BJ8 spark plug reach

[Hoghead]

I notice that my BJ8 spark plugs do not fit flush with the combustion chamber and are recessed about .100"
This does not seem right to me

It is a simple manner to correct this when the head work is being done, and unless I am missing something, is there a reason not to correct this apparent flaw?

 

[Bob_Spidell]

You're in luck:

https://www.myaustinhealey.com/aec960-healey-cylinder-head.html

 

[Marvin Gruber]

So what is the general consensus? Leave the head alone or machine it out when other work is needed.

Marv

 

[Hoghead]

Not sure if the reason for more advance in the BJ8 is a function of the recessed plug as Dimmock states or the different cam and induction system

 

[Bob_Spidell]

Not sure if the reason for more advance in the BJ8 is a function of the recessed plug as Dimmock states or the different cam and induction system

Good point. But is the (log-type) induction system--besides being larger--much different than earlier cars?

 

[steveg]

Going out on a limb, I'd say most folks are in favor of machining the spark plug recesses down enough to bring the plug rims flush with the inside of the combustion chambers -- like the 100-6 & 100 heads. It would be interesting to cc the head before and after and use that to figure out how much it increases the compression ratio.

 

[andrea]

Hi Steve
I have on my 100/6 a late BJ8 head (long story) with recessed spark plugs
my solution is long 18mm spark plugs- I not remember exactly the code of the spark that are on my engine, but tomorrow I return with the code
Cheers

 

[steveg]

But the plugs shown in the photos are 18mm/3/4". The plug holes are approximately 25mm/1" deep. I looked in vain for 1" plugs.

 

[Hoghead]

I do not think that longer plugs are the solution and one has to mill the head if the plugs are to be flush with the combustion chamber.

 

[andrea]

View attachment 26174View attachment 26175
sorry for late reply but I recuperate some photos from my mechanics
that give me this photos and indication --NGK BKR6EIX
I am not sure that it corresponding exactly at the late photos
(it is more long than 19mm )
and if anyone are interested I will control, take apart it from my engine(that runs very well with this sparks)
Note: exceeding threads, must be removed -to avoid damages
cheers

 

[steveg]

Andrea,
According to this cross-reference:
https://www.sparkplug-crossreference.com/convert/NGK_PN/BKR6EIX
the BKR6EIX plug is 19mm reach.

The longer plug on the right looks like a 26.5mm reach plug. See how it protrudes from the head.
What is the part number for the longer plug? Can you send a picture so we can read the part number.
Is the longer plug the one that runs well.
You could put another gasket under the longer one to avoid protrusion.

 

[steveg]

I was looking at these two 26.5mm Iridium plugs in the "6" heat range: NGK LFR6AIX11 and Denso IKH20 (also known as Denso 5344). These plugs are used on WRXs and other turbos and some other modern cars. I have no idea how well these expensive plugs will work.

But I like your plug better because it looks like a more "normal" plug for our cars. It also looks less expensive.

Denso has a useful chart here:
https://www.globaldenso.com/en/products/aftermarket/plug/spec/index.html

 

[steveg]

I measured the thickness of the AEC 960 head at the spark plug hole with a home-made gauge:
24mm

Sighting along the bottom of the hole to the top of the piston at TDC: measuring the depth along the bottom of the hole to the point of interference with the piston: 29mm.
Thickness of crushed sparkplug washers: 1.5mm
Therefore a 26.5mm plug would only protrude about 1mm into the head - well within the 29mm.
To be extra-safe one could use two washers if desired.

To prevent interference with carbon buildup, one would want to tap out the threads before inserting 26.5mm plugs.

 

[andrea]

Hi Steve
I will take off one spark plug from my head and make one photo, inside is impossible to read anything, but sure it is NGK.
I have studied the problem more than one year ago, but never find trace of my indications to the mechanic about the long nose sparks
I remember that I give one reference and mechanic find one equivalent, easy to find on market, I see the sparks when it arrived, and remember
that was more long than normal, I am also very curious--
Same age, for me and mechanic....can be the catalyst?
Cheers

 

[Chris_Masucci]

How about one of these? I'll have to look up each part to see what the recommended gap is supposed to be, but these are all 1" reach with the right thread. Basically any of the LFR# series:
https://www.summitracing.com/parts/ngk-6499/overview/ LFR4A-E no gap listed
https://www.summitracing.com/parts/ngk-6668/overview/ LFR6A no gap listed
https://www.summitracing.com/parts/ngk-7787/media/images LFR6C-11 .043 Gap

https://www.summitracing.com/parts/ngk-6376/media/images LFR5A-11 .043 Gap
https://www.summitracing.com/parts/ngk-3672/overview/ LFR6A-11 .043 Gap
https://www.summitracing.com/parts/ngk-1116/overview/ LFR5A-11 S25 .043" Gap
https://www.summitracing.com/parts/ngk-1119/overview/ LFR6A-11 S25 .043" Gap

 

[steveg]

Eureka - those look like the plug in Andrea's photo above. The "6" would be the desired heat range.

 

[andrea]

Hi Steve
Just at the edn of the journey i take the photo of spark fro my engine
My mechanic are completely wrong spark are NGK LFR5B (more hotter)
he remember that was mounted on a Citroen C5 that is right, and on many FIAT and Mercedes-
easy to find and economic, here in Europe can be find at less than 5 EU each
Photo are very bad Sorry--but you can see that thread is partially removed at the end
your comments are agreed and await
Cheers

 

[Hoghead]

I think that having the plug protruding into the combustion chamber is a bad idea as well, and it would need to be shimmed

Since my head is off I am going to fix it properly

 

[Chris_Masucci]

True, which is why I plan to kill two birds with one stone with these:
https://www.jegs.com/i/JEGS+Performance+Products/555/40355/10002/-1

Indexing plugs for efficiency and smoothness is a good thing.

 

[Hoghead]

I have been indexing the plugs in my bikes for years, but too thick a washer will get you back to the recessed plug problem like the BJ8

These washers would work to solve the BJ8 problem, however you are going to need a pretty thick stack and it will be a bit difficult to get both proper plug indexing while being flush with the combustion chamber

Unless you have the head off you not not really know how thick a shim stack you need
If you have it off, spot face the plug hole and be done with it
If the head is on the car, then not much choice to guess on the shim stack and make the best of a shoddy situation