Originally Posted by Dino308gt4
I'm building a mild SBC 400 and I noticed that the camshaft can move about 1/4" front to back with the timing cover on. I discovered this while installing the distributor. As the dizzy gear contacted the cam gear, it pushed the camshaft forward. After the dizzy was installed, I could move the camshaft front to back by turning the rotor.
I put on a new timing set, and everything is lined up there and the chain is snug, it's just that the cam can walk out from the block while pulling on the chain. Has anyone ever seen this? Will it stay aligned in the block while running, or do I need a button on it? BTW, the cam is not a roller.
Chevy uses, or used, a taper on the lobes that caused the lifters to press the cam toward the rear of the engine. This took up the clearance between the thrust face of the cam's timing gear and the block. It's not a reliable process which has resulted in a many failures over the years, enough that GM has been Class Action sued several times over this issue. I might add they lost all of these suits. The advent of the roller cam came on the heels of the last and biggest suit. So the need to take ZDDP and other such similar high pressure lube compounds out of engine oil for reasons of extending catalytic converter life coincided with the need to do something about the bouncing cam problem that ate lobes and lifters. Ford went through this with the FE back in the late 1950s. But they didn't ignor the problem like GM, they went first to a production thrust button in mid 1958 and finally threw the whole Mickey Mouse solution out in 1962 and went to a thrust plate.
With todays low HP additives in motor oil, ZDDP being an example of organic- metallic compounds removed, the problem of lobe to lifter wear has grown. The aftermarket manufacturers are using better materials to counter the problem, but from an engineering standpoint using tapers on lobes to force the already pretty busy lifters to provide thrust control is really more like wearing gloves because your pen leaks. It just doesn't address the root cause of the problem.
I highly recommend using a thrust button to take this fore and aft load off the lobe and lifter interface. However, the thrust button is not an easy thing to set up. extremely careful attention to details must be paid to arrive at both the correct clearance and a reliable method to maintain that clearance over time. The former is an issue of initial set up which requires following the manufacturer's instructions very carefully. This may require you purchase a set of long feeler gauges so you can check this clearance from the bottom of the timing cover with everything but the pan installed to the engine. You can GOOGLE these things. The latter is an issue mostly of support and long term adjustment. The typical sheet metal timing cover is not strong enough to provide long term support, it will deform which usually opens the clearance, but it also can be deformed inward to reduce or eliminate the clearance. Although this condition places a preload on the cam gear to block interface it cannot cause the damage that another contributor suggests elsewhere in this blog until or unless the preload causes the cam gear to block interface to cut material from either part which would then allow the cam to move backwards and side load the distributor drive gear if sufficient preload contined to be applied. A common means of insuring the timing cover doesn't deform is to replace it with a cast aluminum part which is stiff enough to provide long term support. Or welding a piece of metal to the outside of the area where the cam button rides to stiffen the sheet metal cover. Or using a bolt that screws into a boss on the bottom of some water pumps for this purpose, where the bolt is extended to just touch the timing cover preventing it from distorting. The bolt is then locked in place with a binding nut.