Look closer. The firing order isnt the issue. Its the cap. Most line drawings you find online are wrong. They borrow from the Haynes manual. They show the correct firing order, but the cap is clocked incorrectly. The plugs wont fire when they are supposed to. Dont use Haynes. I made this a while back to show the difference. The pic posted by Cruiser is correct.

 

When you do a side by side comparison of all brands of USA made brass contact distributor caps, other than the color you'll find they are identical right down to the mold marks. I buy whichever one is priced lower.

 

Don't seal it. Splash water can't get up in there. It needs to be vented. If moisture wast to get in from between the cap and distributor it has an easy escapee path once it warms up and expands. Every time electricity jumps a gap a tiny bit of metal is eroded away. That vent lets some of the metal vapor vent away instead of slowly building up and forming a path for electricity to follow..

 

When electricity jumps a gap it arcs and sparks and erodes the metal a teeny tiny bit. In aluminum contact distributor caps that dust it forms is Aluminum Oxide. Aluminum Oxide is one of the abrasives used in sand paper. Besides brass contact caps lasting longer,nobody wants to be making sand paper dust inside their distributor.trace amounts at a time.