Tainan distributor has now responded, as follows
"Thank you for the inquiry.
We have forming taps, but I need to have more information from you.
Hope to hear from you soon.
Thank you."
Uh?
Doesn’t 14MX1.25 adequately specify what is required?
WHY would they need a drawing of the spark plug?
WHAT does “valid inner diameter” even mean?
That shop I told you about near Taipei bridge has a huge inventory of used taps, drills, end mills, and carbide inserts. They will have what you need. I botten extended shaft taps from them too. As for form taps you’ll have to ask, because they are less common.
He’s trying to determine what class of thread you have in order to determine what class of tap H1-to-H12) to give you. Just tell him any class of M14xP1.25 tap he has in stock will be fine as you’re just repairing an old automobile.
Nope. Not going to do that, because it doesn’t seem to make sense.
If M14X1.25 doesn’t adequately define the thread (as I naively assumed), I either have to adequately define the thread, or forget it.
The age of the thread seems irrelevant.
I can do without a forming tap, which probably won’t work anyway, so “forget it” is an option there.
However, if M14X1.25 plus a look at a spark plug isn’t an adequate description, it does make me wonder if the thread cutting tap I was sold is correct.
There doesn’t seem to be much scope for “chabuduo” here, since if it isn’t correct it’ll bugger up the undamaged threads I’m trying to conserve.
Ah OK. having looked at that document, (for which I thank you) its a "closeness of fit " thing, reflecting the tolerance class of the thread. (D classes for this metric thread).
I suppose that isn’t very critical here, It ought to be fairly gas tight, but I’d guess the sealing is largely down to the plug washer.
Also ought to match the original thread parameter, but that might be hard to find out.
How about I send them this?
Just tell them you cross threaded the hole good so thread tolerances have gone out the window and all you want to do now is get your jalopy running again. The first time I fixed a cross threaded spark plug hole on a Honda dirt bike I used a cutting tap which took a good bite out of the aluminum thread because it was the wrong class tap. Bike ran fine though as far as I could tell.
He needs a tap with an extended shaft. Thread class and all that isn’t relevant, it just needs to be the RIGHT thread, that means M14 x 1.25. The normal shaft length isn’t doing the job because the thread is too deep or too hard to reach, reach is the problem he is having.
I mentioned that used tooling store near Taipei bridge because they have loads of taps of all sizes, shaft length, type, spiral, orientation, etc. since they seem to get all their tooling from retired machine shops, and even retired machine shops in Japan (lots of their tools are japanese branded). So if you need some weird tap, they probably got it. Maybe nothing nonstandard (I can’t find any LNMX carbide inserts for example because those are not ISO size, but rather it’s a proprietary insert type made by Iscar).
If you ever make the trip tell them you need a M14 x 1.25 tap, and it needs to be long shaft because the thread is deep.
In fact if you want I’ll go look there, and tell you if they got the tap. If you can pm me and add me on LINE or give me your phone number, I’ll go there tomorrow and look. If they got it, I can buy for you and mail it to you.
Or @QuaSaShao can have his assistant get you a long M14 x 1.25 fully threaded bolt, and you can use that to chase the thread. Your choice.
These say D4 for "Pitch diameter limit"of M14X1.25 spark plug hole taps, though these are cutting rather than forming taps.
spark plug taps | McMaster-Carr
They are also “bottoming chamfer with 1-2 chamfered cutting threads, so they can thread close to the bottom of closed-end holes.” which AFAICT will NEVER be a requirement for a spark plug hole (?!), and they seem to be extraordinarily expensive, if those are USD prices.
CAD drawing download didn’t work, but perhaps one needs to be logged in.
Dunno if I actually do need a long shaft tap (though it shouldn’t in any case hurt). So far I’ve only used the non-tap spark plug hole thread chasers that are sold for use with a standard socket set. These have hex drives which interfere.
I would certainly need a long shaft tap if I need to use a (non ratcheting) tap handle, which would be obstructed by the head, since the spark plug hole is at an angle. I’m assuming, however, that I would be able to use a tap with a socket set (with perhaps a bit less control) and could thus use a ratchet drive, avoiding obstruction by the head.
Going to add a small note or two here that don’t seem to have been covered yet - forming taps work great, but the reformed threads in an aluminum head can still be cracked/weak. After reforming it would be a good idea to take a peek with a scope, (maybe one of those tiny ones for checking ears), to see if the reformed threads look solid or if they have cracks.
