A friend of mine had a basket Norton 16h given to him which came with two heads. What model sidevalve had the non domed shape head. His motor also seems to be fitted with a 120mm stroke crank, therefore the domed piston fitted pops past the top of the barrel by about 3/8 inch. Big 4 crank?

I think he has a Big 4 crank (120mm) and head. I image you could bore a 16H barrel out the other 3mm to give you 633cc.... if you can find a B4 or suitable piston. Or a suitable 79mm piston from say a post war B4 to give 600cc. Ron

email (option): ronpier@talk21.com

I presume the big 4 had a taller barrel to suit. looks like he'll have to make a barrel spacer for the extra height.It has had a hard life the cases have been welded in the past.

Yes I was thinking along the lines of M20-M21 were the same barrel is used. I just measured my B4 and 16H and the B4 is about 3/4" taller. So I guess some longer push rods as well as a spacer. Ron

email (option): ronpier@talk21.com

The left hand head looks like a homemade bodge job to increase the CR. It seems someone had plans to relocate the spark plug as well otherwise why not weld up the TDC inspection hole while your at it?

Keith I'm sure that is a Big 4 head for a flat top piston (I'm not going to take mine off for a look) and maybe someone had designs on using a twin spark mag. I know I couldn't use a 16H gasket on mine so I fashioned one from solid copper.
The timing hole is too useful to weld up. Ron

email (option): ronpier@talk21.com

Keith I'm sure that is a Big 4 head for a flat top piston (I'm not going to take mine off for a look) and maybe someone had designs on using a twin spark mag. I know I couldn't use a 16H gasket on mine so I fashioned one from solid copper.
The timing hole is too useful to weld up. Ron

email (option): ronpier@talk21.com

Sorry, but that is not a military B4 head, best to measure the bore though, will try to post a picture later. The B4 cylinder variations are also plentifull, the 600cc version being slightly less tall then the 633cc, very deceiving.

Cheers, Lex

email (option): welbike(at)welbike.net

OK Lex. Interesting to see your picture. As I said, I'm not about to pull mine apart to remind myself what it looks like Ron

email (option): ronpier@talk21.com

Is that pitting on the cylinder head on the left due to preignition (detonation) or just a bad case of corrosion?

The second plug has more than likely just been used as a convenient way to blank off the hole...If it was an attempt at a twin plug set up it was one doomed to failure...
With twin plugs both plugs would be in the combustion chamber...The whole point of the squish area is NOT to have combustion there, so it would be a very illogical place to fit a second plug. If it worked at all would probably make the combustion worse.......Ian

email (option): ian@wright52.plus.com

Sorry, seem to have lost my head, so no picture, but from memory, the flat piece extends past the timing plug hole, and then gradually slopes towards the valve pockets, no curvy bits like above, that most probably is a 600cc postwar head, that can be used very well on a 633 engine.

But then my memory can be playing tricks with me, and I may be thinking of Indian Sportscout heads, that I've been working on recently, time will tell.

Cheers,

Lex

email (option): welbike(at)welbike.net

The pre-war 633cc Big 4 was found not to produce enough power when combined with the swd outfit and all military versions received a new cylinder head which was continued post-war for 1946/47. The post 1948 engines had alloy heads so it's not from one of those.

The head on the right is clearly 16H as it has the 'Y' form given by the 79mm bore. The left hand head looks to be for a bigger bore.

The pre-war civilian iron head Big 4s (pre and post 1938 redesign) always had flat pistons and kept the same head.

I'd say that the head either has to be from a pre-war civilian Big 4 or possibly a 1946/47 Trusty tractor or stationary engine - I have no idea of their state of tune.

Is the engine the type with open or enclosed valves ? 633cc cylinders for either will be quite difficult to find. A 16H crank would be an easier proposition but most common are ex-WD which differ from those fitted to the enclosed tappet engines (I believe the t/s mainshaft changed along with the timing gears).

Photographs of the other major components would be interesting. I may be able to pin down the crankcases.

If anyone happens to have a spare head for a Big 4 I would be very very. Grateful. John

Still no head, sorry John!

Have made a picture of a genuine 633cc B4 head, and it looks same as above, probably the shadow threw me off balance!



Cheers,

Lex

email (option): welbike(at)welbike.net