Spark plugs with 3/16 x 40 thread.

[BobsModels]

Not sure why the 1/4 32 is in the pictures, it sure would be a bit large on that head, but would work. Looking at the 10-40 from the engine and comparing them to the ones you purchased looks like the new ones are about 3 or 4 threads longer. Not an issue as long as piston clears the plug. A long plug usually fouls a bit less.

When you get them all your questions will be answered.

Bob

 

[timo_gross]

But It's nice to be a Prussian.
But it's better to be a Bavarian.
But It's the best to be a Rheinhesse.

Not sure how, but this popped up.

 

[Bruno Mueller]

I'm not a Hessian. !!!!!!!!!!!!!!!!!!
I'm Rheinhesse, that's something completely different.
After the Napoleonic occupation of the areas on the left bank of the Rhine, the land was divided between Prussia in the north (Prussian Rhine Province), Bavaria in the south (the Palatinate) and Hesse-Darmstadt in the middle (Rheinhessen). The state of Rhineland-Palatinate has only existed since 1946, when these three areas were partially reunited.
Rheinhessen roughly covers the area between the cities of Mainz - Bingen - Alzey - Worms. The Rhine forms the eastern and northern border.

So please, I am Rheinhessen and not an Hessian.
I still have a bit of national pride.
However, the dialect is identical with minor differences.

https://de.wikipedia.org/wiki/Rheinhessen

 

[Bruno Mueller]

Hello boys and girls.

The postman has just brought a shipment from the Chinaman. Two spark plugs of type 10 - 40.

I tried to screw one of them into the cylinder head of the tiny engine. Unfortunately in vain. The diameter of the thread is too big and the pitch doesn't match the original spark plug either.

The original spark plug has a thread diameter of 4.73 mm, the 10-40 spark plug has a thread diameter of 4.85 mm.

This does not match.

Original spark plug has a pitch of 40 Trade per Inches. 10 - 40 spark plug has a pitch of 0.5 mm or 48 Trade per Inches.

 

[petertha]

From your description & measurements, I suspect it may be one of those unfortunate cases where certain items on AliExpress are 'labelled' in a way to make them familiar to (in this case North American) standards or product ID, but obviously it does not match what the specifications designate. I think sometimes they do this to be helpful as opposed to deception. I have seen both helpful & deception. A $25 Mitutoyo caliper ins not a real Mitutoyo caliper, that is deception. But if trying to be helpful just confuses things, that's not good either.

#10 is a nominal thread major diameter. It has a corresponding minimum / maximum range for a specified class of fit. 40 means TPI = threads per inch. By definition TPI for these common threads specifies 60-deg thread profile, but obviously others could be used as long as they are specified somehow. Your AliExpress plug has a different pitch & that is just not going to work. Forcing it any more will damage the threads in your head. The nominal OD might be reasonably close or mostly correct depending on the class of fit. I show 2A in comparison table but 3A would have slightly larger diameter. A plug may be an example where a closer fit may have been contemplated, but depends on what tap was used to cut it to begin with & what the current condition is. I attached a thread calculator link so you can play around.

Class tolerances: (If unsure, use class 2A for external or 2B for internal)Tolerance classes 1A, 2A and 3A apply to external threadTolerance classes 1B, 2B and 3B apply to internal threads.The class tolerance determines the size of the gap between the pitch diameters of the internal and external thread. class 1A and 1B have the largest gap to class 3A and 3B which have the smallest gap. Most commonly used are the classes 2A and 2B. the tolerance classes 2A and 2B are within classes 1A and 1B.

http://theoreticalmachinist.com/Threads_UnifiedImperial.aspx

 

[Bruno Mueller]

It's all very unsatisfactory.

Why does every manufacturer cook their own soup? There don't seem to be any spare parts (ignition units, spark plugs) for this little thing. I have already spent days searching the internet.

I will probably be able to avoid having the cylinder head rebuilt. But then with a spark plug thread for which spark plugs are also available.

The engine is too good to just let it rot away in a display case. It has to run once in a while.

I will make several cylinder heads with different spark plug threads so I can find the best combination.

I have ordered different taps with 10-40 and 10-48. I will cut a test thread in a piece of aluminum and choose the right one.

The Chinese are crazy, but there is almost no way around them.

 

[GRAUBELE]

Hello Bruno

Here is the link to my most frequently used website for thread identification.
It lists all threads relevant to our hobby.
You may already know it.
The site can be switched from English to German, which makes life easier for us in Germany...

https://www.gewinde-normen.de/en/index.html

 

[petertha]

I'm not an expert but I think - 1/4-32 was a very common thread for commercial spark plugs, because for an even longer time 1/4-32 was a common size for glow plugs which were produced by the millions in typical RC model type engines. I don't think 1/4-32 spark plugs were specifically targeting 'conversion' of methanol glow engines to spark, but it certainly would be helpful in that respect. Obviously large displacement model engines developed from chainsaw type engine platforms could also use larger commercial spark plugs. They didn't have to re-invent the wheel.

