Glow plug lengths

[KEITH BEAUMONTParticipant @keithbeaumont45476]

I am about 80% complete making a Jones .605 Glow engine, The notes suggest the use of a "long reach " plug and up to this point I have thought that long reach was a bit longer than the normal standard size. However, doing a seach for such a plug with model aero engine suppliers drew a blank ,but model car part suppliers do offer "long reach". I duly purchased two OS LC3 long reach plugs from a model car part supplier, only to find they are no different in length than the normal plugs in my possesion.

The Jones design is of late1970s vintage and I am now forming the opinion that in those days it was still a custom to mark plugs as long reach to mark them out from the short reach type used in small capacity,1cc and under engines. For whatever reason the model earo engine fraternity have dropped the use of the description"long reach"for the standard size plug, but the model car people still use it.

My contention ,therefore, is to say only two lengths of plug exist,- Short reach and the standard length, also known as long reach.

I am open to be corrected on this by any expert out there.

Keith

[KEITH BEAUMONTParticipant @keithbeaumont45476]

[Speedy Builder5Participant @speedybuilder5]

This is taken from **LINK**

OS and all Japanese plug threads are 4.5 mm long. Always have been. Enya uses the same length in all their plugs from the #3 to the #6. OS does too. American plugs are 5.5mm for long reach and 3.5 for short reach. The Japanese don't complicate things. Why make two when one works perfectly well? The only "long" plug made by OS is the type F which is a four cycle plug but the thread length is 4.5mm.



Enya plugs are rated from hot to cold with the #3 the hottest and the #6 the coldest. The higher the nitro content, generally the colder the plug used. The Enya #3 and OS #8 are pretty much all around plugs for the 0 to 15%nitro user. OK up the about 25 %. OS A3 is a bit hotter I'm told.

BobH

[Neil WyattModerator @neilwyatt]

I have a small collection of plugs. Always thought of them as long ones and short ones…

Neil

[ChrisHParticipant @chrish]

My understanding is that the so-called 'long' type F plug for the 4 strokes is longer as the nose of the plug, the business end with no threads, has to stand proud of the cylinder head so as to receive more heat from the combustion and get and stay hotter for longer while the engine does twice as many strokes as the 2 strokes, unlike the 2 stroke plugs which end flush (or thereabouts ) with the cylinder head.

[KEITH BEAUMONTParticipant @keithbeaumont45476]

Prior to creating this post ,I contacted two other experienced model engine makers and they were both of the opinion that there are 3 lengths of plug available,as I did. Clearly we were wrong and only short and long are the standard. I have several OS F for 4 stroke use and they are the same thread length as other "standard"plugs The longest plugs that I have are the Model Technics 4stroke type. They have the same thread length, but have a bar across the bottom.

I have always understood that the plug should be flush with the face of the combustion chamber and the use of different thickness washers are available to enable adjustment to take place. Also allows use of a long plug in an engine requireing a short plug.in an ermergency..

Thanks for the replies.

Keith

[SpurryParticipant @spurry]

…. The longest plugs that I have are the Model Technics 4stroke type. They have the same thread length, but have a bar across the bottom.

Keith

Those are what are called Idle bar plugs. The idea being that the hot element is screened from the incoming fuel charge by the bar, as in a 4 stroke engine there are only half as many requirements for Glow as the two stroke.

Pete

[ChrisHParticipant @chrish]

Keith, I quite agree that for a 2 stroke the end of the plug should be flush with the surface of the combustion chamber, or cylinder head in most realities.

However, I understand that the end of the plug in a 4 stroke should protude into the combustion space, hence the 4 stroke plugs having that extra length after the end of the threaded section. As there is more time in a 4 stroke engine for the plug to cool down between combustion firings than there is in a 2 stroke engine, the bit that protrudes into the combustion space is more exposed to the heat of combustion than a 2 stroke plug, flush with the surface, would be. This exposure allows the plug to get hotter than it would have done if left flush and thus by the time next combustion stroke comes around it is still hot enough to do the job for which it's there.

I guess the reverse of this is that if a 2 stroke plug protruded then it would get too hot and burn out more quickly; keeping the end flush helps ensure that the plug is a bit protected by the surrounding metal of the combustion chamber and thus can maintain a more suitable maximum temperature.

Chris

[Neil LickfoldParticipant @neillickfold44316]

A lot of the stunt/aerobatic engines used the long series plug with a head that when the standard plug is fitted is flush. The long plug protrudes the head shape surface, and puts the hole of the glow plug about 1.5 mm or so closer to the piston. As wrong as it sounds, it prevented the very rich fuel setting from putting out the glow plug and made for a more consistent flight. K/B made the long series and the standard plug, with the long being K/B 1L or a K/B 1S, with the S being for standard, not short as far as I remember. There were many brands of glow plugs in the late 70's and early 80's. Now not so many around. Now there are 2 main taper seat type plugs. One has the 60 deg included seat angle, the Turbo plug,from the mid 1990's, with an 8mm body thread with 0.75 pitch. The other is the Nelson taper seat plug,(early 80's) with an 11/32-32 tpi thread body and the seat taper is 110deg included taper. There were also in the 70's the flat seat plugs , from the likes of Super Tigre, that used an M8X0.75 pitch retainer nut for the plug insert to be retained.

Neil

[KEITH BEAUMONTParticipant @keithbeaumont45476]

Posted by Neil Lickfold on 12/04/2018 14:50:25:

A lot of the stunt/aerobatic engines used the long series plug with a head that when the standard plug is fitted is flush. The long plug protrudes the head shape surface, and puts the hole of the glow plug about 1.5 mm or so closer to the piston. As wrong as it sounds, it prevented the very rich fuel setting from putting out the glow plug and made for a more consistent flight. K/B made the long series and the standard plug, with the long being K/B 1L or a K/B 1S, with the S being for standard, not short as far as I remember. There were many brands of glow plugs in the late 70's and early 80's. Now not so many around. Now there are 2 main taper seat type plugs. One has the 60 deg included seat angle, the Turbo plug,from the mid 1990's, with an 8mm body thread with 0.75 pitch. The other is the Nelson taper seat plug,(early 80's) with an 11/32-32 tpi thread body and the seat taper is 110deg included taper. There were also in the 70's the flat seat plugs , from the likes of Super Tigre, that used an M8X0.75 pitch retainer nut for the plug insert to be retained.

Neil

If there were a longer reach plug available in the 70s it might suggest that the Jones .605 build notes. which were published from 5th May1978 in ME, did in fact refer to the use of a "long reach"plug. The combustion chamber is dome shaped and has plenty of room for it to protrube. Interestingly, the Chenery Vee-Twin four stroke,of 1983 vintage,also qotes the need for "long reach" Chenery originaly ran the engine with spark ignition,but due to comlaints about it interfering with TV , he changed to a long reach glow plug with no problems. My experience with the the one that I have made is that fitted with the OS F type plugs it would not run continuously. Machining 0.030" off the heads was the answer.

We still have no explanation why Aero plugs just have the type No on the packaging,but model car plugs, as per my OS LC3, have "long reach" on the packet, although they are same length as an OS No 8 aero plug

Keith

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