Correct gap for sparkplugs

[170C]

Internet states the proper gap for a Tempest UREM40E is .016-.021. Do you agree that this would be correct for C-0300's?

All the years dealing with Continental engines and sometimes having my IA's test the plugs with a pressure tester and sometimes not, I had always believed if the gap was correct and the tester showed the plugs to be good that that meant they were good to go. Started seeing the Tempest advertisements with the resistance values,etc. and had never given any thought to that factor. A friend with a early 172 and a 300D was having a good operating engine until one morning recently he couldn't get it to start. Had a local mechanic check things out over several late afternoons and could not find the problem. Friend purchased 12 new Tempest plugs from Tex-Air, put them in and guess what-------the engine fired up on first blade and has been running like a sewing machine since. Mechanic took the old Champions & checked each for resistance. Many had resistance's of 7000 -10,000 om's. So now that I am finished with the IA's annual inspection, I am checking the resistance on my almost 500 hr plugs even though the engine has been running OK.

[blueldr]

Who sold your friend the set of new spark plugs? I've NEVER seen a set of spark plugs from a previously running engine suddenly fail to the point that all of them were so bad that the engine wouldn't start.

[FredMa]

The (approved) TCM manual says the spark plug gap should be 0.015 - 0.021. I don't see the purpose of testing your plugs that are working normal unless you are doing an annual or 100 hr insp. If it ain't broke don't fix it.

[bagarre]

Sounds like some great marketing material that reminds me of the old automotive SplitFire plugs

The only time I've ever thrown away a plug that passed all tests was when I dropped it on the shop floor.

[170C]

Dick, the decision to purchase new plugs was made by my friend, the aircraft owner after all other checks failed to locate the problem. Those were purchased from Tex-Air in Fort Worth as mentioned earlier and the decision of where to purchase was made after checking several other sources including Spruce and the local supplier was actually a couple of dollars per plug less than Spruce. They weren't able to get cylinders to fire and on one test they did pour raw fuel into one or two upper plug holes and managed to get the engine to start. If they let it run until the engine was warm it would restart, but if left to cool down it wouldn't. After doing this a couple of times even this procedure failed to permit the engine to start. Intake hoses were checked, the carburetor was pulled and checked, ignition wiring checked, mags checked,etc, etc. It was very strange that the engine had been running just fine and all of a sudden wouldn't start. Mechanic said he had never seen that exact situation before.

To repeat a story I posted some time back, at one of my annuals some almost 500 hours ago I had my mags overhauled as required. Reinstalled them and upon the first engine start up it ran like c_ _ p. We checked a number of things and I was asked how old the sparkplugs were. I don't recall the exact time, but they had been replaced when I overhauled the engine and had not demonstrated any problems. I went to a local aviation parts source here in the FW area and purchased a set of Unison (now Tempest I believe) plugs, installed them and the engine started and ran perfectly as it has for the past 450+ hours. It is strange how something can occur all of a sudden. Its not normal, but can happen.

[Sixracer]

Any of you guys check the resistance of "New" plugs?
Std procedure in my racing operation.
New doesn't spell good as the old saying goes.
I have several boxes of new plugs that would probably misfire in anything you put them in.
Now let me add a twist to this plug resistance discussion.
What would be the situation if a plug lost resistance after it was used for a short period of time? (heat cycles)
I'll give a Hi-5 to the first one with the correct answer.
BTW: The A/Fuel team I'm associated with replace the plugs after every run, all 16 of them in the dual plug Hemi. That Nitro "Eats" plugs. On a good run between 800 and 1,000 ft down the track they become "Glow Plugs" and some start melting out.

[blueldr]

That "Hemi" aint eaing twenty five dollar plugs is it?

[T. C. Downey]

What would be the situation if a plug lost resistance after it was used for a short period of time?

0 Ohms or no resistance is a direct connection, infinity or an open has a high resistance.

your aviation spark plug has a carbon rod about 5/16-1/2" long in it to give the plug a resistance of about 5,000 - 7,000 Ohms, as the plug electrodes wear the gap widens, and the plug gets dirty, resistance goes up. When you clean and recap the plug the resistance goes down again.

