terraplane spark plugs

Are you sure J8's protruded too far? As I found the opposite as they are short reach plugs.



This was what I said on the report I posted here on my '36 Terraplane with high compression last year:



"Also researched a bit on plugs as all my cars seem to have the short-reach plugs such as Champion J8 or NGK B6S which end up with the plug recessed. The correct plug appears to be NGK B6HS which is 1/2" reach."



I did put the NGK B6HS plugs in and they are just perfect on my head (112 head), although I'm sure the standard '36 head is just the same in terms of spark plug depth as I had both heads on the bench to compare. The J8's being 3/8" reach result in a recessed sparkplug which is no good for efficient combustion, as well as leaving threads exposed in the head due too the too short plug.



So from my investigations, the 1/2" reach plugs are definitely the correct ones to use, and NGK make a very good plug.

Interestingly the Mechanical Procedures Manual for the 1937 models lists the Champion H10 as the correct plug for the high compression head. This is a 1/2" reach plug. The Champion J8 is listed as the plug for the standard head for '36-'37.



The strange thing is that the 3/8" J8 is definitely too short for both the standard '36 Terraplane head and the '38 112 head from actual observation. I can't think of any logical reason for it being specified as it is by Hudson unless to help reduce the compression a little by its being shorter. It certainly doesn't make sense from an efficiency point of view, as it results in a recessed spark plug tip.



As far as valve head clearance is concerned there appeared to be plenty using the 1/2" reach plugs and that was with my head having being shaved as well. Certainly nothing has touched after a fair bit of enthusiastic driving and I don't think that would happen anyway as the measured clearances looked ample.



Well here in NZ we get ripped off, my NGK's cost $US4.50 each!! I bought 14 of the sods, one set for my H8 and one set for the Terraplane.

Kdancy wrote:

About to buy new plugs for the 37 Terraplane PU.
What is the consensus now for the proper plug?

Irrespective of the name and code of the plug, a standard 1/2" reach plug of the correct heat range is what you want.

A Champion J8 is a 3/8" reach plug here in NZ (hard to think it would be any different in the US) and as I said it is too short to reach the end of the spark plug hole in the head.

The NGK B6HS plugs I put in work perfectly.

As a matter of interest why not take a plug out of your head, look down the spark plug hole and get a measurement on its depth? If you've been using the short plugs you should see that the last 1/8" of threads in the head are carboned up due to the short plugs not extending to the full depth of the hole.

walts garage-53 wrote:

The standard head takes Champion J-8, set at 0.025". The aluminum power dome head take Champion H-10 set at 0.025. This is right from the Hudson 1937 manual. The eight cylinders only use the J-8 plugs. Walt.

Walt, 3/8" or 1/2" reach? As I said, I have found by physically fitting that the 1/2" reach is correct for both a standard '36 Terraplane head and a later 112 head.

Don't know how you'd bend an exhaust valve unless your valves are already almost touching the head. The 1/2" reach plug in any case barely extends past the end of the plug hole into the combustion chamber.

On a planed 112 head fitted to my '36 Terraplane, standard cam and new thick valves there is plenty of clearance. The only possible issue could be that with a 1/2" plug the sparkplug tip is too close to the exhaust valve head which could cause the plug to overheat in extreme conditions?

Chuck G wrote:

terraplane 8 You just dont give up! I think i will throw away all my shop manuals and just consult you. J-8 J-8 J-8 get it?

J8 that's what I thought too, until I put the heads all three of them on my bench and actually measured everything to my satisfaction. I ascertained that a 3/8" reach plug does not reach the end of the plug hole ending up with the spark occurring within the plug hole which is hardly conducive to efficient combustion. Who knows why J8 was originally specified, lots has changed since 1936 especially the fuel quality. All I'm saying is, taking a fresh look at something can pay off sometimes. Those who are interested don't take my word for it but check it out to see if it will work for you. My reason to use them was to improve efficiency, nothing else. The car goes like a rocket so something is right. Hill climbing is phenomenal, as good as the Terraplane Eight without actually measuring it.

Denzil, I'm willing to bet my reputation that you will find absolutely no difference in performance or economy between a 3/8 or 1/2" reach plug.