Good morning!

I'm running a set of KE cast aluminum headers on my 242 with a 489. I'd really like to be able to use my wideband on this setup, but I cannot keep the sensor dry. Right now I have a bung in the tail as close to the end of the header as possible, but it's only about six inches from the end where water mixes in. I've tried a 3" extender and it still gets wet. My question is does anyone know a location on the cast aluminum header that could be drilled and tapped right into the exhaust without going through the water jacket? I have a blower ready to bolt on this engine but without AFR monitoring I'm very hesitant.

Thank you,
Adam

Measure from where tailpipe bolts to header and then all the way to end of your exhaust tips. Should be near 18-19” for best performance most 496-540 marine engines with decent cam.

The end of the header / beginning of the tips is only about 7" from my transom. I was hoping someone knew of a location within the header that could be tapped. The only other option I'd have to to extend my tips a foot past the transom and keep them dry to the end, which would look pretty bad.

The dimension i am looking for is end of header (where tail bolts to) to end (outside of boat) of tips

This is about 14", but the last 8" or so is wet.

That's fine. Slip 5" addt'l temporary hose/pipe on end and see if o2 , in it's current location, still gets wet.
If it still get's wet, my reversion reducing collector length addition did not help you. Has helped some others.
It's more of an exhaust wave tuning length thing, than a water distance thing.

Is there a location that's too close? I'm doing the turbos. Seems the three 'easiest' locations are a spacer under the turbo where all the exhaust is coming together (maybe 16"?) or a spacer between the turbos and the tails (after the exhaust goes thru the turbo). Last option is of course picking a spot on the tails and welding in a bung that doesn't leak through both the outer and inner tubes, but it would be 20"+ away from the last valve.

I had wondered about bungs in those given the extremely long runner length. If you get it sorted out keep us posted.

I’m using a 90° bung extender off Amazon with pretty good results. May be worth a shot before taking more drastic steps.

Did you have to weld together the inner and outer tubes to the O2 bung? Thinking about doing my Lightning headers.

How to install oxygen sensor bungs.