But 1/4-32 was just too big for more discriminating shop made model engine builders with smaller engines. 1/4-32 spark plugs would not fit & even if they did, didn't look right. So being the smart & resourceful people that they are, they developed their own shop made plugs. #10, #8, #6 and so on. Marvelous examples of miniature engineering. Most did it for their own engines, but a few offered them commercially. Two I am aware of, both from USA. I suspect not a big market, its a specialty niche market. And it appears there has been some retraction in their availability. I believe the Rimfire plugs referenced on the previous link were developed by Paul Knapp, but someone else can correct me. Smallest is #10 (40TPI). Apparently they are also used in more demanding industrial applications.

Along this timeline, spark plugs were manufactured in Asia. Spark plugs like RCEXL have been around for quite some time & very common format in RC model engines along with their CDI ignition modules. Size is typically 1/4-32 for reasons mentioned. More recently, smaller spark plugs have arisen, mostly corresponding to 'table top' ignition engines, either assembly kits or pre-assembled. If I had to guess, that sounds like what you ended up with. I have heard the quality of these plugs (and engines for that matter) varies, but that's another discussion. I hope this helps.

https://sparkplugs.morrisonandmarvin.com/order.php

 

[Bruno Mueller]

Thanks dieter, I have also been using the table for years.

 

[petertha]

the 10-40 spark plug has a thread diameter of 4.85 mm.

So hard to know but it actually wouldn't surprise me that your Aliexpress plug is a M5 x 0.5mm regardless of what they name it.
Your measured OD of 4.85mm is quite close to the lower range major diameter 4.87mm for 6g.
If you are going to try various tapped hole sizes to match the plug, I would try that.

 

[GRAUBELE]

As an active RC model pilot, I'm quite familiar with the topic of glow plugs and spark plugs with a 1/4-32 thread size.
I also have a 1/4-32 tap in the workshop.
Many of the Methanol glow plug engines are now also offered in a gasoline version with spark plugs.
I have some old 1/4-32 spark plugs in my display case.
I also have new engines with gasoline spark plugs here.
I haven't seen a spark plug with a thread smaller than 1/4-32 since then.
Those will probably be custom-made.

The two radial engines are gasoline engines with spark plugs, the others are methanol engines with glow plugs.
All with 1/4-32 thread.

 

[timo_gross]

Can a 1/4-32 spark plug be modified to be smaller? How involved would it be to make a shop-made plug?

I do not have a good feel of how small the thing is compared to a 1/4-32 plug.

 

[Bruno Mueller]

Can a 1/4-32 spark plug be modified to be smaller? How involved would it be to make a shop-made plug?

I do not have a good feel of how small the thing is compared to a 1/4-32 plug.

In Post No: 19 I have posted a comparison picture of the two spark plugs.
One 1/4 - 32 and the other 3/16 - 40

 

[petertha]

I haven't seen a spark plug with a thread smaller than 1/4-32 since then.
Those will probably be custom-made.

As mentioned, these plugs are #10 which are smaller than 1/4" & have been commercially available for many years. I'm not sure if they are considered 'custom-made' if they are readily available alongside other sizes including 1/4-32, but maybe just interpretation of words.
https://sparkplugs.morrisonandmarvin.com/order.php

And the OP has linked a smaller than 1/4 plug from Chinese commercial supplier. As I mentioned, these smaller sizes are relatively more recent & generally not used on RC engines AFAIK which is maybe what you are saying? Whether it is truly a #10 or some other dimension is to be determined.

 

[Bruno Mueller]

So hard to know but it actually wouldn't surprise me that your Aliexpress plug is a M5 x 0.5mm regardless of what they name it.
Your measured OD of 4.85mm is quite close to the lower range major diameter 4.87mm for 6g.
If you are going to try various tapped hole sizes to match the plug, I would try that.

View attachment 166066

I will definitely try out all the variants.

 

[BobsModels]

Bruno

Why not just make the spark plug, if you have a lathe your on your way. Get some 6mm hex, get some round to experiment with threads until you figure out what they are. There is a kitchen counter top material called Corian in the US, get some scraps from a counter producer. You do not have to press in the insulator just real tight or even thread it in. There are a number of threads on making your own.

Its a great learning experience.

Bob

 

[Bruno Mueller]

Bruno

Why not just make the spark plug, if you have a lathe your on your way. Get some 6mm hex, get some round to experiment with threads until you figure out what they are. There is a kitchen counter top material called Corian in the US, get some scraps from a counter producer. You do not have to press in the insulator just real tight or even thread it in. There are a number of threads on making your own.

Its a great learning experience.

Bob

For me, this is the very last option I want to try. I could use tungsten electrodes from the Wig welder for the electrode.
For me it is easier to make new cylinder heads from a piece of aluminum.