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Austinsevenfriends
parkplug well too long for high cylinder ? - Printable Version

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parkplug well too long for high cylinder ? - creusois - 01-07-2021

Dear all,

First, I'm still sorry about my bad english, I'm working on it, but it's still poor.

I bought several year ago a specific head cylinder at the Beaulieu autojumble. 

I wish to update it on my engine, but I'm really surprised by the size of the sparkplug well. It's really to long for a standard.

As you can see on the photo, the spark plug electrode is still in the sparkplug well.

Do you think that it necessary to machine the sparkplug well to reduce by the top the head cylinder ?


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RE: parkplug well too long for high cylinder ? - Robert Leigh - 01-07-2021

Sparking plugs are available with different lengths of thread. Without being able to measure the holes in your head I suggest that you may need 3/4 in instead of 1/2 inch. I hope this is your problem. Excuse my quoting inches but that is how they are normally defined from the early days. Metric conversion gives approximately 19 mm and 13 mm to the nearest mm.

RE: parkplug well too long for high cylinder ? - Stuart Giles - 01-07-2021

I think the BP6ES plug is 3/8" reach. Does look like the head needs 3/4" 19mm reach plugs.

RE: parkplug well too long for high cylinder ? - Colin Wilks - 01-07-2021

https://www.sparkplugs.co.uk/ngk-spark-plug-bp6es-7811

According to this the BP6ES has 19mm thread length, as do the other possibles BP6E and BP6ESZ.

Creusois, how long are the threads on your plugs?

RE: parkplug well too long for high cylinder ? - Ian Williams - 01-07-2021

ES are indeed long reach plugs so I would suggest the plug wells have not been machined deep enough, this seems to be quite a common problem on aftermarket Alloy heads for some reason.

RE: parkplug well too long for high cylinder ? - Nick Lettington - 02-07-2021

Funnily enough I was thinking the same thing when I was looking at a Speedex head recently. It also got me back to wondering about the TT head thread and Riccardo patent drawing...

The chamber in the late Ruby head is deeper at the plug so that the electrode is in the mix. With the correct long plugs, on both Speedex heads I have the electrode is tucked up inside the threads... although not as bad as the one from the OP.

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The Ricardo patent may be drawn deliberately wrong to confuse competitors, but ithis begs the question s there a benefit to having it a little shielded? 

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RE: parkplug well too long for high cylinder ? - creusois - 02-07-2021

Creusois, how long are the threads on your plugs?

22 milimeters. Really long no ?

RE: parkplug well too long for high cylinder ? - Mike Costigan - 02-07-2021

So that would locate a plug with 19mm thread in a similar position to the one in Nick's alloy head?

RE: parkplug well too long for high cylinder ? - Nick Lettington - 02-07-2021

The plugs I have measure 12mm and 19mm, ignoring the crush washers. There is 22mm of thread in the Speedex head. This means the nose of the plug just sits inside the threaded section. 

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If you deepen the plug well to get it lower, there must surely be a risk of the valve hitting the plug?

RE: parkplug well too long for high cylinder ? - Reckless Rat - 03-07-2021

Creusois. Before you go to the extreme of machining the cylinder head, I would recommend that you check the clearance available from the valve when fully open. The use of an "extended nose" spark plug rather than a standard electrode could mean the difference between good and bad.

Rather than using an extended nose plug, a standard one should sit flush with the combustion chamber such as shown on post #6 by Nick Lettington. With that in mind you could then machine 3mm off the top surface of the head to accommodate a 19mm plug like the ones in the last photo. Good Luck.