oxygen sensor hole repair on cast iron

[Dave WM]

Geez louise, I need to repair the 02 sensor hole on the cast iron down pipe from the turbo. Its hopelessly stripped (not enough left to chase). So I figure heli coil etc... BIG bucks I mean like cheap o kit 100$ good stuff 200+. Perhaps its the hard iron that needs some really good taps to work with? there are some cheap kits on amazon for as low as 35$ but they all have terrible reviews. Took it to a machine shop was told 130$. 

So if any of you guys have some exp with how to fix cast iron O2 sensor hole, please speak up. I have another possible solution, the hole is set in a cast in flange area that had a heat shield bolted to the cast iron. I am pretty sure I can turn a alum piece that woutl fit over the 02 hex and then I could capture the sensor that way. I just can see spending 130$ for what seem like a simple tap and time sert job. 

[Dave WM]

O2 Oxygen Sensor Stepped Weld Bungs (5 Bungs/5 Plugs) M18 x 1.5 | eBay

maybe just weld on some mount ears and use something like this. I have not done much drilling into cast iron, maybe that is the expensive trick, tools hard enough to deal with heat cycled cast iron?

[Captain Obvious]

That type of cast iron is usually easy to drill and thread. Abrasive to tools, but if you're only doing one of them, not a big deal.

So you have a lathe now, right?  Make your own insert. Have you tried threading on the lathe yet?

[Dave WM]

33 minutes ago, Captain Obvious said:

That type of cast iron is usually easy to drill and thread. Abrasive to tools, but if you're only doing one of them, not a big deal.

So you have a lathe now, right?  Make your own insert. Have you tried threading on the lathe yet?

have not tried that. Its a really old hobby lathe, no auto feed etc. Good for small stuff. I think for now I may just use a big washer to capture the top of the O2 sensor hex and then drill some matching holes that can be used to bolt the wash down to the flange where the heat shield use to go to retain. Not a perm solution but good enough for now. I may try one of those cheapo kits, like you said good for one try prob.

I have been thinking (dangerous) about adding closed loop to the existing NA FI. Use a voltage divider to arrive at a .5v bias (maybe get fancy with a zener diode), use that and the output from the single wire O2 sensor to arrive at a 0v difference when AF ration is correct (.5v out from the sensor). Feed that in series with the wiper of the AFM, smoke the computer or have a a closed loop FI. any bets which it is?

Edited August 9, 2022 by Dave WM

[Dave WM]

I used the cheap poorly rated amazon kit, worked just fine. I can only assume the neg reviews were by folks that do not know how to use a tap. It took a while to ream and tap but nothing extraordinary. I ended up using a punch to stake the insert since there is no mechanical lock and I did not use any thread lock (I cant imagine it working in the high heat of a turbo down pipe. 

did another video of the engine running this time using the narrow band 02 sensor hooked to a analog voltmeter. I think I am about done testing without the turbo, so that will be added soon. Will use the NA fuel injection and closely monitor the air fuel mix to keep from running lean. Added a variable resistor to the temp sensor to allow for added enrichment. 

[Captain Obvious]

Glad to hear the cheap kit worked. And I don't think I would trust thread lock either due to the heat.

So do you have the turbo control system? Based on the different distributor, I assume they mess with the ignition timing too? You may be able to account for fuel using the NA system, but unless you're gonna move the distributor with one hand and diddle the pot with the other....

[Captain Obvious]

Thinking about it more, I'm betting that you won't be able to test much of the turbo capability without a load on the engine.

Bottom whatever... Just don't blow your motor up!   

[Dave WM]

roger that about blowing up the motor. At this point I just want to see the turbo get oil and be able to spool up to check for boost. I do have the entire turbo FI setup so eventually the plan is to outfit the the test stand with that setup for testing. I will have to dig into it to see if there is any missing parts, it looks pretty complete. 

I forgot to add the color tune plug, I really want to better understand how that O2 sensor readings relate to actual combustion, I trust my eyes to read the color tune.