spark plug thread depth

[charvey]

Hi All,I have got an 39,iron headed and a 52 alloy headed Ariels both with 1/2" plug depth I thought that the alloy heads had a greater depth of thread but it seems not, I run them both on a mixture of NGK B6HS and B5HS spark plugs which they seem fine on but it was just the depth of the thread which I find a bit odd like I stated between iron and alloy,I did measure the threads and the alloy one is 1/2"regards Clive.

[will_curry]

I'd be interested to see a photo of your alloy head.

[JohnnyBeckett]

hi in my iron head engine i use B 6 HS or 5 plugs and in my alloy head engine i use B 5 ES or 6 plugs and the ES plug for the alloy head has got a longer thread

[ian williams]

in my alloy head NH [non external pushrod tube type barrel ] it has long reach N5C plug .

[charvey]

Hi all,I have measured the thread depth again and it is 1/2" depth why I don't know but it is the same plug for both bikes which makes it easier enclose some photos it's on the VCH but I cannot see why that should make a difference I have a photo of the plug in place to see if the depth is the same on other alloy heads,Regards Clive.

[JohnnyBeckett]

hi how i check my plugs depth was wen i had the heads off and i could see the plug thread reach the end of the thread in the head i got a NH that come with a shorter plug that didn't reach the bottom of the thread in the head, that is wen i started to look in to the right plugs for my Ariel's

[charvey]

Hi John,I measured the thread with a double piece of bent wire and also when the head was off a short while ago checked the plug in the thread and saw it was a good fit in the threads and not sticking out so it is a 1/2" thread depth so Ariel's must do different depth threads in alloy heads, never having had a alloy headed Ariel before only iron it didn't strike me as odd till I ordered some recommended plugs on the forum site for alloy heads and realized the depth of the thread was different regards Clive.

[BobWard]

Your plug thread is recessed a long way down.

[JohnnyBeckett]

hi i can be a aftermarket head or it has had the plug hole depend for some reason all the alloy heads i have use B 5 ES plugs witch are a bit longer thread than the H S plugs

[charvey]

Hi Bob,Thanks for the info yes for some reason or other it does seem that my head has a deeper inset than yours where the plug fits in going on your photo for what ever reason, I don't know why,the previous owner is beyond asking as he is riding Ariels in the sky but I know Chass had a dropped valve a long while ago but I see no reason to reduce the thread because of that,But as far as I know it's the same head as it's had a lot of work around the exhaust guide welding etc.,so I am sure it's the same head which has been on all the time Chass had it, thanks again regards Clive.