ChrisH Posted April 18, 2016 Share Posted April 18, 2016 Can anyone please tell me the correct position the glow plug should be in relative to the cylinder head? By that, I mean should the glow plug sit so the business end is slightly recessed into the head, slightly proud of the head (thus into the combustion area), or sit so that the end of the glow plug is flush with the underside of the head. The reason of asking is that I am building an aero engine from old plans and the drawings do not make this clear. As is both the first engine I have built AND the first time I have had to use a glow plug I am in ignorance of what is the accepted 'norm'. I apologise if this has been the subject of a previous thread, I did check (honest!) but could not find anything. Chris Edited By ChrisH on 18/04/2016 14:29:34 Quote Link to comment Share on other sites More sharing options...
GONZO Posted April 18, 2016 Share Posted April 18, 2016 For a standard plug its flush. If it was recessed the glow element would be shrouded from the fresh fuel charge and also slightly reduce the compression ratio of the engine. The amount of reduction being dependant on the size(displacement) of the engine. On small engines of about 1cc and less the space around the element along with the threaded portion before the sealing washer can be a significant factor reducing compression ratio and thus performance. Hence the adoption of 'glow heads' in small performance motors like the Cox. The exception are 4 stroke plugs which have a nose that sits proud of the cylinder head surface to prevent the fresh incoming charge from excessively cooling the element, especially on rapid opening of the throttle from tick over. Quote Link to comment Share on other sites More sharing options...
Don Fry Posted April 18, 2016 Share Posted April 18, 2016 Gonzo, I am impressed, I've been running these things for over 50 years, and I didn't know that. Quote Link to comment Share on other sites More sharing options...
ChrisH Posted April 18, 2016 Author Share Posted April 18, 2016 Hi Gonzo, thanks for that, very interesting. My engine is a 4 stroke and I have just bought the plug and finished the head and the end of the plug is a good 1mm below the surface of the cylinder head. It didn't look right and now you tell me it is not right, encouraging. I note that the plug has a spigot (nose) below the thread of about 2 mm - so am I right in saying that spigot has to be clear of the head? I can arrange that by taking a little 'hollow' around the plug in the face of the head. Chris Edited By ChrisH on 18/04/2016 17:32:38 Quote Link to comment Share on other sites More sharing options...
GONZO Posted April 18, 2016 Share Posted April 18, 2016 Yes, needs to be clear of the head. But be aware that you need clearance between the piston crown and the plug nose. Don't forget to make allowance for mechanical play and thermal expansion if things look close. Don't want the plug 'pecking' at the piston crown. Quote Link to comment Share on other sites More sharing options...
ChrisH Posted April 19, 2016 Author Share Posted April 19, 2016 Gonzo - many thanks for your information, I will follow your advice. Cheers, Chris Quote Link to comment Share on other sites More sharing options...
GONZO Posted April 19, 2016 Share Posted April 19, 2016 Happy to have helped. Good luck and hope the engine runs well. As an additional bit of info you can always use more than one copper sealing washer on the plug if you need to pull it back out a touch. Quote Link to comment Share on other sites More sharing options...
ChrisH Posted April 19, 2016 Author Share Posted April 19, 2016 I'm going to set the end of the plug flush with the surface of the head then make a dome to reveal the nose of the plug. That should allow the nose to heat up properly and keep the plug away from hitting the piston! Chris Quote Link to comment Share on other sites More sharing options...
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