When you drop the plug one of three things happen, 1 nothing and the plug survives, (rare) 2 the carbon resistor breaks, and the resistance goes to infinity. (open) 3 or the ceramic insulator breaks and allows the voltage to escape thru the wall of the plug and no spark jumps the gap firing the plug.

[FredMa]

A plug with too little resistence where? between the center electrode and ground or through the resister of the center electrode. There is a difference and several different potential types of failure. That is why I think the tempest plug resistence check is only to sell more plugs, it is not a comprehensive check and only takes into account one particular type of failure while ignoring others. A low resistence to ground would cause weak or no spark, doesn't have to be from a dropped plug. A low resistence in the center electrode would create excessive amounts of RFI (radio frequency interference)

[GAHorn]

Internet states the proper gap for a Tempest UREM40E is .016-.021. Do you agree that this would be correct for C-0300's?....

YES...that is "close". The actual specification is .016".

The installation torque is 25-30 ft-lbs....or 300-360 in-lbs. Lubricate the threads with anti-seize, don't get any on the electrodes.

Spark Plug Servicing AV6-R-Nov2004.pdf

I prefer the Unison/Tempest plugs for three reasons over Champions:
NUMBER ONE: their high-nickel content barrels don't rust like Champs do.
NUMBER TWO: their resistor is higher quality than Champs (as depicted in their literature. Regardless of whether or not you think variable resistance in plugs is harmful or not....why would anyone deliberately choose ANY electronic item which is produced in an inferior, non-stable fashion?)
AND...(DRUM ROLL...) NUMBER THREE: They are the type bluEldr would endorse because they are LESS EXPENSIVE. (And now, they are the standard TCM OEM plug, as are Tempest filters. Ram Engines also recommend them.)

One more thing... if your previously serviceable plugs become lazy and no longer "fire" even tho' they pass inspection....???? You MAGNETOS have probably lost their residual magnetism.
Magnetos, if properly overhauled (and not simply field-repaired)... should have their field magnets RE-CHARGED to full strength. It's a commonly overlooked item, but a specification of complete overhaul procedures.

[blueldr]

Why in hell are we still buying twenty five dollar spark plugs when maot of the "experimentals" are using automotive types and doing just fine on them.
When all the aviation radios were transmitting on 3105 KCs and listening on 200 to 400 KCs on the old four legged radio ranges, there used to be a lot of ignition noise on the radios and shielding cleaned a lot of that up. I don't remember any of that since we moved up into the VHF spectrum, and I never hear of the homebuilders having a problem using an unshielded system.

[Metal Master]

