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Different O-360 spark plugs

ccsmith51

ccsmith51

Well Known Member
I have an O-360-A1F6D in my RV-4. It uses RHM40E spark plugs. Some of my RV O-360 friends have REM40E plugs. I see that the RHM is a 3/4-20 thread, and the REM is a 5/8-24 thread. Out of curiosity, why would there be two different thread sizes for an O-360?

Thanks,
 
ccsmith51 said:
I have an O-360-A1F6D in my RV-4. It uses RHM40E spark plugs. Some of my RV O-360 friends have REM40E plugs. I see that the RHM is a 3/4-20 thread, and the REM is a 5/8-24 thread. Out of curiosity, why would there be two different thread sizes for an O-360?

Thanks,
Click to expand...

They both thread into the same 18mm thread in the cylinder.
The thread sizes you mention are to the shielding on the plug harness.
 
Thanks, Gil...

Another friend pointed that out to me. I'm still wondering why there would be different shielding sizes...
 
ccsmith51 said:
Another friend pointed that out to me. I'm still wondering why there would be different shielding sizes...
Click to expand...

The difference is in the sealing boot that goes into the spark plug cavity. The RHM is an all weather (completely sealed) which has larger sealing lip area at the barrel plug, and that?s why it has a ?? thread.
 
I'm curious if anyone has ever noticed a difference in performance? I've never been able to see any distinct difference between the two. Makes me really wonder. Maybe on the bigger older engines that were more exposed to weather more often?
 
The RHM series or 3/4-20 "All Weather" design offers improved resistance to water (heavy rain) and improved flash over protection at high altitude - especially with the Slick Harness where the M-2928 insulating sleeve compresses the air as it is inserted into the spark plug barrel.

Typically, lower powered engines use a 5/8-24 threaded nut and larger, turbocharged engines, use 3/4-20 threaded nut. However, either one can be used. When replacing both the spark plugs and harness, consider upgrading to the RHM 3/4-20 "All Weather" as the price is the about the same. If you have an extremely tight clearance problem then stick with the REM 5/8 size.
 
Autolite 2223

While we are on the topic of shielded plugs, I noticed that Autolite has a 5/8-24 shielded plug for air cooled military engines. It has 18mm dia, and 12.5mm depth, so it will fit. It has a cold Autolite heat range of '3'.
They cost $5.27 @ Amazon :rolleyes: aerospace . com
Has anyone tried them?
 
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