I just finished replacing a headgasket and putting a new head on my z6 and the timing belt will not tighten up and I'm not sure why. I did go from a d15b vtec head to a z6, could the belt have stretched and now it needs to be replaced? I have a z6 timing belt. The block is a z6 I swapped out a bad z6 head for a rebuilt jdm d15b vtec head. I really didn't like the power gains and had to do a headgasket again recently so I decided to switch to another z6 head with a comp cam. And now the belt will not tighten up or is tight on the leading side and loose on the trailing side and when the crank is turned 1/4-1/2 turn it is loose on the leading and tight on the trailing side. Maybe its off a tooth, it idles/lopes like an old cam'd out muscle car. If anyone could help me get this car back on the road it would be much appreciated.