A small step backwards?

There are a small number of references to it on this site. Something about moving it back so the bigger bores clear the camshaft and dimensions of the timing case could stay the same.  But 0.01" (10 thou) must surely be wrong? That's only a fractiom of an overbore.  Is it 0.1"? 2.5mm sounds like it might be useful.

Its called a Desaxe bore

https://en.wikipedia.org/wiki/Desaxe

I now have the Bike for sale on Ebay.

Dave

It also means that when the pistons are at the top of the bore the crank is not at tdc . So there are some small changes to ignition and valve timing . 

Paul Dunstall offered big bore conversion kits to turn both 650 and 750 Norton motors into 810cc monsters. Owners had to bore out the mouths of their crankcases so that the replacement cylinder would fit inside. This in turn did make the top of the crankcases thinner and therefore slightly weaker if the engine was tuned further.

On the 750 engines, because the crankshaft centre line had been moved slightly rearwards, the ignition timing had to be reduced from 32* down to 28* BTDC.  The Desaxe' conundrum.

On the 650 engine, stepped studs were used on the crankcase flange to cater for the rear mounting studs  being closer together compared to those on the 810 barrel base flange. 

As I understand it, and forgive me if I am wrong, I have never had an Atlas but a few bits have passed through my hands, moving the barrels backward shifted the widest part of the bores to a point in the crankcase where they could be moved apart sufficiently to be bored out to 73mm, plus oversizes.

The best way to see this, I think, would be to look carefully at a spigotted Atlas head, the "half-moon " recesses show how the bores were offset to the combustion chambers.

... many ill-informed bits of journalese have been absorbed into the general "wisdom". However I have no knowledge of this particular change. One thing that does strike me is that if the barrels were moved back significantly the geometry of the camshaft / followers would also change as the centreline of the followers would no longer be directly above the camshaft centreline. But I have no idea what if any effect this may have.

One thing that does strike me is that if the barrels were moved back significantly the geometry of the camshaft / followers would also change as the centreline of the followers would no longer be directly above the camshaft centreline.

You are assuming the whole barrel got moved backwards, but it would be easier just to add meat to the barrels at the rear and move only the bore centres back and keep the cam followers etc in the same place. Looking as both 650 and 750 heads together would show which solution they used.  

I can ask Steve Wilson where he got his info from

Dave

If the arrangement affected the ignition timing then it must also affect the cam timing by a very small amount.

The barrels were not really moved anywhere.  It was the centre line of each bore that move slightly rearwards to accommodate the larger bore of the 750. I think that you will find that the front edge of each cylinder liner is in the same position in the 600, 650 and 750 engines.  All the  head studs / bolts in front of the spark plug keep the same location relative to each other.

Nobody has yet mentioned the 850 barrel which has even bigger pistons.  Now where was the cylinder centre line on that engine? As this engine used a version of the 650 crankshaft the bore centres must have been even further behind the crankshaft.

Now the 'killer' point which very, very few people know about. In 1961, inconsistent machining of the 650 & 88SS engine pushrod tunnels was causing problems with the thicker SS rods wearing themselves out inside the tunnels. The factory solution was to thicken up the outside of the front of the pushrod tunnel so that the inside of the tunnels could be widened. This in turn led to the followers moving forward by around 1/16". With flat feet followers this has no effect on valve timing but what happens if the round version is used?

.... how can moving the barrels back have an effect on the "room between the cylinders"?

reply from Steve Wilson, 

The info re moving the block back came from an interview with Hele in Motor Cycle Mechanics magazine, April 1977, principally about the projects he was working on for BSA/Triumph which never came to production; the piece was titled “Paper Tigers”.

dave