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Wyvern Spark plug

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NR6723/06/2022 09:25:24
36 forum posts
10 photos

I am currently machining the cylinder head for my ET Westbury Wyvern. The build notes say to use a M10x1 thread for the spark plug and not the 3/8x24 as per the drawings. I guess thats because modern plugs are metric. I will be running on petrol so my question is are there any recommendations for the plug. I want to leave or remove enough material so the tip is in the correct position. I will want the plug in hand before I continue with that feature. Also what is the correct position for the tip. This is my first IC engine so sorry for the beginner sounding questions.

Andy_G23/06/2022 09:57:36
184 forum posts

The NGK CM-6 plug is often used for engines of that sort of size (it is M10 x 1 thread).

I used one for my little (20mm bore) hit and miss engine:

They are readily available and reasonably inexpensive.

In the absence of any information to the contrary, I'd aim for the end of the plug body to be ~flush with the inside of the head, provided that nothing comes too close to the electrodes in that configuration.

Edited By Andy_G on 23/06/2022 10:05:01

Roderick Jenkins23/06/2022 10:08:37
2201 forum posts
616 photos

+1 for everything Andy says. That's what I used on my Wyvern it works ok.


JasonB23/06/2022 10:21:17
23022 forum posts
2763 photos
1 articles

Same here I use the NGK CM-6 on the larger engines.

I've done engines where the plug is a long way removed from the cylinder in separate valve blocks but they still work OK

noel shelley23/06/2022 10:24:38
1435 forum posts
23 photos

The main thing is that nothing hits, I once came across an engine that would not run ! The owner had put a long reach plug in and the gap was closed as the piston hit it ! Shorter plug ran fine ! Measure piston height and any head clearance and go from there. Plug position will dictate combustion/flame fromt.. If your near me I could lend you a 10mx1 tap. Good luck. Noel.

NR6723/06/2022 21:39:46
36 forum posts
10 photos

Thank you for all the excellent replies. Just what I needed to know. I left the plug entry point a bit high so I can take a bit off to get the plug position right when I get one to install. Coming from steam to IC is challenging so I may have further beginners questions when it come to timing and making a spark. Ordered the tap today. M10x1. 

Edited By NR67 on 23/06/2022 21:41:52

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