Modern Vespa

Does it matter which direction the electrode is pointed?

into the combustion chamber for maximum boom.....

otherwise nope, just wind it in and tighten it up

I'm looking at the burn pattern in the head and thinking the electrode is currently 180 degrees out of position.

What does the head of the piston look like?

Solid uniform carbon.

I've never heard of a plug being out of position in that way in my life. Too long or short sure but never that the electrode was facing a right or wrong way. Never even thought about it. Interesting. Like you, I'm now wondering if that's a thing

https://www.motortrend.com/how-to/how-to-index-spark-plugs/

Maybe it's all academic, especially with a 5 hp engine, but if the electrode gap was facing inboard of the head, it would see the fuel-air mix faster and if nothing else would facilitate easier starting. Currently the gap on mine is in the shadow of the ground electrode as evidenced by the heavy carbon up the middle of the head. No?

I really wouldn't worry about it to be honest. unless it's a high compression and high performance build it won't make a gnats ass of a difference.

flame front travel in regards to indexing is only going to come into play when you got something hot-shit cooking. on a stocker? nah.

Agree! Somethings that can be important if it's a race car where the difference of a couple tenths of a second can make a major difference in race just are meaning less on a engine for daily use.

I recall somewhere, think it was a Harley forum, where someone was claiming you needed to use spark plug wires if the same length or else the timing would be off from the front to rear cylinders. Like the spark running through the wire at 186,282 miles per second is going to be noticed from a difference of 2 or 3 inches of wire!

LOL. People are full of so much shit sometimes. Does it technically take more time for an impulse to travel 2 more inches? Yes. But someone thinks the tolerances on their HD are way more (too) precise than they are

Ask yourself . . "will indexing this sparkplug get me home?"

in theory, probably matters if you are doing some kind of timed test. would you even notice on a 5 Hp engine? probably not. i'm not even sure i would even notice on a 25 Hp engine.

On a head with centralized plug you index open end toward the boost port, where the fresh charge is coming from. On these offset heads it's all wonky, but I'd say that you want the open end to face the squish band since that's where the fresh charge is being pushed from. It also makes sense that you don't want the spark facing a corner of the combustion zone. Stance on indexing depends on which side of 80/20 you stand, which in the case of race engine building is that it takes 80% work to get that last 20% which makes all the difference.

As stupid an idea that this sounds like (to me anyway, and at least one other forum member), apparently people do give a shit- at least in the high performance world:
,
https://www.moroso.com/pub/media/wysiwyg/PDFs/NASA_Speed_News.pdf

Just to clarify, I wasn't reacting to the sparkplug indexing idea with that comment. I was reacting to the idea that modern Harley Davidsons won't run right if one spark plug wire is 2 inches longer than the other

Sometimes I release a bit of air from my tires into a special test unit to measure the oxygen content in my tubes to make sure I'm getting maximum performance and optimal gas mileage. I remember reading that in one of my Piaggio service manuals.

I have some surface gap sparkplugs to play with on a custom head, but this would come into play only after:
* central squish
* squish gap optimized
* central plug
* toroidal combustion chamber shape

Tested it back in the day with my race bike. Zero difference on a small frame with the angled plug. Not worth the time.

Compression ratio was 13.5:1 which is way higher than any off the shelf kit for reference.