(This is where a reforming tap will be better than a plain old bolt as the threads of the tool will be tapered/chamfered in a way to maximize a good re-shaping and minimize scraping/dragging of material.)
If threads look weak after reforming then you could try something like the epoxy/JBweld “re-tap” but chances are that will be of minimal benefit and risk further damage if the tap wasn’t perfectly aligned with the old threads.
However, there’s another possibility, which you’ll have to verify carefully, and that’s to see how deep the standard plug goes vs how much clearance there is below it still at full insertion. I.e. if you can use a plug with a longer base then you can engage the lower/healthier threads for a stronger fit.
Even if it appears good, you’ll still need to verify after re-assembly by sticking a chunk of plasticine or melting a blob of soft wax or something on the end of the plug (i.e. any pliable substance that won’t cause damage if it falls off) and turning the engine over slowly/manually, then pulling the plug to verify just how close you’re getting.
HTH!
Plugs go all the way down into the combustion chamber, where they have to be to do thier thing.
I’m thinking/hoping the main potential benefit of epoxy is likely to be directional correction for the spark plug entry rather than strength.
I wonder if it would be possible to form (as opposed to cutting) threads in “green” epoxy,before it has set, by coating the tap or bolt in release agent (wax, PTFE tape, grease). That would probably be the strongest option.
However, the risk would be ending up with a bolt or forming tap firmly JB Welded in the spark plug hole. That’d be fairly terminal, so I THINK its a risk I’m probably not prepared to take
If threads are engaged all the way then that’s as good as it can get, and if you have enough good threads at the bottom to get it tight enough to deform the washer then you’ll probably be good to go even if some upper threads are sub-par. (Worst-case it blows out one day and leaves a dent in your hood and you go ahead with the heli-coil or similar option after all. 
) Just keep an eye out for the old threads coming loose as you change plugs over time.
What I don’t get is why you haven’t fixed the problem yet. At this rate you’ll still be “discussing” possible solutions when China invades.
Well, could be I’m indecisive (since there’s likely only one shot at this), or don’t know WTF I’m doing, or don’t want to bust it further, or am not in any particular hurry, or have other things to do, or am waiting for a delivery of high temp epoxy, or a forming/cutting tap, or a bolt, or all of the above.
Perhaps you should sue me before China invades, when the legal framework might change.
It’s not your problem. It’s mine. Indecisiveness triggers me.
Indecisiveness certainly a factor, but mostly I’/m waiting for the epoxy, which I have indecisively decided to use.
Meantime I’ll take the sump off, and see if I can get that piston out. Probably not, since that piston isn’t at BDC, plus I’ve got the timing sprocket and belt locked in place and don’t want to move them, but at least I can swab the sump out.
This car has always put quite a lot of ferrous metal in its oil. Assumed it was from the cam, but maybe it was from that damaged cylinder.
If and when I get that high temp epoxy I’ll be testing it a bit, so expect still yet more additional further incremental delay.
Removing the sump on this car is a PITA. Lots of stuff has to come off, and the gasket hangs up on the studs, but I’ve done it a few times since the sump doesn’t drain fully.
Needed done this time because coolant got into the sump during head removal. Not much metallic sludge this time around and oil pump strainer looks clear.
Looks like I might be able to get the piston out, but mosquito happy hour forced a halt so dont know yet.
Bearing looks pretty scratched up and apparently has (metallic? Ferrous?) debris embedded. I suppose they’ll all be like that.
Mysterious bearing cap markings. Possibly Klingon. Or upside down
Local alleged forming tap sources, having been given all the info on the spark plug threads that I had, have gone silent.
I used the word “sample” but didn’t make any attempt to pretend to be an industrial customer (I did this in the UK many years ago to get trade prices, one time accidentally getting some quite expensive bearings free) and maybe they aren’t interested in one-off retail.
So if your kind offer is still open I’d like a (preferably long) forming tap and cutting tap. I’ll PM you
Don’t forget the trade show for bolts and tooling is in Kaohsiung next weekend from the last times I went i recall they had small stalls also selling equipment.
Offer is still open.
This shop sells all kinds of used tooling and the price is very good for the brand and quality. They get their stock from japanese machine shops after all.
Don’t bother emailing. I find taiwanese companies never answer email even if you look like someone they shouldn’t ignore. You have to go to them in person but don’t be surprised if you get sticker shock.
