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Author Topic: Glow plugs in general  (Read 3973 times)

Online ray copeland

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Glow plugs in general
« on: October 28, 2007, 02:31:45 AM »
Looking for information about glow plugs, mainly what is the difference between short and long and R/C.  Also , could a long plug possibly hit the piston and if it could, maybe not when turning by hand but hit when turning high rpm's? I've been told that r/c plugs are made for idling and burn hotter, could that cause lower peak rpm's? I have some old fox engines, .35 ,  .19, and .15 , also norvel .15 and .074. .  Some of the literature i have says to use a standard plug???  Thanks,  Ray
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Offline Dennis Vander Kuur

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Re: Glow plugs in general
« Reply #1 on: October 29, 2007, 12:58:28 PM »
Ray,

I'll try to answer all of your questions.

First, the difference between "short" and "long plugs" is the length of the threaded part of the plug. Typically engines .19 or larger use long reach plugs while engines of .15 size or smaller use short reach plugs. There have been a few exceptions. When R/C first started most R/C plugs used to have an "idle bar" or small bar across the combustion end of the plug to provide better idle operation. Current R/C plugs may or may not have this bar.

Second, a long reach plug installed in an engine designed for a short reach plug (i.e. .15 size or smaller) might possibly hit the piston at top-dead-center or even prevent the engine from being turned over. If it doesn't hit when flipping the engine over by hand then it probably won't hit when running unless there is a lot of slop in the engine's internal parts (i.e. connecting rod ends, piston pin holes, etc.).

Third, R/C plugs typically are in the hotter range of plugs to, as you've been told, assist in keeping the fire lit when the engine is idled down. However, except for speed or racing applications, using a hot range plug doesn't cause lower peak rpm or result in any meaningful loss in top end power for control line sport or stunt flying. Actually when running your FOX engines it is very beneficial to use a hot plug, even though they may not be R/C engines. Most old FOX engines use an iron/steel piston/liner and, for maximum longevity, need fuel with a larger percentage of oil than required by the more modern design ABC (aluminum/brass/chrome) engines. Compared to a cold plug, the hot plug is more tolerant when running with a high oil content fuel and prevents the engine from quitting when running with a richer needle setting.

Fourth, your FOX .35 and .19 engines use long reach plugs. The FOX .15 uses a short reach plug. I have no experience with these specific engines but I ASSUME the Norvel .15 and .074 would use a short reach plug.

Fifth, I would assume that your literature that specifies a "standard" plug is referring to a plug without an idle bar .

Hope this helps.
DennisV
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Online ray copeland

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Re: Glow plugs in general
« Reply #2 on: October 29, 2007, 03:29:22 PM »
Dennis, thanks alot for the information, clears up a lot of things that i could only guess about.   Ray   H^^
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Offline Geoff Goodworth

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Re: Glow plugs in general
« Reply #3 on: October 30, 2007, 04:49:09 AM »
Dennis et al

I only have one small engine these days, an FP10 and everything that I have uses long reach plugs.

I query your use of size as the arbiter.

45 years or so ago, glo-plugs were very expensive in OZ, especially for someone in junior high.  My dad was in the auto parts business and got me some plugs from Champion, I think.  One was short reach and it was too short for an OS Pet.  I tried to be delicate when tightening but eventually, I needed a new head.

Best advice is measure the depth of the head. It's not hard and there are several threads here or elswhere to tell you how to do it accurately.  Assume nothing.

