500cc Lightweight?!

of which I have many- I have mused on the notion of a 500 L/W. The 88 has the same bore as the Electra, and a crankshaft could probably be squeezed into the cases with appropriate machining. 
Biggest headache though would be suitable barrels. The Electra variant is very short, and would not accommodate the longer throw- a barrel packer would have to be ridiculously thick so would have to have some fin arrangement to help with heat dissipation. 88 barrels might have the right bore spacing etc but then you are snookered in terms of (i) having to fabricate a whole new arrangement for the camshaft- 2 in a L/W and 1 in a Dommie. (ii) blooming great big holes where the crankcase mouth meets the barrel flange because of (i)!.
Whatever arrangement there is for the camshaft(s), the cams themselves would likely need suitably re-profiling to suit.
Better off trying to use the Dommie barrels somehow, so a Dommie cylinder head could be used. The L/W heads do not lend themselves to much in the way of porting and bigger valves to accomodate the extra gases involved.
Then there's the con-rods: L/W too short. assuming you have somehow nailed Dommie barrels onto L/W crank-cases, 88 rods seem logical. If you've stuck with L/W barrels- hmm.
And that's before what do you stuff the monstrosity into?
Electra frames have extra meccano struts because of all that extra power (!). Your monster engine will be way too tall to fit into a L/W frame anyway. But will be lost in a f/bed.
Which leads me on nicely to the next madcap idea- a 3/4 size featherbed.
I really should get out more. A LOT more....
 

Perhaps one way to go is to graft an extra cylinder to the two already there. Three jubilee bores running Navigator crank throw must be about 500 cc. Use long con rods. 

Perhaps put the extra cylinder to the right and push the timing chest out. 

Nought but a bit of welding to join bits of crankshaft and camshaft together. 

Bob's your uncle. 

Better move the seat to the right too. 

There is an article in that edition by a guy who built a 500cc lightweight.   He used an 88 crankshaft and Electra cylinders.
It's an idea that's been going around my head for a while.  My idea would be to cast a new cylinder block, in aluminium with iron liners.   The cylinders could be spaced a little further apart creating room for a bigger flywheel.  This would allow a bigger bore for some decent pistons
This would, of course, mean making a crank shaft.  
Cams could be re-profiled if necessary.  Maybe a larger inlet valves along with some port work, although not much scope for that.
The challenge would be to not increase the height of the engine so it would fit in to an unmodified lightweight frame.
The gearbox, that could be a problem.

Why don't you ask Alan Millyard how he would go about it with a hacksaw and an assortment of files.

Think one has been built with a spacer under the barrel. 

This whole thread reminds me of one of Michael Pailins Ripping Yarns where a WW1 POW was attempting to escape from Stralag Luft 24 but the war finished before he could escape. 

The war ending gave him more time and opportunities to carry out his plans.

I could see this 500 light weight firing up just as the second last member of the NOC dies of old age, leaving the constructor with no one to show the creation to. And dieing a sad, lonely, bitter and twisted man.

More likely he would pass away happy and contented that he enjoyed  a special creation that no one else on the planet has.

Ok so here we go.
Bob Tuck does know the guy that built the first one.
It has Electra crankcases and barrels.
88 crank and con rods.
The increased throw can just be accommodated by the barrels.
Fat spacer plate below the barrels and a thick alloy plate above which mimmicks an extra fin.
So as to not make the thing any taller than necessary pistons from a Suzuki GS400 are used as the gudgeon pin is rather nearer to the crown. This means extra bushes in the conrod little ends, more on that later.
Ok so the crank for an 88 is wider than Electra/navigator, so a bit is machined off the sides of the crank.
Also the bobweights of the crank would collide with the cases so a bit off those too.
Crankpins spaced further apart easily sorted by shaving a couple of mil of the sides of the conrods.
Given that the barrels are now raised by a bit over an inch, the usual and clattery cam followers are discarded, bushes set into the baseplate and we now have BMC Mini bucket cam followers and longer push rods.
On the drive side we up the size of the primary drive sprocket. An alloy one designed for a commando is perfect with one set of teeth machined off and the taper is moved. Tricky bit this as it's easy to get this misaligned. More teeth on the final drive sprocket and the gearbox copes easily.
As mentioned earlier, the engine is now taller so won't fit in the frame.
As it happened someone at the time was selling a commando frame cheap as it had been in a prang. Cut off the entire rear section, straighten the downtubes by hand.
Navi frame, cut off the straight tubes that run above the engine and cut off the bottom bits that narrow down to go in the previous downtube. Weld the whole lot together. Half way up the new downtubes weld in a bracket to connect to the head steady.
Engine now fits nicely. To maintain the style an early commando tank is used ( Dommie style).
First trip was to a Norton rally at Bristol Docks around 1997.
Next was the run down to the Dordogne Rally but don't remember what year for sure but could have been 2000. Returning a problem showed up where the little ends started rattling. This was due to an incompetent 'engineer' not doing things correctly so out came the engine and dismantled for final fettling.
Sadly other events took precedence, divorce, house move, all sorts so it never got rebuilt.
Maybe one day soon, when I've finished building the albatross (new house).
H.

Well beyond my skills as well but I love hearing about this sort of thing.

Sorry Henry , forgot it was you. ! . 

Fascinating!  .......is there no limit to Norton Lightweight owners engineering imagination and ingenuity???!!!   Nick

I considered fitting Electra cylinders and pistons but didn't bother, for an extra thirty+ cc's, it didn't seem worthwhile.  Peter didn't mention the spigot on top of the cylinder, or, rather the lack of it on the Electra.  Not an insurmountable problem but more work for not much gain.

The other thing that put me off doing the work was the reduction of flywheel inertia.  The engine, I think, is a bit light on flywheel.  I have managed to add considerable inertia to my Navigator flywheel, nicer to ride over the standard arrangement, I think.

I have a spare pair of Electra crankcases, crankshaft (bare) and inner and outer primary drive cases.
Rare as rocking horse poo, so serious offers only.