The International Cessna 170 Association

I have always used either Champion REM40E or Unison (now Tempest) UREM40E plugs. I now have well over 500 hours on my current Unison plugs and they check out fine for both the electrode condition and the resistance test Tempest recommends, but I know I should plan to replace them sometime in the next year if I do a decent amount of flying. Reading Champion's Spark Plug Application Chart they show the Continental O-300 A, B, C, D, E & C-145 can use either the REM40E or REM38E plugs. Seems the difference is temperature? Would/could one use say 10 40E's & 2 38E's or would that possibly create a problem (someone "may" have 2 new 40's & 10 new 38's he will give me. Just wondering about advisability.

Frank-
I believe that is perfectly fine as they both are approved but I am not a rated A&P so that info may just be worth what it cost. However, an A&P installed different heat range plugs on my 195 which tends to foul plugs on the bottom cylinders so the 37Es in that case were installed on the bottom cylinders. I would not hesitate to install mixed plugs especially if you have some that foul worse than others. All things being equal, I would install the 38Es in the lower plugs. When I looked at 40Es vs 37Es, they look identical except the center electrode sticks out farther on the 37Es. If you are buying plugs, the newer style bonded-in resistors are supposed to be better than the old screwed-in resistors.

Just stick them in and fly Frank, you won't notice any difference. The higher the number , the hotter the plug and vice versa. The 38 is a slightly colder plug and may be more susceptible to lead fouling. You will know the next time you clean them.

170C wrote:I have always used either Champion REM40E or Unison (now Tempest) UREM40E plugs. I now have well over 500 hours on my current Unison plugs and they check out fine for both the electrode condition and the resistance test Tempest recommends, but I know I should plan to replace them sometime in the next year if I do a decent amount of flying. Reading Champion's Spark Plug Application Chart they show the Continental O-300 A, B, C, D, E & C-145 can use either the REM40E or REM38E plugs. Seems the difference is temperature? Would/could one use say 10 40E's & 2 38E's or would that possibly create a problem (someone "may" have 2 new 40's & 10 new 38's he will give me. Just wondering about advisability.

Ole Pokey, the only problem I see is when you rotate the plugs you will need to pay more attention to where the plugs go. you could put a colder running plug in a cylinder that needs a hotter running plug. It is ill advised to swap partial hotter plugs for that reason. Nothing says you cant do it, but it is essentially not a best practice to do what you are advocating. If there is an accident and the investigation yields that you did that, Insurance has greater ammunition to deny a claim. I advocate against the practice you are doing.
Since you have 2 cylinders that run colder, I would start to look for problems in those cylinders. Look at the leads to see if you are leaking power and also look for carbon trails in the magneto caps. I think there may be more to your story, but I would definitely check the leads.

Thanks, turned out that the offer of free plugs had two REM 40's and five of the 38's so I kept the 40's and declined the 38's.

You will never know the difference between the two heat ranges unless you're having plug fouling problems.