Hi Guys.....!!!!

I am trying to make a miniature (scaled down) model engine.
Wanted to know if there is any place where I could get spark plugs having a thread size of M5 or smaller. (Unified threads also would do)

If you are milling your own engine, please do post in detail with pictures !  We haven't seen any such efforts so far on RC India, and it would be very interesting to the audience

Can't seem to find any information on miniature glow plugs via online searches.  Wonder if removing the platinum wire for a good plug, and soldering it on to a plug structure milled by you would be an alternative

There are miniature spark plugs available for many sources. Here is one.

http://sparkplugs.morrisonandmarvin.com/index.cfm?menu=main2

If you want to try out something simple (and one-off build) then use a glow plug. Just precisely cut the glow filament so that you make a spark gap.Worked fine for my pulse jet experiments.

-Ismail

I had assumed he was looking for glow ones, so I ignored all links which referenced gas plug brands like NGK.

That brings up an interesting topic... how do the plugs differ between gas and glow ?

I realize that the thread is about "spark" plugs, so it is most likely gas plugs.  There are many sources for these as Izmile rightly said. 

The reference to "miniature model engine" immediately put my mind into "glow model engines", and finding miniature glow plugs seem to be quite a task !

Gas engines use spark plugs. As the name implies they produce spark between the air gap to initiate combustion. Spark plugs need to generate spark for every combustion cycle and so they require an ignition circuit to periodically do that.

Glow engines use glow plugs. Glow plugs have a platinum filament (or platinum coated filament) that glows when an electrical current passes through it (Glow booster), there by generating the heat required for combustion. Since methanol has a lower flash point the platinum acts as a catalyst to initiate combustion. Unlike gas engines glow engines do not need to have the electrical current through it for all the combustion cycles. Once the engine starts the platinum filament retains the heat due to thermal inertia and so it can re-ignite the air/fuel mixture for the next cycle.

Thank you Ismail bhai !  We need to use every opportunity to bring up technical discussions/nuggets on the forum.

Just one more small bit.
Glow plugs do not necessarily require current to pass throug to initiate combustion. You could start a glow engine without the booster.
What the current does is hasten the checmical reaction (remember your high school chemistry  lessons ? Add heat to quicken a chemical reaction...)
Just a small bit of more information, platinum catlayst fired lighters utilizing alchol fuel were in use even before invention of safety matches   

Is that a reference to fluke/rare cases, or is there someway to make this happen repeatedly (other than manually heating the filament/whole plug somehow) ?

Yes, Manish is correct.

I am not sure if you can make that happen repeatedly (without additives in the fuel). I had experience with my 46LA engine starting without the glow booster. The engine was hot when it used to do that.

Anwar,
Contrary to poular belief, it is not the hot filament which causes combustion, but the platinum itself. The metal acts as a catalyst to break down methanol. The initial heating is to displace other foriegen molucules sticking on to the filament, and hasten the reaction.
With a cold engine, starting the engine without the current is possible but extremely difficult, with the engine hot/warm the chances increase manyfold. 

Capt is absolutely right. The Platinum Filament acts like a catalyst and lower the 'Pressure'/temperature  and which the air + nitro-methane could be combusted. Suppying electric current to filament acts as a 'catalyst' over catalyst platinum.

Remember, Diesel engines dont even need a glow/Spark plug. They entirely depend on the entropy of the compressed diesel and air mixture in the chamber  to kickoff combustion which is sustained cycle after cycle.

One thing more. With Spark Plug, the spark  timing also comes into the picture to govern the engine RPM, and not the amount of fuel mixture alone.
http://en.wikipedia.org/wiki/Ignition_timing

Just waiting Prasad to give the glimpse of his project. Any plan to put it into product, Mr Prasad  ?

I second that. It is true. Its the platinum as a catalyst to break methanol. Even I have witnessed old LA engines being started without the booster when warm.

The temperature of the filament would have to have a larger role in the whole process than just to repulse other elements.  Otherwise, we would see designs that maximize the "contact area" of the platinum filament.  Another question is what happens to the filament while it acts as a catalyst, doesn't it progressively get corroded and ineffective ?

