John is correct with the obd1 however on a 97 obdII, that distributor should not be adjustable, unless someone cut the tabs off the plastic dist body. There also is not a wire to unhook on 96 or later to set timing. The hold down bolt goes thru a slot in the dist and you can only adjust the dist by about 2-3 degrees, just the slop in the hold down bolt to the slot. These are difficult to set because of that, trying to seat the dist gear and the hold down slot at the same time because the distributor will twist as the dist cam gears mesh. YOU MUST USE A SCAN TOOL TO SET TIMING ON VORTEC 5.7 Here at our repair shop we have the factory GM tech2 scan tool. You will be able to read the timing degrees with that. If the timing is off by more than 2 Degrees it will set a check engine light. Once you have the timing correct, you MUST do a procedure called (cam crank variation re-learn). You MUST DO THIS WITH A SCAN TOOL. In the function with the scan tool, with the engine running, scan tool will tell you to quickly rev the engine to 4000rpm basically flooring it in park, and immediately let the engine back down to idle, and then if completed correctly, it will display that on the scan tool. Even if your timing is correct, you must do the cam crank variation re-learn if it has not been done yet. If you ever touch the dist once the re-learn is done, you must do the re-learn again. Sometimes it takes a few times to get the variation re-learn to accept, but also be carefull because sometimes, during the relearn the rev limiter is disabled to allow the engine to rev high enough for the relearn. If you put your truck in park and are able to rev it up past 3500rpms, then your re-learn IS NOT done.