Harley Goes Wobbly on Twin Cam

Since Harley came out with their Twin Cam in 1999, it has gained a reputation as a motor that is fairly easy to modify for significant horsepower gains. Unfortunately, the Twin Cam's reputation for reliability seems to have lagged far behind, and not without reason.

First there was the whole fiasco with the early cam chain tensioners. Seemed as though the life expectancy was anywhere between 20 and 30 thousand miles (though some did not make it that far). When they wore through the tensioner pad, things could get ugly in a hurry. Throw a little hearing loss on the owners part, and you get a cam chain wearing a hole through the cam cover (its true - I just rebuilt one with just that problem).

An early response to the whole cam chain tensioner problem was the S&S gear drive cam set up. Its a legitimate fix, but it comes with its own issues, the main one being gear lash. Of course the whole idea behind the cam chain set up from Harley was to quiet the motors down by eliminating gear lash issues. Only trouble with the gear drives is you now have two sets of gears to get the lash right in order to have a quiet motor; the outer pair which drive the rear cam off the crank, and the inner pair which drive the front cam off the rear one. Although S&S offers and oversize and and under size gear for each pair to aid in getting a good fit, every cam plate will give a different fitment. Too tight on the lash and the gears will whine; too loose and you get the old tappet type noise.

After only seven years in production, the factory came up with a new solution for the early cam chain tensioner failure syndrome. Beginning with the 2006 Dyna models, and 2007 for the balance of Big Twins, Harley fit the Twin Cams with a new type cam chain system known as variously as hydraulic tensioned, or silent cam chains. Apparently the key to this new system is not so much the hydraulic tensioner as it is the type of chain itself. A quick feel of the outside edge of the chains (the part that the tensioner pads ride on) tells the real story. The early chains which were controlled by spring loader tensioner pads are down right sharp compared to the later chains!

All that leads us to that venerable practice of upgrading old motors with later model part. Really, it is a custom that has flourished for many years, and involves everything from putting on an M74 carb in place of your M35, all the way up to the simple but ambitious practice of putting an entire Shovel top end on a Pan. The "late model parts on an earlier model bike trick" has been a staple of the Harley aftermarket as well as the Motor Company itself for years.

The first I became aware of the possibility of upgrading the cam tensioners on '99-'06 Twin Cams was via the Kuryakyn catalog. They even list all the parts you need to purchase from your local Harley dealership to complete the upgrade. Included in the upgrade is the late model cam plate and the new higher volume oil pump, and of course Kuryakyn will be happy to supply a variety of cam grinds manufactured especially for this upgrade.

Checking the Andrews catalog, I see they offer a whole kit with cam plate, oil pump, and everything else needed for the conversion as well as a full complement of cams specifically for this upgrade.

Not to be totally outdone, Harley Davidson, via their Screamin' Eagle division also offer a kit. Harley's kit, however, leaves you stuck using the original "chainsaw" style inside cam chain. The reason they did this was so that you do not have to use a special cam; you can keep the one you have if it is still usable. The down side is that you are forced to keep the chain that contributed to the problems in the first place, and is the one hardest to service to boot. Seems to me it is only a solution to half the problem.

Now, if you are thinking that you lean toward the Kuryakyn promoted solution, I have a couple of cautions for you. Since you are purchasing a cam plate that is not designated for use on the '99-'06 engines there won't be any warning label mentioning that on certain years of NON Softail models, one of the oil holes in the new cam plate will line up with ...NOTHING. That right, if you have a '99 (for instance) Dyna or FLT model, this hole which feeds the balance chain tensioners on "B" motors, will dump pressure oil directly into the cam chest. This will drastically lower your oil pressure, give you noisy lifters, and a flickering oil pressure light at hot idle. The solution is to simply plug the hole; in fact the procedure would have been outlined for you in the instructions if you had purchased a stock replacement cam plate for your early Twin Cam.

this is the offending oil hole

where the boss would be on B motors - note the pencil is pointing at open space

the solution

The next caution has to do with cam sprocket alignment. From what I have seen so far, I am led to believe that Harley has machined every pinion shaft from 1999 until now incorrectly. Every shaft is machined for the diameter of the cam chain drive sprocket just inboard from the portion that rides in the cam plate itself. This machining includes a "flat" on one side which positively locates the sprocket. This machined flat is the problem. Admittedly I have only checked a half dozen crank assemblies, but the range from 1999 to two brand new fresh out of the shipping crate 103" cranks. The specific problem is that the "flat" is machined deeper (further inboard) that the rest of the diameter for the sprocket by about .015". This causes the cam chain sprocket to pull in on that side when tightened. It gives said sprocket a similar amount of lateral runout, which in turn makes sprocket alignment between cam and pinion, shall we say ...interesting.

undercut deeper at the flat

There's not much you can do about the situation, as far as I can see, outside of being aware of the issue so that you can split the difference and minimise the misalignment between the two sprockets. If you were to simply follow the factory service manual and check the sprocket alignment, adjusting to within the Harley mandated .010, you may not think to turn the motor over 180 degrees and re-check. You may not think to, because the manual doesn't mention it. Whoever wrote the manual assumed (as most of us do) that the sprockets would install "square" on the shaft. Oops.

note gap between straight edge and top sprocket

rotate engine 180 degrees and note difference in gap

Now you "get" the title that I chose for this piece; "wobbly" indeed. If that were the only place the factory went wobbly on the Twin Cam, it would be one thing, but it seems to be only the "tip of the iceberg", so to speak. How about the flywheels? Why is it that the specification for flywheel runout on my 1946 Knucklehead is within .001", but on a new Twin Cam it has become four times as much at .004"? Is that progress? Were the machine tools that much more accurate back then?


A comparison of a 1999 Twin Cam service manual with its 2009 counterpart is revealing. In 1999 the spec for flywheel runout was .002 maximum. By 2009 that spec was doubled to .004". Not only that, but the service wear limit (deemed acceptable by the factory before repair) in 1999 was .003 for flywheel runout, but .005" ten years later. Note that the .003" runout which was the limit before replacement in 1999 would be within new setup spec for 2009. In 1999 the spec for pinion shaft runout was .003, and no service wear limit was given. In 2009 that spec, though renamed (to protect the guilty?) is .010" with a whopping (or should I say wobbly) service wear limit of .012!

My conclusion? Harley reached its zenith for quality and engineering with the Evo motor. Of course they reached their zenith in just plain "class" much earlier with the Knucklehead!