plug hours

[David Price]

Question: An A-65 running on 100LL with shielded plugs; How many hours of usage can be expected on the plugs for normal wear before replacement?

[lktiller]

I'm interested in these statistics also. I seem to replace plugs every year with less than 100 hrs. The electrode wears oval shaped more than what the IA will allow even with a re-gap. They are not carboned.
I do run mostly autofuel with some mmo. Occasionally 100LL.

How about the Titanium plugs? Anyone? I ran those over 300 hrs with no problems.

[Robert Lees]

We use unshielded plugs, but the principle is the same: use the plug manufacturer's recommendation, which in the case of Champion, is their plug gauge (see picture). Our plugs last about 500 hours before replacement. 100LL used exclusively on A65-8.

[NY86]

It is critical to properly rotate the plugs to ensure longest life. If you're getting less than about 400 hours, you're doing something wrong. When you rotate the plugs, make sure you are reversing the firing polarity. This is more important than going from top to bottom. The easiest way to remember is to move each plug froward one cylinder in the firing order. Clean and gap the plugs then check them with the mentioned gauge. If they don't fit in the hole, they're good to go, regardless of what your IA may tell you.

[David Price]

re: make sure you are reversing the firing polarity.

Confusion! Always thought that polarity was which side of the curcit the positiave leg went to, or the neutral leg; ie: reverse polarity. Help!!!!

[Robert Lees]

NY86:

You are telling us what to do.
You are not telling us how to do it.
We all fly 4 cyl hoz opposed engines, please tell us what you mean.

Rob

p.s. which John are you?

[NY86]

Originally posted by Robert Lees
NY86:

You are telling us what to do.
You are not telling us how to do it.
We all fly 4 cyl hoz opposed engines, please tell us what you mean.

OK, I'm not sure how much you want me to tell you, so at the risk of offending someone, here's the whole 9 yards:

Magnetos are alternating current generators. Each revolution of the armature produces a positive voltage peak and a negative voltage peak. The first plug in the firing order fires with one polarity and the second fires with the opposite polarity. The third is the same as the first and the forth same as the second. Depending on the polarity, either the center electrode or the side electrode(s) are eroded. So, if the plug fired the same polarity all it's life, one electrode will wear and the other will not. To get the longest life, you want to balance the wear.

Plugs on the bottom of the cylinder are more prone to deposit buildup than those at the top. No magic, just gravity at work. If you clean the plugs, it really doesn't much matter if you put them back in the top or the bottom, but what the heck. While you're at it, might as well.

So, we want to move each plug forward one cylinder in the firing order and also switch it from top to bottom, or vice versa. The firing order for a Continental 4 cylinder is 1-3-2-4. So, you can move 1B to 3T and 1T to 3B, then 3B to 2T and 3T to 2B and 2B to 4T and 2T to 4B, then 4B to 1T and 4T to 1B. Or, and this is easier to remember, you can accomplish the same thing if you exchange 2T with 3B and 2B with 3T. Then do the same with Cylinders 1 and 4.

p.s. which John are you?

I'm not sure how to answer that. There are so many of us...

I'm the John at NY86 airfield...

[tcraft]

Well I thought that was wat the answer was; having maintained 20+ elderly tractors all with bendix or slick Mags; we would "rotate" the plugs on July 4th weekend, along with a oil change. They all got new plugs in January.

I remember reading about this for airplanes somewhere; is it covered in aircraft maint training still?

bob.

[David Price]

John, Now this makes since. Did not know that each plug position fired with a differnet polarity from the mag. Thanks!