Has anyone tried to mount a derby cover that fits an M8 touring bike to an M8 softail? I usually don't like a lot of the HD collection engine covers, but I kind of like the Kahuna derby and timer covers. The timer cover is listed as fitting my 21 Street Bob, but they don't make a derby cover in that collection for softail. Anyone know if the touring version will actually fit, despite what HD lists on their site?

Any idea why they are different?

Zach

 

Looking at the Harley parts diagrams, I have my doubts. The primary cover's are quite different on the derby mounting face, and none of the part numbers match up.
But if the bolt holes match, with a little creativity, it should be possible to make it work and seal. Though it might not sit flush mounted on a softail primary cover.

 

Won't fit.

Late model touring bikes have a derby cover that is 7.75" in diameter.

Late model Softails have a 6.4" dia derby cover that's a real oddball. The outer primary has five standoffs that fit inside of recessed pockets on the derby cover, and an alignment notch in the top hole so you can't mis-clock the cover. The gasket surface is recessed on the inside of the cover, similar to how Sportsters are made.



As to why they are different, I wish I knew.

Best I can say is that HD goes out of their way to change the sizes on various accessories in order to force customers to buy them over again when they change bikes. That comes from a couple of "industry insiders", and it seems reasonable to me.

I can see good reasons why HD got away from the 3-hole design from the Shovel/Evo era. Those had a tendency to warp, so going to five holes made sense.

But they also increased the dia to 7.25" when they went to five holes. Why? Your guess is as good as mine. The real annoyance is the five hole timing/points cover for the Twin Cam motor. There is no reason HD couldn't have stuck with the same size they used on Shovels/Evos, particularly since on the TC, the timing cover is purely decorative. Then they changed again when they went to the M8 engine. Again, for no reason. They could have used the Shovel/Evo size timing covers all the way through.

They maintained the 7.25" dia derby cover through most of the Twin Cam era, then went to 7.75". Again, I can't see any engineering reason to make that change. It was just to force customers to buy them over again.

The M8 Softail version is really something. It feels like some designer at HD decided to see how absurdly complicated he could make it, and that's what popped out. They even changed the mounting bolt size. Every other model uses 1/4-20 bolts. M8 Softails use 10-24's. Took me quite a bit of trial and error to reverse-engineer those covers. They included a little trick in the geometry that isn't obvious at first, and unless you know it, they will leak.

Anyway, bottom line is what you are hoping for won't work, sorry to say.

 

Thanks Omaha. I was at a dealer over the weekend and saw the difference immediately when I looked at a touring bike. I agree with all your comments. I do like the larger derby covers, and see no good reason why they had to put a smaller one on the softail. I went ahead and picked up the Kahuna timer cover. I found it hilarious the instructions indicate 1 hour install time. Good grief.

Zach

 

Is that something new? I've never seen "install time" in the instruction sheet(s) included with the various HD accessories I've purchased.

 

First time I remember seeing it as well, but I rarely buy HD accessories.

Zach