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Old 03-26-2010, 08:20 PM   #1
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Default NPT fitting not threading in far

My turbo came with a 3/8 NPT fitting drilled in the oil drain flange. I tried to screw my 3/8NPT to AN fitting into it and they didn't go in very far. I know they aren't supposed to go in all the way, but the one went in 1.5 turns and the other 2.25 by hand. Is that far enough to seal? Can I run a 3/8NPT tap deeper into the hole to enlarge it?
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Old 03-26-2010, 10:05 PM   #2
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Are you sure it's NPT and not BSP? The two are close but not identical, and sometimes that'll manifest as an NPT fitting not going in very far.

If it is NPT, then you could tap it deeper (tapered), but it's easy to overdo it and wind up with something that won't seal. I'd stick a wrench on it and tighten it to snug first, and see how far it goes. There's no real pressure in there, so I wouldn't think you'd need many threads to be secure.

As for tapping -- is it the center housing that's tapped directly, or a bolt-on flange? I dunno if I'd want to put a tap on the center housing, because you'll never get the swarf out properly without disassembling it, and that opens up the question of rebalancing it properly after reassembling. If it's a bolt-on flange, then it's pretty easy to take it off before tapping and clean properly.

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Old 03-26-2010, 10:21 PM   #3
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Just wondering, what turbo?
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Old 03-26-2010, 11:10 PM   #4
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It's a GT2560 from a Flyin Miata kit. I ran my tap into it and it turned down pretty far before it got tight, so I don't think it is BSP. I didn't put any pressure on it because I didn't want to cut any threads yet. It could be a combination of a used flange (npt deforms slightly) and a low quality thread on the fitting. The threading is only on a bolt on drain flange, not into the turbo directly.

I did some research and it looks like NPT fittings are only designed to be turned in about 3 turns by hand anyways.
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Old 03-27-2010, 12:19 AM   #5
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Use a little teflon goo (not tape), it's purpose is to lubricate the threads and coupled with a wrench, you should be able to get a good turn or two more out of it. If it's a crappy flange and you have the tap, chase the threads with just an extra .5-1 turns of the tap to clean up the threads, doesn't sound like it'd hurt anything.
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Old 03-29-2010, 06:32 PM   #6
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Thanks for the responses. I'll try this at some point and see what happens
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