Head / Block Leak (Possibly)

[Steve Jones]

Agreed. And tightening the plugs down to 30 lbf.ft seems an awful lot. Hand tight and a good nip seems a lot more reasonable.

Steve

[Charles P]

I would check that the steel plate on the exhaust manifold is absolutely flat. It may have distorted slightly when the exhaust pipes were welded on.  With this type of manifold set-up there is quite a lot of strain on the fixings into the block, unless they are good and tight you could get leakages.  If I was getting a hissing noise my first instinct would be to look at the plugs.   Have you tried turning the engine over in neutral on the starting handle (with the ignition off!) and listening carefully to the spark plugs?

Putting a bit of water in the plug wells on a cold engine helps to identify bubbling at the plugs. If it's a '37 head bored out for 18mm plugs, the washer may not sit flat due to ropey machining. 
Steel manifolds are frequently a bit bowed and I find that my manifold bolts do loosen over time unless tweaked.

[John Mason]

If the hissing sound is near to where the top water outlet is check the hose clips. A small water leak dripping from hose onto a warm head will make this noise as it boils and evaporates . The head is the first part of the engine to warm up.

John Mason.

[John P]

Not going very well and my hopes of an cheap and easy fix didn't come to anything.

As suggested I put some soapy water around each of the spark plugs and there was some degree of leakage from all of them.

The new seals from Green Spark Plugs arrived and I tried both types.   The budget ones, which are a "U Section" ring didn't help at all but the copper ring types did reduce the leakage and Cylinder 1 and Cylinder 4 now seem OK although to achieve this I did have to tighten them more than the suggested finger tight plus 1/4 turn.  Cylinders 2 & 3 however still leak slightly.

I did wonder if the pocket in which the spark plug sits is really too small in diameter causing the outside diameter of the plug to rub against it and push the plug sideways.  The HC Head which has been drilled and tapped for 18mm plugs has a pocket diameter of 25.11mm whereas the original low compression head which was on the engine when I got it has packets which are 27.94mm.   The 18mm plugs have a body diameter of 22mm so I would have thought 25.11 would be fine but I can't see anything else.   The sealing face in the head was spot-faced when the head was originally modified for 18mm plugs but I guess this could be out of square with the thread.

Unless there's no alternative I'm reluctant to get them machined as it always cost more than I expect and takes longer meaning more time without being able to use the car. (But see below).

However, I have now found that the hissing sound seems to actually coming from the carburetor inlet and occurs when Cylinder 1 is on it's compression stroke so I'm guessing this is probably a leaking inlet valve.   I've tried to check the tappet clearance but, working on my own, it's almost impossible to do with the engine in place.   I did however manage to get hold of the tappet screw and with No 1 at TDC there is apparent movement up and down so frustratingly it's not just a sticky valve.

I suppose the only way forward is to take the engine out and the head off so I can investigate it properly but it's getting increasingly difficult to justify continuously spending money on a car I hardy ever get to drive.

John.

[Chris KC]

John, I wouldn't lose sleep over the odd bubble at the plugs, unless they are totally failing to hold pressure. 

If you suspect a valve isn't closing / sealing properly that certainly merits further investigation.

You'd do well to acquire the art of checking the tappets - with feeler gauges - 'blind' - it's a bit of a fiddle but beats removing the engine from the car every time you want to check them. Feel for the head of the tappet screw and slip a suitable sized gauge across it. If it won't go in try a thinner gauge. Making sure you've got the correct tappet and the correct position on the stroke, of course!

If the tappet clearance is good, it's head off, visually check the valve seat and lightly grind the valve in as required. Then re-set the tappets.

It involves a bit of stooping over the car but it's all do-able with the engine in situ.

[Reckless Rat]

I would try a smear of Loctite 5920 on the plug threads and interface. It should seal any leaks.

[John Cornforth]

Hi

I don't know if it's relevant, but here are some of NGK's recommendations for spark plug torque:

18 mm Flat seat in Cast Iron 25.3 to 32.5 lbs-ft
18 mm Flat seat in Aluminium 25.3 to 32.5 lbs-ft
14 mm Flat seat in Cast Iron 18 to 25.3 lbs-ft
14 mm Flat seat in Aluminium 18 to 21.6 lbs-ft

This ought to give a gas-tight seal, so any bubbling suggests a problem. I have sometimes found the seating areas in the head a little bumpy, but you say these have been spot faced.

[Hugh Barnes]

And use the rule of 9 (or 8 depending on your definition) when checking any given valve. I can do no better than to quote from t'internet:

When number 8 valve is raised (meaning the engine is at the top dead center for that cylinder), it's generally recommended to check the valve clearance of number 1 valve. This is based on the "Rule of Nine" or "Rule of Eight," which states that when one valve is fully open, you should check and adjust the clearance of the valve whose number, when added to the open valve's number, equals nine (or eight)..

hth...

[John P]

Just wanted to thank everyone for their advice.

To be honest I was beginning to loose faith in ever getting the car reliably on the road but at least I've now got a bit of motivation back.

Will report back in due course.

John.

[AustinWood]

If the car is running OK I wouldn't worry about it.
With more use the noise may go away or may get louder and so be easily identified or may just stay the same.
I assume it's quite loud otherwise couldn't be heard over the usual noise of an Austin Seven engine,