Any SI - Damaged Spark plug hole thread

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Tue 28 Mar 2023 18:03
Any SI - Damaged Spark plug hole thread - edlithgow

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

Oh THE SHAME!

Never done that before.

My potential options seem to be, in descending order of cost/ascending order of 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 theoretically superior but I probably wont be able to find it in Taiwan and don't really want to wait for shipping.

www.amazon.com/Sealey-MS003-Reverse-Action-Thread/...O

www.amazon.com/Powerbuilt-640811-Back-Tap-Thread-R...W

Anyone used one?

Might be able to find (b) here.

www.amazon.com/ARES-Limited-Access-Perfect-Confine...L

(c) would be a last resort.

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Tue 28 Mar 2023 20:30
Any SI - Damaged Spark plug hole thread - paul 1963

Oh Ed, the trials and tribulations you go through in order to keep the old girl running along with your sometimes crazy hairbrained ideas do make me smile!

In this case you really need to take the head off inorder to effect a proper repair, I take it you've stripped the thread? Really needs the attention of a machine shop to fit a helicoil insert, can't see your normal tool kit of sunflower oil, tin foil, bamboo skewers or bits rescued from the local dump sorting this one mate.

Must add I absolutely mean no offence, all my above comments are made in jest ( apart from the machine shop one), your cleary a huge eccentric and I love a eccentric.....

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Wed 29 Mar 2023 01:53
Any SI - Damaged Spark plug hole thread - edlithgow

Dont THINK its that bad, (the damage, rather than my alleged eccentricity) though its hard to be sure.

It looks like the sort of limited damage that thread chasers are advertised as being appropriate for, IF I could find one.

Presumably if the threads are bad enough to need a Taiwanese Machine Shop (there must be SOME good ones, surely?) I wouldn't stand to lose much by chasing them, other than perhaps getting swarf in the cylinder.

Avoiding that seems to be largely why type (a) is superior, plus it uses the probably less damaged threads at the bottom to get the line right.

Edited by edlithgow on 29/03/2023 at 01:57

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Wed 29 Mar 2023 03:05
Any SI - Damaged Spark plug hole thread - Ana William

Cross-threading a spark plug can be a frustrating experience. However, it's important to fix the problem correctly to avoid further damage to the engine.

Option (a) - a fancy expanding "bottom up" thread chaser - may be the best option if you can find one. This type of tool works by expanding the threads from the inside out, effectively reforming them to the correct shape. This should provide a more accurate repair than using a solid thread chaser.

Option (b) - a conventional solid "top down" thread chaser - can also work, but it may be more difficult to achieve a proper repair. This type of tool can only re-cut the existing threads, which may not be enough to fix the cross-threading.

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Wed 29 Mar 2023 08:37
Any SI - Damaged Spark plug hole thread - skidpan

Put some axial hacksaw cuts in a spark plug and run that down

I have put a saw cut down an old bolt before but only to clean out the threads when I did not have a correct thread tap (flywheel bolts on a Zetec are one example, Ford use a fine thread). But I doubt that this would reform the metal adequately.

Only real way to sort a wrecked thread is to have an insert fitted. Used to be quite common in the 70's on Plastic Pigs, the aluminium engine did not appreciate cross threaded plugs put in by ham fisted owners.

Here is a tip given to me by a mechanic many years ago. When fitting plugs (especially those in difficult to reach situations) do not insert them with the plug socket. Get a suitable piece of rubber hose and press it down the insulator and then insert the plug using this. It will easily screw them in if lined up but will not cross thread them. Once in torque correctly.

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Wed 29 Mar 2023 09:46
Any SI - Damaged Spark plug hole thread - craig-pd130

Only real way to sort a wrecked thread is to have an insert fitted. Used to be quite common in the 70's on Plastic Pigs, the aluminium engine did not appreciate cross threaded plugs put in by ham fisted owners.

Reliant's engine was relatively advanced for its time in being lightweight, it's just a pity that it was used in Reliants! Same with the Imp engine, a good design that got a bad reputation due to poor manufacturing and ignorant owners.

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Wed 29 Mar 2023 13:21
Any SI - Damaged Spark plug hole thread - edlithgow

"Get a suitable piece of rubber hose and press it down the insulator and then insert the plug using this. It will easily screw them in if lined up but will not cross thread them. Once in torque correctly.

"Thanks. I normally fit them by hand. Must have had a lapse this time.

The hose trick would be useful and applicable if access was poor

Edited by edlithgow on 29/03/2023 at 13:25

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Wed 29 Mar 2023 14:24
Any SI - Damaged Spark plug hole thread - edlithgow

Trick for thread insertion (but I suppose it'd be applicable to thread chasing too) -

Fill the cylinder with shaving foam

Then when finished, turn the engine over and skoosh all the foam out, hopefully taking the shavings with it

I suppose that's why they call it shaving foam

Stolen from here

www.youtube.com/watch?v=B3irFj2XeCM&t=5s

Actually a bit alarming, because (having apparently done quite a few before) it fails initially, and he has to use what looks like one of the bottom-up chasers AND a stud extractor to get it to stay put.

