Engine still knocking....

[Stuart Giles]

Most people live with the backlash. But as you've seen (or felt with a.005" guage) due to the uneven load around its rotation, the cam wheel doesn't wear evenly. If you cut a new keyway in the cam gear at 90 degrees to the existing one, the hard work will be done by relatively unworn teeth. Obviously, this makes 'dialling in' the cam timing a must. IMO, even without a mod like this,  it's worth checking the cam timing with a 360 degree protractor rather than relying on the 'dots' lining up; not least because after many decades of tinkering, the gears in any motor may well not be a matched pair from the factory.