Recently purchased a 2018 softail slim which I believe has the wide primary on it. I am just learning now that a narrow primary is available from the 2019 and up softail .Can someone tell me what the difference is?, is the whole primary narrow or is it just narrow at the portion of the derby cover and if so, by how much?

 

It’s just the area of the derby as far as I can tell. Also the derby covers are different between the two types so that’s something to be aware of when looking for custom covers.

 

I can't tell you all the differences, but I can say that the "narrow profile" version has a 7.75" dia derby cover, while the previous version has a 7.25" dia derby cover.

This is from my page on derby cover sizes:

Reading between the lines on your question, I don't know if you could take a narrow profile outer primary and swap it out on your bike. I'd be interested to learn if that is possible.

 

On my 2012 SG, I replace the stock primary cover with the narrow primary cover. It was only narrow by the clutch area.

 

If you think about it, there is no real scope to make it narrower elsewhere other than the clutch because the internals, primary, compensator & clutch are all the same!
We have two 18 Model year M8 Softails and my wife did not like the position of the clutch bulge compared to her twin cam deluxe it replaced as her leg would pretty much touch it when stopped with feet down, so fitted the narrow primary which stopped this problem.




As said it's a direct replacement you do need to get a narrow replacement Derby inspection cover, fixing fasteners & gasket as these are all different. The fixing screws for the outer primary cover and gasket are identical!
You also need to swop over the damper weight ( The round cylinder in the middle of the outer primary cover) unless you get another, which is probably pretty pointless as again this is the same part!

 

Speaking for myself, that's helpful and thank you!

 

That’s some great info, thanks to everyone for the reply’s.