Edited by edlithgow on 29/03/2023 at 15:12

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Thu 30 Mar 2023 15:44
Any SI - Damaged Spark plug hole thread - edlithgow

Sort-of-interestingly, these guys actually seem to have got better results with the DIY-modded spark plug than with a conventional top-down thread chaser, perhaps because it seems rather lightly modified, with very shallow grooves perhaps in turn producing a shallow cut.

How to Avoid and Repair Damaged Spark Plug Threads

If its good enough for mangling a Mercedes, it could be good enough for screwing up a Skywing

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Thu 30 Mar 2023 19:11
Any SI - Damaged Spark plug hole thread - Oli rag

I’d definitely give the moded spark plug a try Ed, like others, I’ve had success repairing damaged threads with something similar.

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Fri 31 Mar 2023 03:08
Any SI - Damaged Spark plug hole thread - edlithgow

I’d definitely give the moded spark plug a try Ed, like others, I’ve had success repairing damaged threads with something similar.

Thanks,

I got a “FORCE Professional Tools” Spark plug hole thread chaser in the nice Kaohsiung tool shop.

(Its not actually the best Kaohsiung tool shop, but the laoban in that one gets a bit dismissive when shown pictures without the correct Japlish “Comin in ere with your fancy foreign fantasy tools” stylee, so I tend to leave it till last).

They looked quite surprised to see it when they got the thick, thick dust off

350NT

“Applying company with 13/16"standard deep socket” which American atavism seems to be about 21mm in the modern world, which I think is about a standard plug socket, which I should have somewhere

This gives me a bit of a dilemma. Should I use this or a modified spark plug first?

Decisions, decisions

Hoping shaving foam and grease will limit chip ingress, though I’d be happier if I had compressed air to blow it out

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Fri 31 Mar 2023 09:20
Any SI - Damaged Spark plug hole thread - bathtub tom

Use thick grease in the slots and that will, hopefully, collect any bits. With luck, any bits that do fall onto the piston will be small enough to be blown out the exhaust valve and not wedge themselves down the side of the piston. Would it matter, how long do you think you can keep it going?

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Fri 31 Mar 2023 14:20
Any SI - Damaged Spark plug hole thread - edlithgow

Use thick grease in the slots and that will, hopefully, collect any bits. With luck, any bits that do fall onto the piston will be small enough to be blown out the exhaust valve and not wedge themselves down the side of the piston. Would it matter, how long do you think you can keep it going?

I THINK the shaving foam should be effective with aluminum which shouldn’t be heavy enough to sink through it, or hard enough to do very much damage if it does. As for longevity, no idea. Overdue for inspection now so might be a matter of days
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Sat 1 Apr 2023 05:14
Any SI - Damaged Spark plug hole thread - edlithgow

Since I have apparently lost my plug socket, I went to the nice Kaohsiung tool shop for another. My original would also have worked with the double-ended chaser gizmo with the rubber insert removed, but plug sockets have apparently moved on a bit, the rubbery insert is rather firmly bonded, with a magnetic steel liner, so I had to get another deep 21 mm to go with the gizmo.

IF the gizmo works I won't need the plug socket, but its the start of Tomb Sweeping Holiday, so all the tool shop owners will be away setting fire to graveyards and getting bitten by snakes for a few days, and I thought I'd better get one "in case"

Irritatingly could only get 12 point, which I never do if I can avoid it. Hopefully wont be applying much torque to a plug, but could be an issue using it with anything else.

Also got shaving foam. Gilette Foamy Mentholated. Maybe Aloe Vera would have been a better UCL, but too late now.

Running out of stalls. When I’ve finished this coffee I might have to go and finish destroying my cylinder head…

Edited by edlithgow on 01/04/2023 at 05:36

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Sat 1 Apr 2023 11:39
Any SI - Damaged Spark plug hole thread - edlithgow

Felt like it wasn’t working, so I stopped.

I assume that, if you get it right, it gets easier with depth, as you hit the undamaged threads. After a few turns it hadn’t, so I assumed I was cutting new ones.

I suppose I’ll have to order the fancy back tap thing from the US.

Perhaps followed by an insert kit

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Sun 2 Apr 2023 08:16
Any SI - Damaged Spark plug hole thread - edlithgow

People who order stuff from the US tell me its likely to take a month "because USPS now have Covid as an excuse"

Going to look into taking the head off.

I've only done that on an OHV engine (BMC B series 1800) and I did it in a lockup, so I could pull the head up with a Spanish Windlass toward a scaffolding pole propped between the walls, (which got quite bent) while I whacked the cast iron head with a BFH.

Took a while

Doing it in the street to one of these fragile wee aluminium OHC 3-cyls might end badly.

But most things end badly. Thats why they end.

At least it predates those horrible stretchy head bolts that you can't re-use.

I think

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