Until about 1940, most SV engines (Ford V8 hot-rod is an exception) were built with the piston rising to just below the cylinder's deck surface. The remaining clearance + the gasket thickness is the quench distance, per classic Ricardo dictum.
Around that date, Indian started to make the dome rise above the deck in the 640 Sport Scout "Bonneville", and intrude into the head volume (and removing a crescent section of the head to permit this). In 1948(?) Harley followed suit with the improved WR piston & head, with the production K/KH models using the same principle but with chamber shape refinements.
AFAIK no high performance SV engine was built without pop-up after that date.

However: Prof. Gordon Blair's software model for the SV engine makes no provision for pop-up, and shows a transfer passage (in elevation) very much like the pre-1940. It also has quite a restriction in X-area (in plan view) compared to most SV heads.

How does he get enough compression with the chamber not partially filled near TDC? The WR/K pop-up height varied over the years from about 3/64" to 3/16" before going to a radiused dome. Based on the 3/16" figure (Branch's recco for the std. KR piston) the dome is partially in the chamber for almost 46 degrees!

Any thoughts?

email (option): sales@victorylibrary.com

I haven't seen Blairs model and don't know anything about what he was proposing in making it...
'Pop up' pistons aren't really about raising the compression ratio, only maintaining compression ratio while at the same time improving breathing...
Having the 'pop up' allows the 'throat' between the valve and the cylinder to be deepened, improving its ability to pass gas while at the same time maintaining the compression ratio at the same figure, or thereabouts, as it would have been without the 'pop up' but with a shallower, more restrictive 'throat'...
'Reliefs' in the top of the cylinder face also help breathing to a degree but do reduce compression...
Harley Davidsons experiments regarding compression ratio versus gas flow (the two conflict in the side valve with an improvement in one compromising the other generally) concluded that gas flow was of a greater priority than compression ratio when seeking to boost power.
'Pop ups', to a degree allow 'the best of both worlds' in that particular area of the combustion chamber.
Do you have some more details of what Blair was trying to achieve and how he went about it?.....Ian

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

I took a look at some info. I found on the net done by Blair..Don't know if it was the same you looked at. However, I got the impression his work was focussed on valve layouts primarily....Maybe he just used a 'generic' sidevalve layout when looking at this if gas flow in the chamber wasn't his primary focus?. As you say, all later tuned sidevalves (not that there were many) featured 'pop up' pistons and it could reasonably be considered a 'must have' feature in a 'normally aspirated' sidevalve..Perhaps he assumed pressurised induction or fuel injection which would make this feature less relevant?
Take a look at the Gas Gas competition sidevalve currently in production if you haven't already....Fuel injected and lots of power for the size. There is a school of thought that with modern knowledge and materials, fuel injection etc. etc. the sidevalve may yet make a comeback due to its simplicity and relatively low cost of its manufacture....Ian

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

Is this why a 16H is argued by some to out perform an M20 Ron

email (option): ronpier@talk21.com

On paper they make virtually the same power at virtually the same revs and are virtually the same weight...So their performance will be virtually the same overall...Neither has enough power to pull really tall gearing, though there may be some variation due to differing gear ratios at different points in the rev range...I would argue they are more or less the same thing taken in the round...I haven't yet heard a logical explanation of how the Norton achieves its suggested performance advantage....Ian

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

That's similar to the last (fully developed) 1968-69 KR which had a 7/16" pop-up large radius domed piston. See the diagrams and pictures at http://beautyofspeed.com
That shape not only fills the chamber nicely, but also provides a better path from the roof down into the bore at the far end of the cylinder, and the valve-side radius is less obstructive to charge approaching across the deck surface.

I wonder if there's enough material in the M21 alloy head to machine a dome relief?

My replica H-D ULH heads are too thin to do this, so I'm stuck with the next best: the dome's pop-up is a conical section with 20 degree sloped sides and head to match.

email (option): sales@victorylibrary.com

You can alter the alloy M20/21 head for use with a domed piston...I currently have exactly that set up on my own bike...
However, this set up doesn't equate to extra power in the 16H (over the M20) which you would think it might, but it may be mitigated by other, less good features...
The two engines achieved similar outputs but are quite different in many ways.
The Norton is far more 'undersquare' at 79x100 (M20 82x94)..The cranks are different weights, the M20 has a 'wider' cylinder head, different valve sizes and positions, different plug position and different cams....Also the exhaust and inlet arrangements differ....In fact it's surprising the end result is so similar!..

....Ian

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