Glow plug position

[creast]

Hi Guys,
I am currently building the David Parker Vega V twin aero engine (work in progress) and am at the point of machining the cylinder heads. I have trial fitted the glow plug after tapping the thread and hadn't realised just how far back the plug is from the combustion chamber, approx 3mm at a guess. The head is machined according to the plans so I am carrying on regardless but would like any input on just how close the plug end should be to the chamber?
Can anyone advise?
Thanks in advance.
Rich

 

[xpylonracer]

Hi Rich

Have you fitted a short reach plug when it should perhaps be long reach ?
I suggest the plug body may need to be flush with the combustion chamber surface.

Emgee

 

[mu38&Bg#]

I wouldn't worry about it. The early OS fourstoke engines had plug recessed about that much with a ~3.5mm through hole. If it seems to be a problem you can change to a four stroke type plug (O.S. "F") with the extended nose.

Greg

 

[creast]

Thanks for the comments guys.
I am using a 4-stroke plug by ModelTechnics #P4S.
Length from nose to copper washer is about 5.5mm (7/32")
Pic shows how recessed the plug will be when the head has been finished.
Rich

 

[Jasonb]

Plenty of glow engines have the plug at the top of a recess, tends to help keep the element out of the cool incomming mixture. The plug comes to the end of the thread in this one so is tucked up at the top of that rounded recess

 

[creast]

Thanks Jason Thm:

 

[mu38&Bg#]

A two stroke head usually has a significant squish band like the image above. This isn't the same thing. However, the Vega design is perfectly fine. The OS engine were much more shrouded than that and they worked well. Newer OS four strokes (and likely all other makes) are designed such that the tip of the plug protrudes into the combustion chamber. I see the P4S is only .5mm shorter than the OS F plug, but it should make little difference. http://www.os-engines.co.jp/glowplug/plug04.html

 

[creast]

Thanks Dieselpilot, I feel more comfortable about it now. I was going to try it as is anyway but I like to identify problems before they happen.
It may be a month or so before I can try to fire her up.
Although I know a few people have made this design I haven't been able to enter in any form of dialogue or contact them for exchanging info.

Cheers
Rich