Curious??

[CAD]

recently replaced my spark plugs and came across something unusual, bought a set of plugs for my 88 2.9L they gave me (Autolite 105's). But, the plugs that came out of the engine were (autolite 765's). The difference is that the 105's have approx. 1" of threads and the 765's have only a 1/2" of threads. I took the 105's back and replaced them with the 765's, since thats what was in there originally. My question is, why the big difference in thread length. Do I have different heads from stock (the parts house said that the 105's were the only plug they listed for my truck or is something like a poor man's anti fouling adapter? The plugs that came out showed no signs of fouling and except for normal wear were in pretty good condition.

 

[ab_slack]

I recall encountering something like this and was wondering myself. I don't think it was on my BII with the 2.9L but may be recalling incorrectly.

When I saw it I thought, gosh one must stick in further than the other, but upon closer look the seat was the same distance from the end of the thread. One plug had threads the full length (3/4" to an inch) from the end to the seat while the other had threads for like 3/8" to 1/2" with the gap between threads and the seat narrowed so even if the full length in the head was threaded it would still go.

I don't know if the head has threads the full length or not. It may be that it didn't have threads and thus they were just saving money using less material and cutting less threads. Which may be okay I guess since it is same amount of thread engagement.

Or maybe the head was threaded all the way and they were cost reducing for the same reason at the expense of having less thread and thus putting all the stress on just the one portion of the thread.

Or maybe it some idea for keeping them from seizing as easily by having less thread engaged.

I don't know. In my case it was just one single plug I was using for test purposes.

 

[SenorNoob]

Doesn't the length of the metal have something to do with the heat range? Or is that entirely inside the cylinder that matters?

Edit: This is direct from Autolite's site for a Ford Ranger 2.9

IRIDIUM XP XP105 Copper Core, Double Platinum, Fine Wire Center Electrode, Iridium Enhanced, Nickel Plated Shell, Trimmed Side Wire; .044 Gap
DOUBLE PLATINUM APP105 Copper Core, Double Platinum, Nickel Plated Shell; .044 Gap; Replaces APP765
PLATINUM AP105 Copper Core, Platinum; .044 Gap; Replaces AP765
RESISTOR 765 Copper Core; .044 Gap; OE or equivalent plug;