This is the aluminium head gasket of the type fitted to 350cc OHV engines (Model 50) between 1956 - 1959.
In the accompanying photograph you can see it fitted up to barrel of the 56-59 design type (although actually I no longer have a 350cc barrel of this type in my own collection - and an ES2 barrel is actually shown . . . but the design of fins for the two types are the same). Also included in the accompanying pictures is an exploded diagram from the 1957 Spare Parts Catalog for this style of engine (although again, the picture is of a 500cc ES2 engine is shown - but the 350 Model 50 design was the same - with just the barrel spigot diameter being less on the 350cc model - which these head gaskets are designed for.
These are manufactured for us by a well established UK gasket manufacturer.
Fitment and Background Information:
This aluminium head gasket is designed for the later 1956-59 Model 50 350cc engine, as shown in the pictures. This 350cc model was re-introduced in 1956 to supplement the 500cc and 600cc models already being offered post-war.
However, the 350cc Model 50 was actually a resurection of the same popular model that had been offered alongside the 500cc version for many years pre-war. But for some reason, the 350cc version was not included in the Norton line-up when the Norton OHV engine was redesigned in 1948, Norton's waiting until 1956 to re-introduce this model.
Note: If you have a pre-war 350cc Model 50 - then like the visually similar pre-war 500cc engines - no head gasket was shown as being fitted, and you should not require this item.
From a curiosity perspective - if you do own a pre-war OHV 350cc model (or pre-war 500cc OHV model for that matter) - and have found your engine has been fitted with a head gasket (copper or alloy) - then I would be glad to hear from you!, our email is email@example.com. I have only previously had one pre-war 350ccc in my ownership, and as per the pre-war spare parts list - that did not have a head gasket fitted.
Useful Hint when fitting cylinder heads - either with or without an alloy head gasket: It is important when fitting the head gasket to these OHV models to ensure a good quality spanner or socket is used on the head bolts and the bolts are tightened down in an even 'X' sequence. I have never used a torque wrench (either on OHV or SOHC heads), but continue to evenly tighten down until I am unable to apply any more pressure - this normally being with a 12" ring spanner or socket.
However, it is most important to go round the cylinder head bolts again, once the engine has been ran for a short time and is warm. I used to find this particularly true of those OHV models with aluminium cylinder heads fitted with an aluminium head gasket as listed here. This can sometimes be fiddly - and may even entail having to remove the petrol tank to get full access to the head bolts. This normally pays dividends, and I have found I could sometimes get another 1/4 turn of each head bolt when warm. I also found failing to do this could sometimes result in premature failure of the alloy head gasket - particularly the area between pushrod tubes - which seemed most susceptible to 'blowing' the head gasket out.