Damaged Spark Plug Hole Thread Chaser

Was rushing to catch a train and I suppose I must have cross-threaded one of the replacement plugs


Never done that before.

My potential options seem to be, in descending order of cost/ascending order of likely availability.

(a) Fancy expanding “bottom up” thread chaser

(b) conventional solid “top down” thread chaser

(c) Put some axial hacksaw cuts in a spark plug and run that down.

(a) Seems superior IF I can find it in Taiwan (don’t really want to wait or pay for shipping.).

This kind of thing



Apparently this topic is similar to “Spinach? (Seems like an easy one)”?

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Pardon my ignorance, but what would be the purpose of this?

It’s been a while but I pulled the head and chased the thread from the inside because the other, outside end of the thread had been cross-threaded so badly. Only downside was I had to make my own replacement gasket. I like “a” though. That’s what I’d try now, though it’s a bit tricky to use apparently.

Mimics the action of a conventional top-down thread chaser, only probably not very well.

The axial cuts (greased) give somewhere for displaced swarf to go, and the edges of the saw-cuts give cutting edges.

One could perhaps temper the steel a bit, but I havn’t looked into that since I hope I wont have to use this last-resort approach.


I see, thanks!

You’ve got two a’s and are missing d!

(e) Show no mercy, drive that sucker home!

I have one (a), mentioned twice.
Count them using, say, your eyes, closing one each time


Not that I’m much interested in MAXPOWER for this application, but in any case there is seldom 2 foot of clearance for a breaker bar inside an engine compartment, and if there was, alignment is kind of important here, and wouldn’t be achievable with this thing.

Size really isn’t always everything. You’ve been misinformed.

For a (2), give it the ugga-dugga’s!

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Maybe ask @sulavaca for some professional help, if you’re out of your depth replacing a spark plug.

See above?

the one and only time I had to rethread a spark plug I used the standard tapping tool as below.


I didn’t even know about the bottom up tool, but reading a few reviews online, you have to be carful not to get a cheap one as if it brakes it can drop in the engine,
also depending on how bad the treading is you can still have difficulty lining up.

I would stick with the tried and tested and take my time.

Ah, should it be a(3) or e(2) or f? Let me know.

  1. a(3) ask @sulavaca for some professional help
  2. e(2) ask @sulavaca for some professional help
  3. f ask @sulavaca for some professional help

Is it an M16 thread?

Spark plugs don’t make good taps. They’re hollow in the middle and are prone to snapping.

If you’re tapping aluminy-yumyum then be careful not to drop chips into the cylinder, as they’re hard to get out.

I’d use a traditional tap in most circumstances, along with grease to try and trap the chips from dropping into the cylinder.
Of course, this is all assuming easy access to the plug hole.
Worse case then the head needs to come off.

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It’s tough picking up the correct thread lead from the cross-thread though, right?

I think its F- (only with caps) OFF
See long ago, in some other threads far, far away

A starting tap will be tapered, which will help, but it’s obviously important to keep it centered correctly when gingerly feeding it in. This does require some skill and experience definitely helps.

It’s definately the first step though. Worse case scenario the head has to come off anyway.

When tapping aluminum especially, then it is really important to work the tap back and forth in small increments as well as remove it every now and again, to clean off all the chips, as the chips will clog the tap and act as a binder, potentially ruining the job entirely. Iron/steel is much easier to tap as it’s far less “gummy”.

The story (which I find fairly plausible) is that taps are designed to cut new threads, and if you give them a chance, they will, especially in soft metal like aluminium.

I don’t want that.

It may be possible to avoid this with skill (which I have no reason to think I have) or luck (ditto, or I would’nt be in this situation).

The flipside of that is that some Taiwanese mechanics and tool shops might have heard of taps, so you can probably get them,(as your Shopee link confirms, thanks) but they probably won’t have heard of the other options, (or would care if they had) so you probably wont be able to get them.

Its not news to me that Taiwan Spectacularly Sucks for this sort of thing.

I dunno. I might just ship (a) in, though it will involve quite a delay and will be expensive for a tool I hope to never use again.