Cheers, Geoff
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Offline Dennis Vander Kuur

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Re: Glow plugs in general
« Reply #4 on: November 06, 2007, 09:33:54 AM »
"I only have one small engine these days, an FP10 and everything that I have uses long reach plugs.
I query your use of size as the arbiter.
45 years or so ago, glo-plugs were very expensive in OZ, especially for someone in junior high.  My dad was in the auto parts business and got me some plugs from Champion, I think.  One was short reach and it was too short for an OS Pet.  I tried to be delicate when tightening but eventually, I needed a new head. Best advice is measure the depth of the head. It's not hard and there are several threads here or elswhere to tell you how to do it accurately.  Assume nothing.
Cheers, Geoff"

Geoff,
I don't claim to be an engine guru or expert but don't mind sharing my limited experience with others when they ask. I've accumulated almost 200 engines during my 45 years of flying model airplanes. They range in size from a COX .010 up to to a piped Randy Smith PA.65. For all of these engines I've found the statement I'd made previously to be true (i.e. Typically engines .19 or larger use long reach plugs while engines of .15 size or smaller use short reach plugs. There have been a few exceptions.").

Granted my sampling is limited in size, but I found these engines of mine use the following plugs:
SHORT PLUGS
O.K. Cub .049
Royal Spitfire .049
WenMac .049
O.S. Max .06
O.K. Cub .074
Enya .09
O.S. PET .099
O.S. Max .10
Fox .15
K&B .15 Greenhead
O.S. Max .15 III

LONG PLUGS
Enya .19 4004
Fox .19
O.S. Max .19
HB .20
K&B .20 Sportster
O.S. .20FP
Magnum .25
O.S. 25FP
Thunder Tiger .25GP
K&B .29 Greenhead

I own a O.S. PET .099 (picture attached). After seeing your post I pulled it out to check the plug. It has an O.K. brand short reach plug in it. Out of curriousity I pulled out a bunch of plugs and used a dial caliper to measure the distance between their seating shoulder and the end of the threaded part of the plug. I also measured the thickness of several plug washers (as this also affects the distance the plug protrudes into the engine's head). I have attached a picture that shows the plugs placed on a chart with their dimensions. It seems obvious from this data that there is no "standard" for a short or long plug thread depth dimension between plug manufacturers.
 
So for someone trying to decide which glow plug to use my personal opinion is:
Follow the manufacturers recommendation for what glow plug to use. This is the safest bet. Lacking a manufacturer's manual, I would start with the guideline of "Typically engines .19 or larger use long reach plugs while engines of .15 size or smaller use short reach plugs. There have been a few exceptions."
If you have the proper measuring tools then definitely follow Geoff's excellent recommendation and measure both the depth of the head and the length of the plug. If you don't have these tools then install the plug, turn the engine over gently to make sure the top of the piston does not hit the bottom of the glow plug. If the piston touches the protruding plug then you need to try a plug with a shorter reach ( adding an additional glow plug washer may also work).
DennisV
« Last Edit: November 08, 2007, 02:11:25 PM by Dennis Vander Kuur »
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Offline Bill Little

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Re: Glow plugs in general
« Reply #5 on: November 10, 2007, 04:03:59 PM »
The only "physical problem" I have ever heard of is using a plug that is too "long" and the piston hits the plug.   I seem to remember my buddy using a gold "McCoy" short plug in his old K&B GH 35 w/Missle Mist and running it "wide open". :D

(granted, I am not talking "run issues" here)
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Offline Brian Hampton

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Re: Glow plugs in general
« Reply #6 on: November 10, 2007, 08:09:09 PM »
The only type of engine to get really concerned with mechanical interference is with something like the old Max-III's where the plug is offset into the piston baffle slot. With Schneurle type engines there's almost no problem. I just tried a long reach idle bar plug with no washer in an OS Max 10 and the piston hit but a normal long reach also with no washer cleared. With a washer it would certainly be quite ok but probably far from ideal.

The ideal length is when the end of the plug is level with the combustion chamber surface but a bit of variation doesn't matter. Strangely, the OS plugs are really a medium length (so are the Enya plugs) yet they never seem to fit properly in an OS head, they're too short and are at least one thread below the surface. A normal long reach comes about half a thread above which I feel is better suited.

