Archive for Street 750

Do you wear your Harley-Davidson plug left or right?

Posted in Harley-Davidson with tags , on November 8, 2013 by bleiglass

Get excited, I am talking of the spark plug of your engine. And as we are mainly taking about Harley-Davidson, its all about the position of the spark plug in a V2.


We all marvel, pretend to know everything and nod approvingly when we hear pseudo-technical terms like Knucklehead (36-47), Panhead (48-65) or Shovelhead (66-84), but who can really explain the actually existing technical differences in detail? The names are just related to the distinct shape of the rocker boxes, and the average Harley owner will be able to categorise these engines into : old, older and very old.


Here we are again, its all about design, but I like sometimes more background information (and trivia) on the actual technical path chosen. The “Vorsprung durch Technik” gene must be part of me. But back to the subject of this post.

When I saw more detailed photos of the new Harley-Davidson Street 750, I was trying to figure out if this engine was just a baby copy of the V-Rod engine, or if it was a breed of its own.

V-Rod engine

My take: the Revolution X engine shares very little with the V-Rod engine, except for the fact it has water-cooling, like millions of other engines in bikes and cars do. Its not a miniature V-Rod, though some screws and levers may well come out of the hughe Harley toolbox.

Left side view of Street 750

You may have read my comparison of the Street 750 and the Honda VT750s, and while analysing V-Rod details, it suddenly struck me: the spark plug position. While the V-Rod has them plain in the middle of the cylinder head, the Street (and the Honda) have them on different sides of the bike. Front cylinder left, back cylinder on the right side. You may say: so what, the V-Rod got it right with the help by Porsche, as we all know that a centred spark plug enables a more homogenous explosion of the compressed fuel mix. This is also why some cars like Alfa Romeo have “twin spark” engines, trying to overcome the deficit of a off-center position of the spark.

Right side view of Street 750

But that is not my point, if you look at the Sportser Evolution engine, or any big-bore V2 Harley-Davidson engine, the spark plugs of both cylinders are both on the same side, the left!

Sportster_883_Low_LHS_1280Sportster Evolution engine

103 Twin cam engine

I do not know if there is a simple explanation to this build, except the one that comes to my mind: it is just cheaper to build.

Let me explain: in a V2, you want the injection and intake positioned centrally between the cylinders, and the exhaust at the front and back. If you take the front cylinder head and use it for the back too, and just turn it around by 180 degrees, you achieve that, BUT the spark plug suddenly ends up on the other side of the bike. If you want the spark plug on the same side, you have to design and build 2 different cylinder heads (front and back) with their respective valve positions, and that increases costs.

Over to you, tell me what you know, as there must be many more reasons for either build.