Originally Posted by heavymetalhudson
how far does the rear center freeze plug go in and can it effect the cam?
Just enough to plug the hole and leave about a 1/8 inch lip of casting around the outside of the plug. Always check the cam with the gear installed to be sure the plug has not pushed the cam forward such that the thrust surfaces have a clearance.
There is not a lot of space to miss on this if the cam is pushed forward you run the chance of having lifters in contact with more than one lobe, disaster will occur if this happens the cam and lifters will certainly be ruined and it can cause collisions between the pistons and valves, so check and be sure of plug does not push the cam forward such that the thrust faces of the gear and block have a clearance. The thrust faces need to make contact when the cam is pushed backwards. Chevy did not use a positive thrust plate to prevent forward motion of the cam, the natural tendency of the slight angle on the lobes that cause the lifters to rotate in their bores and the distributor gear's engaged tendency to pull the cam rearward was considered sufficient force to keep the thrust faces in contact. The reality of this has been a so-so experience. It works OK with mild cams, but as the cam timing becomes more radical, spring pressures higher and vibrations put into the cam from high power thrusts on the crankshaft tend to cause a flat tappet to jump all over the place taking out lobes and lifters. If if this describes your build with a flat tappet cam then it's a pretty good idea to use a cam bumper for the aftermarket roller set ups to quell the shakin' goin' on.
The use of a thrust bearing between gear and block requires machining the gear or a custom gear with a short thrust face to accommodate the thickness of the added bearing, failure to do this risks having multiple lobes hitting any given lifter and it will affect the distributor gear engagement which can at the extreme leave the oil pump with an inadequate drive connection that can fail. It also pulls the chain out of alignment and may cause the cam gear to strike the inside of the timing case cover.