Hello, Evo Forum!!

You all know me as @0maha , but as I've hinted at here and there, I just started a new gig where I'm making custom derby covers. In order to get the word out, I became an HDF sponsor. They insist that you create a new account when you become a sponsor, so that's why I'm now "Omaha Derby Covers".

I'm threading a needle here with this post. Even as a sponsor, you aren't allowed to start a thread to openly pitch your stuff. No no pitching here.

What you are allowed to do is start a thread to discuss technical information. So here is my highly technical post on derby covers. And if you're into engineering like I am, this is actually a little interesting, even if a lot of this is obvious to those who know.

With the exception of some models in their final model production year, all Evo big twins had the three hole derby cover that they inherited from the Shovelhead. The derby covers are 6.375" inches in diameter, with three 0.280" dia mounting holes to accommodate three, 1/4-20 mounting bolts. The mounting holes are positioned at 12, 4, and 8 o'clock, located 2.812" from the center of the cover. Oil seal is maintained by way of an o-ring that fits in the outer primary, and the derby cover has an interior flange 0.500" wide. The outer lip of my three hole derby covers is 0.140" tall, although that isn't a critical dimension.

The stock derby covers supplied by the MoCo were chrome plated aluminum, and had a mild domed shape like this:



From my perspective, the three hole Shovelhead/Evo derby cover is the best one ever, because they are relatively easy to make, and you have a lot of freedom on how to make them. For example, there is nothing important about the dome shape. It is just there for style. You can make them flat, or even concave, so long as the center of the cover doesn't extend into the outer primary. I do a lot of these with a flat shape (the "Evo Speed Shop" design uses it), like this:



Evo Sportsters, don't work that way. Those covers are more complicated. They have a gasket surface inside the mounting holes, a little spring retainer ring in the center, and a bulge on the outside to accommodate the clutch cable. Making those is a bit more involved, and because of the position of the gasket surface and the retainer ring, you don't have as much design freedom in terms of shape. All Sportster derby covers have to be domed.



My guess is that HD used four mounting holes on those simply because the clutch port would be in the way if they used three. It was easier to just use four.

Things continued to change over the years. If you've never seen one, the derby cover design on Milwaukee 8 Softail models is really something. The outer primary has five posts that the cover fits around, and the top (12 o'clock) post is even keyed so you can't install the cover incorrectly. It has a Sportster-style inside gasket surface. It is also the only time they use something other than 1/4-20 mounting bolts. I try not to take it personally, but HD really made my life difficult when they came out with those.



Just to dust the last corner, here's my quick and dirty way of figuring out which derby cover you need:

1) If your cover has three, four, or six holes, there's no uncertainty. HD only made one version of each of those.
2) If your cover has five holes, there are three possibilities: The 7.25" dia Twin Cam style, the 7.75" dia Milwaukee 8 bagger style, or the 6.4" dia Milwaukee 8 Softail style.

Because of the way HD handed the transitions (ie, some Evos have the 7.25" dia Twin Cam styles, and some of the last Twin Cams have the 7.75" Milwaukee 8 style), it is best to measure to make sure.

And that's what's up with that. I'll be around.

Cheers!

 

Lookin' good, Jeff!

 

I have a serious question. I know that different metals have different expansion rates. How do your bronze castings compare to the chromed Harley primaries? Just concerned about leakage after a several heat cycles. I am not a machinist, so perhaps this is a not a concern and I'm showing my ignorance.

I am considering telling my wife I want one for xmas. Of course I have to tell her what the URL is and which one to purchase.

I like the USMC Eagle, Globe and Anchor. But without the "First In - Last Out" banner.

 

Your wife called, and told me to tell you "You deserve it!"

I thought a lot about that question (the expansion rate stuff) when I started this. When you do the math, the actual amount of movement differential (between the outer primary and the derby cover, that is) from heat cycling is infinitesimal, and the o-ring is easily able to handle it. In practical terms, between my bikes and the handful of friends I made these for during testing, we've got about 30k road test miles behind us, and nary a drip in sight.

Plus, in the course of developing these, I've bought countless derby covers, both from HD and from various aftermarket sources. One thing for sure is that they generally aren't that flat. Well, I mean they are flat, but they aren't precision flat. Makes sense. Most of them are die cast, and there's always going to be a little warp to them. Not a lot. Not enough to notice with the naked eye. But put them on a reference table and measure, you'll be surprised.

Mine on the other hand are as dead flat as a Haas CNC machine can make them.

My God, listen to me. I'm such a bore when it comes to this stuff. Here's the short answer: Not a problem.

 

I'll add to the technical discussion before I peruse the website for a derby cover with a nekkid lady on it.

There is an optional gasket for the cover. It's sort of a real stiff, tar-paper kind of thing. I had one in my Road King when I bought it. I think the cover leaked without it. I also know I had trouble finding another O ring so I just keep reusing everything.

Carl

 

I have seen some of Omaha's work in person

Very Nice

WP

 

I feel like I have a reputation.

Got you covered.

https://omahaderbycovers.com/product...uck-las-vegas/

Funny story: I had that on my display rack at Bikes, Blues and BBQ. This couple came by with a pair of young boys in tow. Maybe eight and nine years old. The boys couldn't stop staring at it, much to mom and dad's amusement.

 

I'm old fashioned meself & remember when naked wimmins were painted sprayed & engraved on bikes before the ***** whipped sados got conned by the skull 'Acceptable' bike graffiti ... time to get naked wimmins back on bike parts.....
ya all too PC fellas !!!... ''No **** in Easyriders' will be next !! hahahaahhaahahah

 

Yes... Even Widmans changed their Shirt... Wussies!!


But... No **** in EasyRiders???? Heaven Help Us...

 

Correct me if I am wrong but HD slipped the narrow primary in staring with some of the touring model lows. Then made it standard on the touring bikes. The narrow primary uses a larger derby cover. My 2016 RU came with the narrow primary and the derby cover off my 2013 would not fit.