Indexing plugs

[LakeFever]

Maybe its another example of Mercury hiring ex OMC engineers.

lmao you win the internet today

[lbhsbz]

Most plugs use M14x1.25 threads....which means each thread is spaced 1.25mm from the next. 1.25mm = 0.050" (roughly).

So, a 0.010 washer would be 1/5 of a turn, .020 washer would be 2/5s of a turn, etc...

[LakeFever]

What direction works best on the mercs?

[bakerfreak]

What direction works best on the mercs?

I'm guessing they should open toward the intake side, but JMHO.

[David]

The opening in the electrode is supposed to face the injector

[Chaz]

Which way is the intake or injector ... when a two-stroke usually has two transfer ports apposed from each other and two or three boost ports facing the exhaust port.

The correct answer , two stroke or four .. is to face the exhaust. There is less likelihood for the spark to be "blown out" by turbulence.

The picture below shows the plug shielded from the intake charge. But at the expense of leaving a sharp edge and threads exposed around the plug. No bueno.



SEE ALL 5 PHOTOS
Old-school thinking: The open end should face the hotter exhaust valve, making it easier to start the fire. This orientation also protects the plug from fuel wetting caused by a rich air/fuel charge. This is unnecessary with modern chambers, ignition, and fuel management. For example, on this Gen IV LS7 chamber, the plug is already biased toward the exhaust by design.

[LakeFever]

I always figured this indexing thing to be a bit snake oily because you get spark and light the fire or you dont. In boosted motors or alcohol builds i can see this being a requirement or at least wanting to ensure you cover all your bases to aid in any trouble shooting efforts.

Sort of like the cryo thread where i was convinced. chaz you ever get any measurable gains out of plug indexing?

[Chaz]

I had a 598" big block that was built so tight that I had to index them to keep a gap, so that motor was a given.
On my 706" motor, 4cc dish, room to spare .
I made a sparkplug rack that held 48 plugs. When they went in the motor where ever they landed vs indexing them .. it was night and day on how they appeared. I found it easier to "arrive" at a tune up when I could see a trend developing (good or bad) to clean up before I made a move. So I would say it's more of a tool to allow you to see what you have, rather than a 10 HP gain directly. Although, I'm sure there are some combo's out there that benefit (HP wise) from the spark starting at a certain point and working out from there ...
A black dude I nitrous raced with for years, said it the best : Frosty will get in there and cut your **** to death if you ain't lookin ..

On two-strokes, I think it's a good idea to have the cool charge blowing on the back and sides of the ground strap. Rather than face the gap towards the boosters. Less cooling, fuel washing the gap .. etc.
Until we get a little guy to go inside , survive , and come out and tell us what he saw .. being able to consistently read plugs is the next best thing.

[David]

I pulled this from a Youtube video. BRP and Mercury both require the plugs to be indexed on the DFI engines. It's easier on the BRP as I only do it every 3 years.

[MrG]

Ive had good luck with NGK BUHW surface gap in My looper carb engine never fouled them even in the long idle zones they are about 2.50 a plug at Autozone

[LakeFever]

I ran those surface gap plugs in my first merc v6 too. Worked good for me. Chaz?

[David]

I got the plugs in today. The 200XS ROS plugs are very hard to reach, and of course I had to try plugs multiple times. It’s not like changing plugs in the old days, but at least it’s a two stroke and light enough for my Ally