It should say the bore and stroke on the top right of the left side crank case. That will let you know if it's a Big 4 or not.

email (option): horror@blueyonder.co.uk

Grahame, pre-war Nortons didn't incorporate a model code in the engine or frame number. I'm not sure exactly how far back the factory records go - from memory to numbers in the low 20000s in the early 1920s.

A 15000 number would probably date from WW1. Does it have a forward facing magneto drive or is it at the rear of the cylinder ?

Based on what I've seen in the past, I'd suspect a WD16H engine rebuilt with new crankcases at some point and with the omission of the 'W' prefix.

A photograph of the engine with a close up of the numbers would really help.

Hi
I cannot get to the engine until tomorrow night to get an overall picture and check to see if the bore/stroke is there but I do have a shot of the number which should be below. The mag mounts at the rear and at first glance the overall look is that of the 16h.
Thanks for the help
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email (option): Packagingmuseum@gmail.com

email (option): Packagingmuseum@gmail.com

I'd say a WD16H with later-war thick flange barrel.

The number font is not Norton nor one usually seen on WD rebuilds. It looks like a style I've seen on 16H engines in France. Do you know where it came from ?

Hi Rik and horror
I cleaned up the barrel flange at the point you suggested and it revealed that it is a 79 x 100 bore, so a 16h as you suggested. I also found a spot of green paint so it might have been in a military bike.
The lack of model type and the odd number font was what really what threw me, hence the query.
I have owned the engine for five or so years and I got it here in Ireland. From the condition of it I doubt it was brought recently into the country.
As far as I am aware the Irish military didn't use the 16h so it must have come in privately. My thought was that if it was a big 4 then it might be worth salvaging but as it's a 16 then all it is really good for is a giant paper weight, and a family member is looking an engine for just that so he can have it.
Many thanks for the help and information
Grahame

email (option): Packagingmuseum@gmail.com

You can send it to me. I'll pay postage and a bit on top

email (option): j@clogmaker.co.uk