Leaking head

[Ashley Bremner]

I have an Ariel 350nh red hunter that is leaking oil out of the bolt that holds the head on by the spark plug,rear bolt. The oil is bubbling out of the bolt through the washer it appears. The engine has recently had a new head gasket but the leak is steadily getting worse. The bolts are well tightened after first couple of hundred miles of running in new piston. It would suggest that maybe the gasket wasn’t good or has blown. The barrel has always been a bit oily before the new piston and valve replacement. The surfaces of head and barrel were dead flat. Any other possibilities other than getting the head off? Ash

[Roger Gwynn]

If it ain't leaking past the gasket then it can only be via a crack in the head, which would be most unusual. I assume your bike dates from 1952-55 or at least the head and cylinder do, if not you should not be using a gasket.

[Julian Murphy]

As Roger has said, no head gasket on these engines.

Who fitted a gasket ???

The head locates down onto the barrel spigot, not on what you think is a gasket face.

When the head is loosly assembled (no bolts), you should see (with a torch), or be able to measure a few thou gap between the head and the barrel face.

You might find that the bolt heads, under the rocker boxes, have caused some cracking of the land around the washers. Cured by making a 'top hat' washer.

[Roger Gwynn]

I never even considered it might be an alloy head, please provide more details of the model and year than Red Hunter, even the 350 was made for 26 years and varied a little over that time!