Big 4 alloy cylinder heads

Hi, Thanks Derek

I should have been a bit more specific: both heads are the later aluminium type, the bike is a 1950 model-year (late 1949) version with the 113mm stroke. Its interesting to learn about the earlier engine though, as I previously had another late Big 4 which was fitted with the iron-head long-stroke engine. Thanks again.

Previously wrote:

Hi Nicholas!

Quoting from Tich Allen's Unapproachable: The Story of the Norton, and concerning the WD Big Four - 'under test the poor old Big four engine, although used in its 1938 form with enclosed valves - or despite that innovation, was short of power.A tuning operation was therefore undertaken. The combustion chamber shape was altered to one more in accord with Ricardo principles of confining the chamber to the area over the valves, thus promoting turbulence by squish action. This modification necessitated a slightly longer cylinder barrel so that part of the engine was no longer interchangeable with the civilian version.'

Thus probably explains your different cylinder head, if made of cast iron; though I have never seen one. I assume the Aluminium head carries this idea forward.

Best regards

Derek Ambler

Hi Richard. Thank you for your comment. That does sound a likely explanation. I did have a slightly domed GPM piston in the engine but it was rattling badly, even though the piston clearance was (just) in spec at 12 thou. The Garrett manual says 6 thou over the standard 7 thou is the limit. I'm fitting a different barrel (+30) with a new flat-topped Hepolite piston. Previously wrote:

If both the heads are alloy then they must be from 1948-on engines. Could it be significant that low octane 'pool' petrol was discontinued in 1953 ? From that point onwards, compression ratios began to increase on most vehicles.

There is a fairly common view that most sidevalves can stand a compression increase on modern fuels. Obviously, you'll need to do the standard checks on valve to head clearance at full lift and also make sure that if the piston is domed, it too has enough room.