Bob, if you fully screw the plug into a late 12 port head on your
bench, and the look at the combustion chamber, you'll see that
although the plug is fully seated & sealed, there are still exposed
head threads between the end of the plug and the chamber.
I.e. The plug doesn't protrude into the combustion chamber fully as
per every westlake head design. Therefore it needs to be fired earlier
I'll try and take a pic.
Chris
Sent from my iPhone
On 04/10/2010, at 1:04 AM, Bob Spidell <bspidell@comcast.net> wrote:
> Vewwwwwy interesting.
>
> re: "A fully screwed in spark plus will not screw all the way into
> the combustion chamber."
>
> If I am interpreting this correctly, this means the plug is not
> fully seated in the plug hole. Is that correct? If so, you'd
> probably have leakage around the plug threads; i.e. the copper
> gasket would not be crushed and hence would not seal. All the plug
> gaskets I have removed appear to have been crushed. Or, does this
> just mean the plug threads are too short?
>
> Also, would a hotter plug compensate somewhat for this?
>
> bs
>
> --------------------------------
> Bob Spidell - San Jose, CA
>
>
> There is a huge ignition static advance anomaly between the factory
> specs for a BJ8 and every earlier 12 port 3000.
> The closest explanation for this i can offer is that the later 12 port
> heads are machined incorrectly by the factory where the spark plugs
> screw in. If anyone on the list has a bare later 12 port head, you can
> confirm this. A fully screwed in spark plus will not screw all the way
> into the combustion chamber.
> It screws in all the way on every other westlake designed head ive
> ever seen There are a few threads still exposed in the head on the
> late 12 port heads. Meaning the plug has to fire earlier because it is
> further away.
> I thought this issue was the reason for the bizzare amount of static
> advance on a bj8, but apparently the same late model head is used on
> pre BJ8 cars.
> Either way, a total of 34 - 36 total advance "all in" at 3,000 -3,500
> rpm is correct for all bmc c series engines. How you achieve it is up
> to you (I.e combination of static advance plus distributor advance)
> I have very little advance in my distributor compared to a std BJ8
> (changed the distributor cam, and no vacuum advance)
> Chris
> www.myaustinhealey.com.
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