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Clearing spark plug thread

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AJW19/11/2020 14:22:56
314 forum posts
121 photos

I'm working on an old Villiers engine which has been running with an incorrect spark plug, it was too short on reach.

This has resulted in the threads getting carboned up and not accepting the correct longer threaded plug. I have turned up the old short plug to be used as a 'sort of' tap to clear the thread, and have applied plus gas in the hope that it will soften the deposit but wondered whether I might be better using something else?

It's a big plug with an 18mm thread and didn't want to purchase a tap purely for just clearing the carbon!


Clive Brown 119/11/2020 14:59:31
580 forum posts
23 photos

Would an 18mm plug thread chaser @ £6 from Machine Mart be within budget?

Or a wire shaped like a hairpin with outwardly bent sharpened points and lots of patience.

Edited By Clive Brown 1 on 19/11/2020 14:59:47

Speedy Builder519/11/2020 15:27:53
2182 forum posts
152 photos

Use an old long reach spark plug and file the sparking end so that it "cuts" the thread clean.

Steviegtr19/11/2020 15:32:01
1769 forum posts
235 photos

I have a set of Dentists picks which are perfect for that type of job. Cheap ones on ebay can be got.


Dave Halford19/11/2020 15:35:39
1127 forum posts
11 photos

Assuming the head isn't alloy, a Dremel size wire brush or a tube brush

old mart19/11/2020 15:36:52
2465 forum posts
169 photos

Those dentists picks are very useful.

AJW19/11/2020 16:51:50
314 forum posts
121 photos

Thanks guys, it is an alloy head (seems very soft!) I did start picking it out but thought there must be something to dissolve/loosen it.

I shall set to and get picking and use my modified plug!


John Olsen19/11/2020 23:10:23
1120 forum posts
92 photos
1 articles

Well, don't use oven cleaner, which would dissolve the alloy.

I would try Speedybuilders idea.


Steviegtr19/11/2020 23:34:48
1769 forum posts
235 photos

Let us know how you go with it. I was an absolute Villiers nut when younger. 8E, 9E, 2T, 3T, 4T had them all. Used to tune the hell out of them.


AJW19/11/2020 23:53:44
314 forum posts
121 photos

Must admit if was going to try anything it could be citric acid? No science behind that just wondering!

This Villiers engine is not from a motorcycle it's a model 25HS direct coupled to a 2kw generator, dates from 1964. It has been rebored but looks like it's not done much running since as the bores hone marks are clearly visible.

I'm enjoying the project, nicely made engine.


Ian Hewson19/11/2020 23:56:05
281 forum posts
25 photos

I used an old spark plug that I cut a spiral groove down the thread with an hacksaw when I needed to clean out spark plug threads. That worked well for me.


Ady120/11/2020 02:49:31
4128 forum posts
576 photos

cut an old spark plug in half? or just less than half, with an angle grinder, at the leading threads part, so they are square on and full size instead of tapering

then screw it in to gouge the garbage straight out of the thread kinda thing

Edited By Ady1 on 20/11/2020 02:54:36

Maurice Taylor20/11/2020 08:54:43
162 forum posts
30 photos

Hi ,What’s the point of trying all these things ,when the proper tool is £6 at Machine Mart.If you damage it ,a helicoil kit will cost a lot more also a lot more time than the 2minutes this tool will take. I have one of these .


J Hancock20/11/2020 09:01:09
495 forum posts

Ahh, we're back to that definition of a 'engineer'.

Anyone can do it for £6 , an 'engineer ' can do it for nothing.

martin perman20/11/2020 13:30:00
1920 forum posts
81 photos
Posted by Clive Brown 1 on 19/11/2020 14:59:31:

Would an 18mm plug thread chaser @ £6 from Machine Mart be within budget?

Or a wire shaped like a hairpin with outwardly bent sharpened points and lots of patience.

Edited By Clive Brown 1 on 19/11/2020 14:59:47

I'm an engineer smiley but still bought the above which has worked for me for many years on my Stationary Engines and it has a 14mm plug thread on the other end. An engineer cannot do it for nothing, cost of correct material, electricity and time all adds up wink

Martin P

martin perman20/11/2020 13:31:04
1920 forum posts
81 photos

Edited By martin perman on 20/11/2020 13:32:17

AJW20/11/2020 13:48:10
314 forum posts
121 photos

Success! I had the old short reach plug on which I turned the body down to less than the thread root diameter, where upon it fell apart!

But no problem, I ground a cutting edge on the end and used one of those 'easy out' screw removers to drive it into the thread, picked out what I could with a dental type device and eventually managed to work the short stub through the head casting.

All because someone had fitted the wrong plug, but got there in the end.


martin haysom06/12/2020 22:49:57
11 forum posts

i got one of these engines [ attached to a landmaster mk 1 ] i seam to remember it had a short reach spark. could be wrong it was repaired years ago by fitting a brass plug

Mick B106/12/2020 23:04:46
1800 forum posts
91 photos

Stick the new long reach plug in the lathe and turn away the thread that would foul the fouling. If the short reach plug had enough thread to avoid blowing out, so would the longreach with a shortened thread.


Ah, you sorted it already...wink

Hopper07/12/2020 04:05:43
5032 forum posts
111 photos

The way that motorcycle mechanics worldwide have been doing this for the past 120 years or so is to take a sparkplug and put a small hacksaw cut lengthwise through the end of the threaded section. Screw it down the hole and it makes a good enough tap to clean up aluminium threads and remove carbon.

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