1994 GMC Suburban k1500 with a tbi 350, I'm replacing the distributor cap and rotor. Upon removing the old distributor cap, I discovered there are two sets of threaded holes for the distributor cap, something like half an inch apart. As luck would have it, I did not notice which set of holes the old cap was attached to. The cap seems to fit either set, but surely advancing or retarding spark by that amount could cause some issues. The two distributor cap positions must be almost 10 degrees apart. Is there a way to know which set is correct? Or do I just have to chance it, pick one (I have a 50% chance of being right) and set the timing with a light as soon as it warms up?

My worry is I'll guess wrong and have the timing 10 degrees too advanced, so the engine tries to turn itself backwards and busts something. Or if I'm wrong the other direction and the timing is 10 degrees retarded, will it even start?

 

Engine won't turn backwards. The cap is only going on one way.

 

Ok, good to know. In further news, it was at 11 btdc.