I sent a set of Champion RHM 38 E spark plugs back to Champion because they would not fire properly in the tester and the engine was misfiring on run up and in flight. All of the plugs showed virtually no electrode erosion. This is abnormal for a plug with 407 hours on it. In view of all that I have been reading and being told regarding the resistance particularly in Champion plugs I started my own investigation. Understand that I have sold and installed Champion Plugs for over 40 years. Not any more if I can avoid it. I went down to the shop and grabbed a Mixed bag of about 30 miscellaneous rejected plugs from our scrap plug bin. I set all of these plugs out neatly on the bench. I cleaned and gapped all of them. I checked and charted all of their resistances. I tested them in my ATS spark plug tester. I found plugs that had virtually no wear of the electrode but these plugs were rejected for some reason. In every case these poor performing plugs had High resistance from the center inside the barrel to the center electrode on the end of the plug, in some cases as high as 30 meg ohms. Others with resistances as low as 6,000 ohms in every case these plugs sparked poorly under pressure in the tester. I found other plugs with barrels that were rusty, well worn and would pass right through the Champion Go No Go gauge but would fire perfectly. All of these plugs had resistances less than 1200 ohms. Talking with the Champion tech rep he told me their tolerance was 900 to 1200 ohms resistance but had no published information on performing this check. I have started charting and testing the resistance all of the plugs I look at during engine work. The new Champion plugs (Tempest URHM 38 Es were not available at the time) I installed in the Seneca engine now have about 100 hours on them. One plug has started to have resistance drift and is now reading 5,000 ohms resistance. It still “bomb” tested ok. Any plug that I have found with over 5,000 ohms resistance fails the “bomb” test. Heated or otherwise. One of my favorite things to do is show an owner one of his high resistance plugs in the ATS tester and watch the spark jump from the spark plug wire to the shell of the plug rather than at the electrode. It looks like a testla coil if you know what that is. The spark plug will not act like this in a champion tester because of the ceramic insulator on the end of the test lead if you have ever seen one you know what I am talking about. It also will not do it with a small plug like a RHB 40E because the plug is so short. But it makes a cool light show with a RHM 38E. AS part of my test in the Seneca’s right engine I took all of the bad RHM 38E High resistance plugs which were causing the engine to run rough. Removed the resistors and installed resistors from the very worn out RHB 38E’s I had. Stuck them in the engine and the engine now ran normally. My conclusion The Tempest Plugs run better the resistance is not drifting and they cost less. However I am nursing the old Champions along in my 170A. When I am forced to buy I will buy Tempest. I have many more stories about these plug issues and could go on for hours. I am quite unhappy with Champions response to all of this. Champion does not make Bad plugs the just install poor resistors that drift.
The following letter and plugs were sent to Champion for evaluation. Two of the plugs had recently been replaced that is why I only sent 10.
All of these plugs were installed in TCM TSIO 360 KB, installed in a Piper PA 34-220 Seneca. I would make more $$$ selling Champion plugs. They cost me more than Tempest so there is a marginally greater mark up.
Letter to Champion
These 10 spark plugs, Champion RHM 38E’s, all have high resistance values over 20,000 (20K) Ω ohms. Eight of the plugs have 2000 date codes and, per the log books, were installed December 30, 2000, calculated time of 407.1 hours. One of the plugs has a date code of 2002. The other has a date code of 2004. The only other log entry shows 3 new RHM 38E plugs having been installed in May of 2005, calculated time of 301 hours.

Only one of these plugs shows any significant electrode erosion. All of the 2000 date code plugs have 400 hours on them. Normally I would expect to see significant plug erosion on all of the 2000 date code plugs. They all spark poorly in the tester.

The 2002 and 2005 date code plugs test over 20K Ω ohms, and also test poorly in the tester.

Of the other three spark plugs installed in the engine, two were new and installed at the last annual. One had a 2004 date code, tests normal, and has less than 1200 Ω ohms resistance.

I would expect plugs under normal use to show erosion of the electrode, and last from 400 to 500 hours installed in this engine. That being said, seven of the plugs could be said to have given good service. However, I believe they are responsible, because of the high resistance, to have damaged the ignition harness, which was found to have burned through the center insulation of the #6 lead about half inch in from the magneto.

I have never in the past, nor has anyone I know, checked the resistance of plugs on a regular basis. I am now going to do so regularly. I would like to know how soon the resistance starts to exceed what seems to be an acceptable level. I have read several articles in the past week regarding this issue. But, after thoroughly reading through the Champion Service Bulletins (information), have found no mention of it.

The final issue is that, on this airplane, I now suspect that I have a similar issue with the left engine, but have not yet begun to examine it. I fully expect to find the majority of the plugs will have high resistance and poor spark quality when tested.

Looking at other plugs I have retired from service, I have found several which demonstrate no erosion, but do have infinite or high, over 20K Ω resistance and poor spark quality. Several show normal erosion, resistance less than 1200 Ω ohms and, other than having been retired because the electrodes passed through the no go portion of the CT-482 gauge, have good spark quality.
Final comment for this forum: Champion eventually quit responding myself, the Seneca owner and several others writing Champion on these issues. Champion bottom line was that the plugs needed to be replaced even though they showed no electrode wear. This wasn't my issue I knew they need to be replaced
Regards,
Jim