Cheers
Brian
Adelaide, South Oz
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Offline phil c

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Re: Glow plugs in general
« Reply #7 on: November 16, 2007, 03:28:05 PM »
a long reach plug in an engine with that needs a short reach plug kills a lot of power.  The smaller engines need the smaller glow plug cavity, and any exposed threads in the head do terrible things to combustion.  The difference can easily be 1000 rpm or more.

You can shim the long reach plug with an extra washer or two and they will work pretty well.  Take the head off and see exactly where the end of the plug comes in the head.  Long reach used to be 7/32 in.(7 threads) and short reach 5/32 in.(5 threads).  Some of the OS plugs are half way in between so you really have to look inside the head and see where things end up.
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Offline mdamico

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Re: Glow plugs in general
« Reply #8 on: July 17, 2022, 06:52:41 PM »
I bought my OS Max H60F Engine new back in 1973 and the instructions that come with it say to use an OS No. 7 Glow Plug, which in those days came with an Idle Bar. My question is did the OS No. 7 Glow Plug come in both long and short lengths, or was it always short? I say this because if you look on eBay you can see that the No. 7s they're selling on there now are short plugs. I "think" my engine is supposed to take a short plug. If anyone knows for sure please let me know because I need to replace the plug in my engine. It has a short plug in it now with an Idle Bar and I "think" it's an OS No. 7, but not 100 percent sure.
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Offline Motorman

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Re: Glow plugs in general
« Reply #9 on: July 17, 2022, 08:43:11 PM »
Don't know about '73 but todays OS plugs have the number stamped on the side. Maybe you're old one does too? That's a big engine for a short plug. Look in the plug hole and see if any of the lower threads are brown/black, this would mean you could use a longer plug.   


Motorman 8)
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Online Dan McEntee

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Re: Glow plugs in general
« Reply #10 on: July 17, 2022, 10:59:08 PM »
   The OS numbering system has changed a couple of times since 1973. You would need to research what heat range it was back then and then see how that equates with the system that they use now and choose accordingly. Is it considered a hot, medium, or cold plug, that is your first concern. Most OS plugs these days are sort of a "midreach" length and are about 1 pitch longer than a short reach, and about 1 thread pitch shorter than a long reach. Why that is, I don't know. The idle bar is mainly to keep the onrush of cold fuel from overwhelming the element when the throttle is reduced to idle. I use them in stunt engines because I think it helps protect the element from junk and give carbon a place to collect instead of on the element.
  Type at you later,
   Dan McEntee
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Offline BillP

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Re: Glow plugs in general
« Reply #11 on: July 20, 2022, 12:56:40 PM »
Your Fox 15 CL uses a short plug and all Fox larger displacements use long. You can run an idle bar on the CL engines but I ran both ways and saw zero running difference. I also cut the idle bar off and that didn't make a difference either. If memory is right Fox advertised both rc and standard as medium heat and their gold plug as hot. Under a scope the standard longs had a larger diameter element wire and the rc idle bar version had thinner wire...the thinner wire glowed brighter than the standard.
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Offline Steve Helmick

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Re: Glow plugs in general
« Reply #12 on: July 22, 2022, 10:54:15 AM »
I would only note that OS glowplugs are now produced in a metric length, which is neither long or short, but more like a "Medium" length. IDK if all models of OS glowplugs are the same length now or if there are still long/short in some models or just what they're doing, because I don't typically use them.

So, I'd say if Geoff is running an OS glowplug in his OS .10FP, no problem. It probably would work fine with a long reach glowplug also, but having the nose of the glowplug extend into the combustion chamber will make the glowplug act hotter. That might be a good thing or a bad thing...most likely good. I have only used the A3 OS glowplugs that came with some sizes and/or vintages of the OS LA series engines. They work fine. Pretty expensive, though. I normally use Thunderbolt long idle bar glowplugs in the Magnum XL .25 and XLS .36, .46LA and a T-bolt 4-cycle plug in the OS .46VF, PA .51, ST G.51, etc.  y1 Steve   
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