time, as may the ones requiring a tapped hole.

The best solution is probably to replace the head, but in the case of hard to obtain parts like the 300 heads sometimes that might not be feasible, and if the engine is reasonably fresh it's hard to justify a teardown. One thing I've done which worked well was to tap the hole out larger and then find a larger plug which was close to the same heat range such as a Motorcraft BTS8, which is a plug for a boat motor. In this particular case it was the #1 plug on an Olds 215, but since the engine had been rebuilt it could very well have been #8 when it was damaged. One of the previously mentioned self tapping inserts had been installed at rebuild time but by the time the plug had been pulled a couple of times it had deciced it liked the plug better than the head. Due to the different coeficients of expansion between steel and aluminum this is likely to be a problem with any insert.

The stock plug uses a 14mm x 1.25 thread. Because the hole had already been enlarged once, I went to an 18mm x 1.5 sized plug because a 16mm one would have been too small. At the local machine tool supply I bought the correct tap, which would have been around $30-40, and tapped the head in the car. If chips are a concern you can stuff a shop vac hose on blow up the tailpipe and turn the crank so the right exhaust valve is open. I would also recommend buying the right tap drill and making sure the hole is the right size before tapping it. These tools may also come in handy later on, so it's not too hard to justify it as a tool purchase, especially as it costs less than a head. Apply cutting oil liberally, as the aluminum can gall the drill or tap and make a mess of things.

The seat may need to be recut. For this I made a fairly simple tool which fits the hole and is run with an electric drill. It was made by turning some stock to fit the hole on one end, then a shoulder, then a smaller diameter on the other end which would fit a drill. Before turning the pilot I drilled about a 1/4" hole in from the side, and then turned the shoulder so that it exposed the edge of the hole, thus forming a cutting edge. Because of the aluminum being somewhat soft, and because all I really wanted to do was level the surface, the lack of relief at the cutting edge didn't stop it from taking off the high spots and I ended up with a perfect seat.

The repair was as good as new aside from needing an odd plug in that hole and a different socket to tighten it. The substitute plug was even in the right heat range.
Jim Blackwood

This thread was discussed between 29/09/2001 and 03/10/2001

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