Good Spark Plug Maintenance = Better Engine Performance

by Visitor gashaka on ‎12-13-2010 02:20 PM


Sometimes the smallest things get overlooked regarding gas engines. One thing that seems like a small detail but has a major effect on gas engine operation is the spark plug. Spark plugs operate in severe conditions and require regular maintenance to provide desired engine performance.

The voltage required to produce a spark in the electrode gap has a direct effect on the life of a spark plug. Some things that are a main factor that affect the voltage required are the:

  • air-fuel ratio
  • ignition timing
  • electrode wear
  • fuel BTU
  • electrode gap

The service life of a spark plug varies for different applications therefore proper maintenance will help extend plug life and reduce down time. As the gap increases on a plug the voltage required for spark increases dramatically. A worn plug can require more voltage then the ignition system can supply. A cleaned and re-gapped plug requires more voltage then a new plug.

Some other things to remember when performing spark plug maintenance are:

  1. Inspect the plug for deposits. A light brown or a beige deposit indicates normal operation. Deposits that are gray or black may be caused by excessive oil, or a substance that is introduced through the fuel system, or incorrect air-fuel ratio.
  2. Look for eroded electrodes. An eroded plug is a worn out plug.
  3. Always install a new spark plug gasket when installing or reinstalling a spark plug.
  4. Install the spark plug into the seal by hand. If the spark plug cannot be seated by hand, there is a problem with the threads.
  5. Do not use a thread tap to clean spark plug threads in the head. A tap is designed to remove metal and eventually could result in thread failure.
  6. Do not mix types of spark plugs in an engine.
  7. Do not use anti-seize compound on spark plugs, most of the heat is transferred through the threads and seat area.
  8. Do not over-tighten a spark plug. Over-tightening the spark plug can cause cracks or can yield the metal which loosens the shell.
  9. Never reuse a spark plug that is cracked.

Good spark plug maintenance will result in a better running engine and help reduce down time.

  • Do you have any additional tips you've used successfully?
  • Do you have experience with unique applications that require different types of maintenance (not listed above) or more frequent maintenance as a result of the environment?

Please post below and share your thoughts.

by New member arun801980
on ‎12-14-2010 02:37 PM



Can any one let me know, how much should be the minimum Gap to be maintained in spark plug for good working condition in case of GAS engines?



by Visitor gashaka
on ‎12-14-2010 03:13 PM

arun801980, There is no 1 specification for electrode gap, proper gap is determined by plug type, compression ratio etc. Without knowing engine model, mfg. and plug there is no single answer.

by Visitor GARYM1955
on ‎12-15-2010 06:16 AM
Good Morning Kent, Can you call me about this Spark Plug article? Gary Minker 561-969-9245 Thank you, Gary
by Visitor johniejay2004
on ‎01-07-2011 03:35 PM

it depends on a number of factors

by Super Contributor
on ‎02-02-2011 06:26 PM

A few additional comments,


When inspecting plugs, some additional things to look for,


Alignment of electrodes, when regapping make sure electrode surfaces properly line up, just bending the ground strap down to close the gap can result in misalignment and accelerated electrode wear.

Discoloration of shell above the threads, look for darkening or bluing, indicates combustion gas leakage and on resistor type plugs can cause internal resistor failures or shortened life.

Condition of porcelain, staining or deposits can indicate oil leakage, especially in engines with enclosed plug bores.  Make sure porcelain is clean and dry and connection is clean with no corrosion.

Thread condition, if threads are rolled, damaged or plug doesn't thread in by hand until gasket seats, use a seat and thread cleaner tool, not a standard tap as pointed out above.

Resistor type plugs need to be properly checked for correct resistance during removal and inspection.  See appropriate service manual.


Plugs that fail early may be caused by faulty ignition primary or secondary components. 

Spark plug extenders need to be clean, make good electrical contact at both ends, and fitted with proper rings and spacers if originally installed.

Ignition coils have a finite life, they degrade in time and as they degrade can cause shortend plug life.  Coils that are excessively hot, have burn marks or discoloration, are cracked or with damaged connections need to be carefully inspected and replaced as needed.  This is especially true of engines operating at lean fuel mixtures for low emissions.

Magnetos and electronic ignition modules can have outputs with low energy levels.  Repetative plug failures require further investigation.

Engine operating conditions dictate changes in recommended maint intervals, running lightly loaded or very lean will reduce service life and maint intervals need to be adjusted accordingly.

Use a torque wrench, always, and correct socket.  Dropping a plug in the bore almost always results in the gap being closed.

Plugs are fragile, handle accordingly.  Know which cylinder they came out of so when inspecting if an abnormal condition is noted you know which hole to go look in.


