Big 4 cylinder head.

Submitted by chris_howard on Wed, 27/06/2018
Forums
Singles

Hi could anybody enlighten me to the function of the 'compression plug' screwed into the head of my 1950 Big 4. I'd also be grateful if anybody could suggest a supplier for the compression plug sealing washer as my poor old beast has spent a large part of its life with a spring washer in its place slowly weeping all kinds of muck into the fin valleys.

Thanks

Chris

Submitted by alan_throssell on Wed, 27/06/2018

Permalink

Previously chris_howard wrote:

Hi could anybody enlighten me to the function of the 'compression plug' screwed into the head of my 1950 Big 4. I'd also be grateful if anybody could suggest a supplier for the compression plug sealing washer as my poor old beast has spent a large part of its life with a spring washer in its place slowly weeping all kinds of muck into the fin valleys

Hi Chris, I always believed it was for checking for top dead centre, be very careful when fitting a suitable sealing washer! I replaced mine with a thin copper washer and the tip of the plug started touching the piston when the engine was running. That night explain why you had a spring washer, put there to gain the required clearance.

I sorted it by shortening the plug very slightly.

Regards, Alan

Submitted by chris_howard on Wed, 27/06/2018

Permalink

Thanks for that Alan it would definitely be convenient for finding TDC. Trouble I'll have, when I get it back from having the face skimmed is getting it out without damage, but that's part of the fun of having and old bike.

Thanks again

Chris

Submitted by richard_payne on Wed, 27/06/2018

Permalink

On the iron head models, the centre plug had an 18mm thread in common with the sparking plug. Did they alter that for the the alloy heads ?

The easiest option is a sparking plug crush washer either 18mm or 14mm, as applicable...If you're worried about clearance, two probably wouldn't hurt.


Norton Owners Club Website by 2Toucans