The International Cessna^® 170 Association

Dear Members:

At the last annual I switched from Champion plugs to Autolite (Unison) plugs and almost immediately started having fouling problems during the taxi out to the runway. I can tell from my engine monitor that the fouling always occurs on the bottom plugs. Since I burn almost exclusively no-lead auto fuel, the A&I at my airport who has a very good reputation and tons of experience restoring classic aircraft, etc., thinks that it is oil fouling caused by oil seeping onto the lower plug after shut-down. This seems likely as it can't be lead fouling as there is no lead in my fuel and it only happens after first start-up after the airplane has been sitting overnight. At run-up, I can usually get the fouled plug to clear at run-up RPM, but, sometimes it takes full-power to get the fouled plug to clear. Once clear, the plug stays clear and all operations are normal. The mechanic didn't think this issue was anything to be concerned about and said I might wish to try a plug with longer threads under the theory that the plug will stick up in the cylinder a little more and be less prone to oil fouling while sitting.

Questions: 1. Is there any experience to show that the Autolite plugs are more prone to fouling than Champion?
2. The engine is 400 hours from TBO and has been generally leaking more oil everywhere, but, consumption hasn't changed noticeaby. Could this oil drain onto the plug internal to a cylinder be a byproduct of normal aging? Or, is this something to be concerned about? The compressions on all cylinders are good.
3. The plugs are Autolite UREM40E's. Does anyone know the model number for plugs with longer threads and has anyone tried them and why? Do you think they are worth a try?
4. If I do change to a longer plug, can I change to the longer plug on only the bottom plugs or do all the plugs need to be the same?

Appreciate your thoughts.

Thanks,

Curt - N4288V

You have been given WRONG information. You do not wish to change to a plug with "longer" threads.
You only want to use plugs which are approved for your airplane. The thread-length of approved plugs are not selected for such matters as you are discussing. They are matched to the depth of the cylinder head.
Plug HEAT-range is affected by the length of the center electrode ceramic and therefore the ability to pass heat off to the cylinder head. BUT.... that is not a subjective matter you can go to the parts counter and simply decide to "try" another length plug.
The method to select a "hotter" plug that might burn oil off the electrodes better would be to select a plug within the same model-family with a higher number. (A REM44E would be hotter than a REM40E.)

BUT IN ANY CASE IT WILL NOT LIKELY CHANGE YOUR SITUATION. This is because if your engine is oil-fouling the lower plugs, ...until you cure the cause of the oil fouling,...the new/hotter plugs will also be fouled by oil. The most likely cause for oil fouling of lower plugs only after shutdown is due to worn valve guides. Oil in the rocker box will leak down thru the valve guide/between the guide and valve-stem, and into the cylinder. This only happens after shutdown because while the engine is actually running the cylinder pressures are blowing past the worn guides and into the rockerbox area (and out the crankcase breather as excessive crankcase gasses.)
The fix is to "top" overhaul your engine and/or repair the valve-guide to valve stem clearance.

IF oil fouling happens even during a running engine's operation, then it's most likely due to worn piston rings that allow oil to pass by the rings and foul the plugs. But this would likely foul BOTH upper and lower plugs.
The "easy" fix for this problem is not hotter plugs....but to switch to fine-wire plugs. Fine-wire plug electrodes heat up faster and burn the oil off more efficiently than massive electrode plugs.

There is no reason Autolite/Unison plugs should perform any differently/worse/better than Champions if the model number of plug is the same. (I recently switched to Unison UREM40E's because I wanted to try their nickle-alloy plugs to see if they don't rust/discolor as badly as the Champions. I'm very pleased so far.)

A fast tutorial on plugs: http://www.sacskyranch.com/faqsparkplug.htm

Again...DO NOT select a different thread length (Reach) plug other than that specified for your engine.

I switched to Unison plugs almost a year ago because my Champion ones had a little over 500 hours on them. New plugs made a big difference in how the engine ran and to date they have not rusted as the Champions had done and so far no problems with them.

If you were lead fouling bottom plugs (Lycomings) you could install RH(B) plug these you can get in the same heat rating. we use these plugs here cause all we can get here for fuel is 100LL. If you are fouling do to oil this is another problem that needs addressed. and spark plugs are not going to fix.

If you have an IA that recommenda putting long reach spark plugs in short reach cylinders, your first action should be to find a new IA.

When you try to remove those plugs after a hundred hours or so, you would probably remove the helicoils or inserts with them.

This isn't the first "good" A&I that I've received bad advice from. I'm not going to name names, but, this mechanic really does have an excellent reputation here in Alabama, his shop is always full, he's apprenticed several other good mechanics in the area and is known by other members of the Cessna 170 association who share the opinion that he is a very good A&I.

That said, I always get second and third opinions, because I've found that all mechanics make errors. I found over the years that mechanics seem to have strong areas and not so strong areas and I've even switched mechanics for annuals for no other reason that I wanted another person with a different approach to take a fresh look at my plane.

The combined knowledge of the members of this association is outstanding and I truly value the advice.

Thanks,

Curt - N4288V

CBogle wrote:This isn't the first "good" A&I that I've received bad advice from. ....

Curt

Often I've found a person will make general recommendations as to how or what to do about something not really intending for their simple advice to be taken literally. An assumption is made by individuals that the first thing would be to insure that any any work or modifications to a specific piece of equipment followed the manufacturers guide lines and in the case of aircraft has an approval bases as well

You may not have received bad advice for your car or lawn mower it just wasn't good advice specifically for a C-145/0-300.

Some of my most expensive lessons in life have been as a result of acting on the advice of a single source. As you have noted I seldom act on the advice of a single individual whether that be for my airplane, car or lawn mower.

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BTW there are a few things I do which I won't mention that does not follow the conventional wisdom of this forum .

Bruce:

Couldn't agree more...thanks. Curt