Chain or Gear driven Cams?
TwinCam and M8 models.
The question is what should you choose with a cam chest upgrade. Chain driven camshafts or sprocket camshafts. In 1999 the Twin Cam came out with 2 camshafts and 2 chains. To keep the chains tensioned, 2 mechanical tensioners are mounted on the chains. Mechanical means with a spring. In practice, this system is not really reliable and the tensioners wear considerably. These tensioners have to be replaced around 35,000 km. If you stick to this, there is nothing wrong.
Gear Driven sprockets from S&S
The "Silent Chain" from Harley with mechanical tensioner 1999-2006
The Harley "Roller Chain" with hydraulic tensioner 2007-2017
The M8 models have almost the same only a larger tensioner.
S&S came on the market with the gear driven camshafts and hallelujah what a great solution. The Harleys has used this in the previous models. No more problems with tensioners, no timing changes due to the stretching of the chains and the gear system also ran much lighter than the chains with the mechanical tensioners. Another advantage is that especially with fast camshafts, the wear of the chains and tensioners is faster than with original camshafts, and that gears are less affected by this.
But there are also disadvantages to the gears. First of all, it makes noise. That was also the reason why Harley switched from sprockets (Evo) to the "Silent Chain". For environmental reasons, the Twin Cam had to be quieter and the chain brought less mechanical noise.
A second disadvantage of gears is that if it goes wrong, it also goes wrong.
A mechanical tensioner from 1999-2006
A hydraulic tensioner
2006-2017
The most important thing with a gear driven camshaft or camshafts is the crankshaft runout. This may not be more than 0.003 in inches or max 0.07 mm. You can imagine that gears do not like it when the backlash becomes less than zero due to a swinging crankshaft. The first Twin Cams, the 88 ci models have a good crankshaft and Gear driven camshafts are a nice option. In 2003 Harley removed the double Timken crankshaft bearing for economic reasons and the crankshaft can move more on the single bearing. This usually does not benefit the run-out of the pinion shaft on which the camshaft sprocket sits. Before you buy gear driven camshafts, you must first measure the crankshaft runout. If you buy an S&S kit with an S&S crankshaft, you safely choose gears.
The 2007 and later Twin Cam models come with the roller chain driven camshafts and hydraulic tensioners. An upgrade which was a good one, because these tensioners only last 70,000 km. Unfortunately, the 2007 and later crankshaft on the Twin Cam models was as straight as a banana so gears in a 2007 and later Twin Cam are not recommended with a standard crankshaft. It is possible, but first measure the runout properly. If you have an S&S crankshaft, or a rebuilt one with a Timken Lager upgrade, you can.
Basically the same story for the M8. There is not much wrong with the original chain and tensioner, but sometimes with the crankshaft. So measure first and then decide.