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1998 civic dx d16y7 with factory exhaust manifold and cat. I just need something to cover the downstream o2 sensor hole for a week. I have a spare upstream o2 sensor. Is it the same dimensions threadwise as the downstream? Same number of threads and same thread pitch?

I was going to leave it unplugged for my road trips next few days. Just want to know if mechanically it's not going to be different dimensions as upstream o2 sensor.

Cannot remove old downstream o2 sensor from old cat. It's like welded seized tried soaking in Pb blaster hammering heating

 
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Yes.
All these are the same as an old 18mm spark plug thread.

 
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Ok. Also the new aftermarket exhaust manifold's cat's bolt hole isn't threaded so can't transfer this stability? bracket from old cat to new cat. What is the brackets purpose that attaches cat to engine mount, stability of the down pipes? Do I need it? I'm thinking the stability will prevent stress from vibration and motion therefore prevents cracks.

 
Rep Power: 518

You're right, the bracket needs to be in place.... I'd think the weight of the converter hanging off of a constantly vibrating engine, along with the rest of the exhaust movements, can eventually cause it to crack and break if that extra support bracket is missing from the bottom.

Run a tap through the hole and cut new threads in it, or maybe find a bolt size that will slip through the hole and use nut/bolt/washers to secure it.

HTH