Van's Air Force
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Why don't we use 18mm spark plugs instead of fooling with spark plug adapters?
- Thread starter FORANE
- Start date
Vern
Well Known Member
I tried some and had poor performance. I actually had a precautionary landing to replace them on a trip because of gaps closing on several.
Toobuilder
Well Known Member
Unfortunately, there is not a wide selection of reaches, resistor and heat ranges. Some are happy with the Autolights, some not.
blaplante
Well Known Member
I assume you are asking about 18mm non-aviation plugs?
Well...
a) you aren't going to find them in the local auto parts store when you need one on some cross-country
b) I can't think of the last time I saw an 18mm automotive plug. 1936 International pickup truck, maybe?
Well...
a) you aren't going to find them in the local auto parts store when you need one on some cross-country
b) I can't think of the last time I saw an 18mm automotive plug. 1936 International pickup truck, maybe?
rmarshall234
Well Known Member
It’s a good question….I recently bought a set of AutoLite automotive plugs that were recommended here, for my NOS Electro Air electronic ignition system. They weren’t in stock at NAPA but arrived the next day and were cheap. 18mm. Lyc O-360 parallel valve. It was suggested (here) that I removed the cheesy automotive gasket and replace with an aviation grade M674 copper gasket which I’ll be doing and sounds like a good idea. I’ll be opening the gap some and am really happy I won’t be using an adapter to install them.
When Mattituck was building my engine, they asked if I wanted 18mm or 14mm spark plug holes. I specified 18mm even though I knew I would be using an ignition that supported auto plugs. This was a mistake, but I didn't know at the time, obviously.
As others have said, the selection of 18mm plugs is very limited.
As others have said, the selection of 18mm plugs is very limited.
Toobuilder
Well Known Member
I'm guessing these are the Autolite 386. These are a bit hotter than optimal for most, and "I think" the ONLY 18mm resistor plug available today.
FORANE
Well Known Member
Yes. I figured they aren't very common. I think I saw some tractor motors use 18mm.
FORANE
Well Known Member
My x-340 was built with 14mm plug holes. When I recently had to replace a couple cylinders, the new cylinders I installed were 18mm. So, I have 4 spark plug adapters currently.
I have been using the Autolite 386 plugs in two planes (RV4 and Glasair both with Lycoming 360s) for nearly two thousand hours. They have performed flawlessly. They are available from all four of the auto part store chains near me and all are under $3 each. The nice part of that is my "plug cleaner" doubles as my shop waste bin. To test longevity I ran one set until I got an intermittent miss which occurred at 317 hours. I change them at 100 hours since you can do all eight for less than $20. I have several friends running these plugs as well. I really like not having plug adapters. Just one testimonial but I am very pleased with these 18mm plugs.
rmarshall234
Well Known Member
They are the 386s. There was a second plug that was suggested as well but I never checked availability because I wanted a hotter plug anyway.
Toobuilder
Well Known Member
I’m considering running a test set in one side to see how they hold up in my application. Curious what configuration or use case you have that makes a hotter plug more desirable?
PilotjohnS
Well Known Member
do you regap the autolite plugs? and to what value?
rmarshall234
Well Known Member
I have newly OH’d chrome cylinders and am still pushing oil past the rings and there is a slight tendency to foul the lower plugs. This with REM40Es gapped at .016. When I open the gap on those plugs to .019 the situation is much improved but I don’t want to make my mags work that hard.
I have a NOS Ted Rose Electro Air EI system that I’ll be installing with the AutoLite plugs. The manual says I can open the gap to .035 but I’m going with .032 initially. There is some experimentation going on here, that’s for sure.
Toobuilder
Well Known Member
That is probably the only benefit to hot plugs. All other use cases (sustained high power, Lean, ignition advance, leaded fuel, and a host of others) are made worse with hot plugs. Once your oiling clears up, keep a close eye on things!
RV6_OKLA
Well Known Member
An FYI from E-MAG website.....
18MM Thread Auto Plugs: NOT RECOMMENDED. We hear of customers occasionally using auto plugs that are manufactured with 18mm thread (avoiding the need for spark plug adapters), we have at least two reports from customers who tried 18mm thread plugs – with very poor (near disastrous) results. We recognize there may be a great many plugs that will perform perfectly well in this environment, but we also recognize that some do not. We recommend that most customers stay with plugs that have a known service record. IF you choose to experiment with other plugs (not recommended), do so with caution.
18MM Thread Auto Plugs: NOT RECOMMENDED. We hear of customers occasionally using auto plugs that are manufactured with 18mm thread (avoiding the need for spark plug adapters), we have at least two reports from customers who tried 18mm thread plugs – with very poor (near disastrous) results. We recognize there may be a great many plugs that will perform perfectly well in this environment, but we also recognize that some do not. We recommend that most customers stay with plugs that have a known service record. IF you choose to experiment with other plugs (not recommended), do so with caution.
Toobuilder
Well Known Member
While the instructions from the MFR should be given due consideration, there is a lot of things to question in this statement. For example, does the physical size of the hex wrenching surface define the operating characteristics of the plug? Probably not. If I had to guess, the fact that there is almost no selection of RESISTOR 18mm plugs drove someone to a NON resistor unit and that scrambled the brains of the Pmag (I’ve been there, done that). That OR a user used a Autolte 386, and had jumpers out in an edge case engine, hot day, and suffered pre ignition due to the hotter than optimal heat range of that particular plug.
All informed speculation on my part, but a few words from the MFG would go a long way to clear that up. Unfortunately, some of us are like children and don’t like to be told “no” without a good reason - often at our own peril (guilty as charged).