Mike L

‎03-23-2011 04:34 PM - edited ‎03-23-2011 04:36 PM

I always find it good maintenance practice to change distributor and plug wires when the spark plugs are changed.

Something I learned from my 9th Grade shop teacher, Mr. O! 

Jim G.

by Visitor MJGLX
on ‎06-23-2011 03:42 PM

Hey Everyone,

Great article! Our company runs a couple of G3520Cs on 50% CH4 LFG and I had some questions about plugs and options. Right now we are using the Denso 243-4291, and are going through them quickly. Does anyone have experience with the BOSCH p/n 7306 extreme duty plug? Would they stand up to LFG, or are they designed mainly for pipeline quality natural gas? Also, while reading the above comments, I didn't see any mention of applying di-electric grease on the terminal to coil boot. Is that a no-no because the plug is sealed inside the valve cover with no real chance of water, or would the grease break down because of heat??

Thank you for your time,


by Super Contributor
on ‎06-23-2011 06:20 PM

I have some customers who have had positive experience with the Bosch plugs and others who didn't notice any difference.  The two major impacts on spark plug life are gap erosion and heat.  For the gap, start with the smallest recommended gap, a larger gap requires more energy and results in a higher rate of erosion.  Low energy fuels tend to require higher energy ascross the plug gap due to the lean mixtures and poor combustibility of the fuel.  Low emissions settings increase the demand for plug energy.  Heat is usually an issue if proper care isn't taken for the plug threads, seat and gasket.  In general, use a new gasket every time, the plug should thread in easily by hand until the gasket seats, and the seat area of the head shold be clean.  If not, use a proper thread and seat cleaning tool.


On using grease, we used to on older systems with external coils, plug leads and boots.  In uder the cover systems grease usually ends up melting and running down the insulator, in some cases contributing to flashover.  A very light film of grease can be applied to the o-ring, as long as it'c compatible with the o-ring material.


Mike L.

by New member dgosselin
on ‎06-24-2011 08:29 AM

This may be old news for many, but great new news for others.  Many power plant owners may have a combination of G3516A/B's in their fleet, and some G3520C's.  You can get a great deal more out of the life of any Cat spark plug by taking the "reached condeming limits" for the 20c, but can easily live in the lower BMEP engines of the A/B's, sometimes from 2000-4000 more hours.  It's just another great way to keep this part be a gift that keeps on giving !

by Super Contributor
on ‎06-24-2011 01:05 PM

From a man who truly knows how to manage costs on CAT gas engines!  Great point Denis.

by Visitor MJGLX
on ‎06-27-2011 09:03 AM

Thank you for your help, it's greatly appreciated. Is there any way you could explain the "reached condemning limits" for the 20C? Is that a plug pull and gap verification kind of thing? What's the recommended gap typically on LFG applications for the 20C? We are in the process of requesting all of the correct manuals, so I'm short on specs.

Thanks again,


by Super Contributor
on ‎06-27-2011 10:32 AM

By "condeming limits" we generally are speaking of the plug energy levels as displayed in ET.  Higher BMEP engines have a higher energy demand from a spark plug and as the gap erodes may reach the maximum

allowed energy levels before misfire occurs realitively short in their life.  As explained above many factors affect a spark plugs service life.  As Denis pointed out, if you have a mixed fleet, some newer style higher BMEP engines and some older style engines with lower maximum cylinder pressures then while continuing to use the older plugs in the higher BMEP engines could lead to misfire, moving them to the older engines will let you get some of their available service life out of them.

by New member elizawinters
on ‎01-18-2012 03:54 PM

Thanks for the great information on how to maintain caterpillar gas engines. My company has invested a lot of money into them and I would love for them to last and maintain their power. Thanks for all of the great tips. I really hope that they work!

by Contributor MKS
on ‎02-17-2012 12:54 AM

it should be also a good addition in spark plug topic.

all plug should be checked for accurate input voltage as describe in manual.

inappropriate input voltage can shorten plug life.

by Scott1234
on ‎09-11-2015 01:23 PM

Does anybody have experience with pre-chambered plugs?


We have a 3520C running on well natural gas at a 2MW generation facility. We are having issues with cold starting the engine (we are in Canada) with pre-chambered plugs. We are finding we have to put in regular plugs, get the engine to temp, then change them out to pre-chambered. 


It is my understanding that we need the prechambered plugs to get the HP output we require (100% output), however we would like the generator to run in a 'peaker' method, thus frequent shutdown/startups. Although we are told we need to run pre-chambered plugs, is there any issue with running the regular plugs?


any help would be great. 


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