Hi Jeff,
I’ve read your problem and some of the solutions and I think I can help a little. First, as some of the comments you’ve received suggest there are several sources for knocking noises; rod bearings, piston slap, fore/aft cam knock and cam gear. Cam gear knock, once you hear and understand what’s causing it, is a sound you will never forget.

Picture in your mind what is going on inside an engine as it goes through the motion of firing. The crankshaft gear turns the cam gear and as the cam turns it starts pushing the pushrod and lifter causing it to collapse the valve spring. The valve spring gives more and more resistance to the turning system of components; finally the valve is fully open and as the camshaft lobe passes top dead center all of the force held in the valve spring is now pushing on the backside of the cam profile trying to push it forward. If there is a fair amount of backlash between the cam gear and crank gear the force causes the cam to move faster - banging into the crank gear. At idle it sounds like a low pitch non-consistent rattle and goes away above ~1500 rpm. The valve job you did could have caused the noise (or knock) to become more audible, especially if you replaced the valve springs.

Any engine that uses gears will have this problem; some engines seem to broadcast this sound more. The key to reducing the knocking noise is to reduce the backlash between the cam and crank gear.

Your problem reminds me of a problem I worked on a few years back when I was an engineer for one of the three American auto manufactures. We used a fiber gear on an engine that after 6,000 to 10,000 miles would develop a cam gear knock. For what it’s worth here’s what we found:

The fiber gears are made from a phenolic resin and cotton material, think of the cotton material as what blue jeans are made from. If the cotton material had moisture in it and when it was combined with the phenolic resin during manufacturing of the gear blanks, those gears will shrink over time. The engine acts as a dryer, shrinking the material just like throwing a pair of jeans in a dryer for hours and hours. The amount of shrinkage we found was over one millimeter or about .030 of an inch in a 30 hour dynamometer test. If you consider this against the tolerances used for engine parts it is an incredible amount.

If you had the blue print for the gear (or even a new replacement gear) and find the O/D dimension you could check it against yours. If the O/D of your gear is more than .010” undersize I would bet money the gear has shrunk. (Remember the O/D of a gear doesn’t touch, so it wouldn’t wear).

The backlash of the gears should be in the range of .003”/.006”. If you check the backlash and it’s greater than .010 then it probably is the gears. Note that if the backlash varies as you go around the gear (more than .005”) then you have a wearing problem, and this probably is caused by the metal crank gear. (Remember the cam and crank gear system has points where the work is being done, and the wear happens at these points).

The fore/aft movement of the camshaft should be restricted to about .003” to .007” of an inch. The cam and crank timing gears are helical causing a force (while turning) against the thrust plate. The only force I can think of trying to move the camshaft away from the thrust plate is again the valve springs forcing the camshaft rotation in the opposite direction (due to excessive backlash in the gears). Simply put the only way you will get fore/aft banging is if you have too much backlash in the gears.



Jim