What causes the filaments to only last a few runs in many cases ?  Some plugs do not even last one tank, others stay on for years of regular/repeated use.

Yes,

Absolutely. The temp plays a crucial role for igniting the fuel. Old LA used to start only when they were warm. Extremly difficult (almost immpossible) to start when cold. (without igniter )

Over a period of time, the element does get corroded and gets grey.
Usually, the plugs get burnt out earlier if the engine is run tooo lean, which greys out the filament and eventually breaks it down.

Here is a simple explaination -> http://www.newton.dep.anl.gov/askasci/chem00/chem00726.htm

I think gold also can act as a catalyst to oxidize methanol... No wonder this hobby is expensive!

And now all those who read this would know why those plugs cost so much?
And mind you those are not pure platinum but Pt alloys.

Anwar to answer you query on what happens to the Pt element
There are two types of plugs, one with entire filament made of the PT alloy and other with Pt alloy plating. Obviously the plated on deteriorates faster as the plating corrodes with commbustion, but the one with entire filament of the alloy last longer. Again the content of nitro, compression ratio, lean runs, and over all the quality of the plug itself all contribute to the performance and life of the plug 

Hi....everybody ….!!!...Thanks for the prompt response.

Ya, making it from scratch... I will surely post the pics. but only after it is completed...... will take some time tough....

It is a SI engine (petrol)
The main problem is the Thread size.
Due to some design constrains, I am left with an 5mm dia space only in which I have to fit the spark plug.

Infact, the ones shown in the rimfire page is exactly what I am looking out for. (Actually I had checked out that page earlier) The Z3 with a thread size of 10-40 fits in. But I am really Skeptical about ordering them from overseas myself. Besides at 21$ each, they are going to cost me a bomb...!!!... (and I need atleast 6 of them)
I am looking out for something that I could purchase locally....... maybe if somebody has imported something similar in bulk.

Initially you may feel that getting small spark plugs might be easy.... just Google it and you will get hundreds of options..... but it not really that way..... most of the stuff that you get is really of no use.... and for most of the small models that you see, people have had to fabricate it themselves.

Even I have thought of making them myself (there are many sites that tell you how to make one)... but then getting the ceramic insulation done properly is a problem.

Another option is cutting up a standard motorcycle spark plug, grinding down the insulation (which is again a big problem) and then refitting it into a smaller casing.

Trying to modify a glow plug would not be of much help.... the only useful part would be its casing, which I can easily fabricate myself.

But It would save me a lot of effort and time if you all could help me find something that small.
Thanks once again.

Prasad,

I am sure you know your stuff. I would say, if you have the capability to mill your own engine then making a spark-plug should be a piece of cake.

Since you need 6 plugs, I guess, you are building a multi-cylinder engine. Do post the pictures of your engine. Would be interesting to see.

-Ismail

Digging out this rather old thread......
.....
.......... but it took me a very long time to finish the engine...
(actually, a bit unfinished ..... it's without a silencer ..... but I had to put it up any way)

I have started a new thread for the actual engine over here http://www.rcindia.org/self-designed-diy-and-college-projects/the-interlaced-delta-engine/

On this thread I intend to write only about the spark plug.
The plug that I have used is the RimFire Z1 plug from here http://sparkplugs.morrisonandmarvin.com/index.php
The thread size of 10-40 UNS is also rather odd (most of us wont be able to tell off-hand what it is). The gap between the 2 pistons is just 5mm ... and this was the only one that gets into it.

The attached pictures will give you an idea of how tiny these plugs are.
The second picture shows the plug along side another plug from a small car over here.

Attempting to convert a glow plug into a spark plug by snipping the platinum filament would not work otherwise. The glow plug which normally carries a current of 2-3 Amps at a voltage of 1.5-3 Volts cannot sustain the insulation required in a spark plug with sparking voltage of 700-2000 Volts occurring several times each second. Which is why the central electrode of the spark plug is insulated from the body (shell of the plug) by porcelain which has a high dielectric constant.