Cone Knuckle Part 2

When we left off in part one, we had looked at and discussed four of the five issues relating to running Knuckle heads of a Shovelhead Cone lower end. We talked about rocker arm ratios, lifter blocks, cams and pushrod tube angles. That leaves the toughest issue for last; oil return from the top end.

Perhaps a review of how the stock Knuckle oil return system works is in order. Starting on one end of the system, the Knuckle breather gear has a hole that is not found in any later breathers. This hole is timed to provide crankcase vacuum to a passage (also unique to Knuckle cases) which connects to the Knuckle lifter blocks. That is what the fifth hole in a Knuckle lifter block gasket is for. In this way, crankcase vacuum is applied to the pushrod tubes and the aluminum housings which give the Knucklehead its name.

Note that this passage in the Knuckle lifter block goes to the top of the lifter, with no drain hole into the gearcase such as on a Pan or Shovel. In fact, if you have replaced your Knuckle lifters with any aftermarket lifter (with the exception of the ones from S&S, which are made correctly) there is a good chance that you have a hole drilled through the lifter which will bleed off the vacuum that is needed to evacuate the heads. This hole is not supposed to be there, and could very well be the root cause of your smoking and oil consumption on your stock Knuck.

The reason that this vacuum in the pushrod tubes and Knuckle housing is critical, is that the 'spring cups" depend on that vacuum to "suck" the oil, through the 3/16" O.D. Steel lines uphill into the housings. If you have ever had a Knuckle head apart, you are no doubt familiar with the seals that are between the aluminum housing and the tin covers. These not only seal the assembly from leaking to the outside, but they also separate the two sides of the rocker arm to keep vacuum on one side only.

Compare this to the oil return on a Shovel, Pan, or Evo. All of them return the oil from the heads via a passage through the cylinder directly to the crankcase. They basically depend on gravity aided by crankcase vacuum when the pistons are on the upstroke (or crankcase vacuum aided by gravity, if you prefer).

So, what are you to do, if you have a set of Knuckle heads, and a Cone lower end you are dying to put them on? It looks as though there are a few possibilities. One which I have performed is to rework the steel lines coming out of the spring cups so that they don't run uphill as much as in the stock application. From there grafting on soft lines and routing them externally down to the crankcase roughly adjacent to the timing plug converts the drainage from the spring cups into the gravity/crankcase vacuum style like Pans and Shovels. The oil from the aluminum "knuckle" returns down the pushrod tubes.

A second method comes directly from a comment received on what I posted in part one of "Cone Knuckle." Richard writes that he has been running a Knuckle-Cone motor since 1986. He uses Shovel lifter blocks with a brake tube brazed to the intake pushrod tube and plumbs this into the fitting on the crankcase that returns oil from the primary in stock configuration. This puts timed vacuum to the heads much like the stock system and so avoids any mods to the spring cups, etc. I think this is an absolutely brilliant idea, and though I had considered the idea of using the primary return, my thought was to go directly from the spring cups to the primary return. Richard's system is much simpler and cleaner! Thank you Richard!

But, now there is one more viable option. It turns out that I have a reader who works for S&S Cycle. After reading part one, he informed me, and got permission for me to break the news, that S&S is introducing a complete Cone Knuckle engine this summer. The S&S version will be called the Kone Knuckle, and will feature appropriate passages and machining to use Knuckle lifter blocks, and hence a stock style oil return system. Plans are to also make the cases available for those of you who have a bunch of Knuck parts stashed away for just such a project. This will make the whole process relatively painless.

Keep in mind though, that excess clearance between the Knuckle rocker arms and shafts can be a source of excessive oil in the top end, enough that even an otherwise correctly working return system will not keep up with. A rebuild of those parts may still be in order.

So, just one last thought. Whoever said that Cone Knuckleheads couldn't/shouldn't be built ....well, aren't you glad that neither Richard or S&S listened to them.