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Thread: Honda GX31


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    Honda GX31

    I'm considering building a GX31,,I've got one question before I pull the trigger.. I know the engine will run in any attitude, but can it be mounted inverted? With the sump arangment they use it would seem possible, but I am worried about oil colleting in the head while the engine is not running. And since the number of scale aircraft with a cowl that would hide this engine when it's mounted upright is very limited. Anyway If I can't hide it by going inverted I just might pass,,,,but damm they sound good!
    He who dies with the most toys,,,WINS!

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    RE: Honda GX31

    I too would be interested in any input on this subject. I have one I want to put on my new 100" Eidecker by SR Batteries. I think this engine with a straight pipe on this airplane is going to be awesome. I'll mount it upright if I have to, but it would be great to mount it inverted if it's possible.

    Thanks,
    AV8TOR
    If it is not SCARY, it is NO WHERE NEAR powerful enough!!
    All R/C planes have expiration dates---> It's just not printed on them anywhere!

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    av8tor1977's Avatar
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    RE: Honda GX31

    I'm hoping this animated picture works here. Only way to find out is to try to post it and see.

    Anyway, from studying it, it appears that the engine would work for a short time inverted, but in the long run all the oil would run out of the sump into the valve cover area, and the pump wouldn't have access to it...

    Oh well...
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    If it is not SCARY, it is NO WHERE NEAR powerful enough!!
    All R/C planes have expiration dates---> It's just not printed on them anywhere!

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    RE: Honda GX31

    Ok, the animation didn't work here. The two "L" shaped things in the oil sump move up and down and inject an air/oil mist into the passageway in the crankshaft. This feeds the bearings. The oil is then ejected from the bearings, and when the piston comes down the pressure forces the oil through the trap door below the crankshaft, up and around to lube the valve train area and cam, where it then drains back into the sump.

    AV8TOR
    If it is not SCARY, it is NO WHERE NEAR powerful enough!!
    All R/C planes have expiration dates---> It's just not printed on them anywhere!

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    RE: Honda GX31

    I have a Dynaflite Decathlon that I bought for this engine. By the numbers it looks perfect for that engine. The right weight, wingspan and wingloading. However looking at the firewall and holding that big 4 stroke in my hand I have reservations. Once I get it built and size it up a bit better I will have a better sense of it's suitability. If it looks shakey, I will come up with another airframe for this engine because I really like the idea of flying this power plant.
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    RE: Honda GX31

    Thinking of converting one also.

    Honda GX31 31cc Mini 4-stroke Engine REGULAR PRICE USD$175.95
    SALE PRICE $99.95
    http://www.thelawnmowershop.com/
    (under Brushcutters & Engines)

    This web page below should help answer most of your questions.
    http://www.carrprecision.com/Pages/prod02.htm

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    RE: Honda GX31

    In a 4 stroke engine the crankcase is filled with oil that is circulated by an oil pump... The oil pump is located at the bottom of the crankcase when the cylinder is pointing upright (on this particular model)... Thus if you mount this engine inverted you will starve the oil pump and your engine WILL fail...

    On the other hand a 2 stroke does not have oil in the crankcase... the crankcase acts as part of the intake system, which is why you have an oil/fuel mixture so that the moving parts inside the crankcase recieve the lubrication they need....

    The other thing is that yes, the oil in the crankcase will seep past the rings and fill your head full of oil if left sitting for even just an hour or two...

    Unfortunately 4 strokes all have this limitation... So I personaly would not attempt to mount this motor inverted...

    Now... if you were to find an engine that could be mounted on its side.. and that was a suitable set up for you, then that would be the way to go...

    Otherwise I would find myself a good 2 stroke as they don't really care which way you mount them....
    F6F HellCat - Proof that if you have a big enough engine, even a brick can fly!

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    RE: Honda GX31

    Gotta be careful making a blanket statement like that Neither do at least 2 of the four strokes that have NO oil in the crankcase, Shindaiwa 25, 34, and 80cc, and Stihl 64.9cc.. I converted these...[8D]
    Stihl also has a few smaller engines, I have not converted those....
    So far they all run about like a G23 except the 64.9 Stihl, it is close to a G62 in performance...And weight..
    RC Ignition

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    RE: Honda GX31

    In this case the engine is somewhat of a dry sump design, so the oil wouldn't seep past the rings. It would however, drain out of the oil tank into the valve train area and therefore leave the tank dry. Same result; no oil being pumped.

    Anyway, I'm going to use the engine on my Eindecker and mount it upright. I just won't fly it inverted! Normal positive "G" maneuvers and aileron rolls won't present a problem. Even full size aerobatic airplanes without inverted systems can be flown negative momentarily....

    AV8TOR
    If it is not SCARY, it is NO WHERE NEAR powerful enough!!
    All R/C planes have expiration dates---> It's just not printed on them anywhere!

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    RE: Honda GX31


    ORIGINAL: RCIGN1

    Gotta be careful making a blanket statement like that Neither do at least 2 of the four strokes that have NO oil in the crankcase, Shindaiwa 25, 34, and 80cc, and Stihl 64.9cc.. I converted these...[8D]
    Stihl also has a few smaller engines, I have not converted those....
    So far they all run about like a G23 except the 64.9 Stihl, it is close to a G62 in performance...And weight..
    I was not aware that there were such 4 strokes without oil in the crankcase... How are these engines set up to provied their lubrication?
    F6F HellCat - Proof that if you have a big enough engine, even a brick can fly!

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    RE: Honda GX31

    Honda specs the GX31 and GX25 at unlimited inverted opperation.
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    RE: Honda GX31

    Interesting... Mind if I ask where you got that info?

    Thanks,
    AV8TOR
    If it is not SCARY, it is NO WHERE NEAR powerful enough!!
    All R/C planes have expiration dates---> It's just not printed on them anywhere!

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    RE: Honda GX31


    I was not aware that there were such 4 strokes without oil in the crankcase... How are these engines set up to provied their lubrication?
    The Stihl 4-mix engine uses 50:1 two stroke fuel, but has valves like a four stroke engine. The crankcase vent is connected to the intake manifold and sucks fuel/oil charge into the crankcase for lubrication.. as I understand it, it then expels it back into the intake to be burned? I have no idea. somewhat similar to a four stroke glow I guess.. excelt it doesnt rely on blowby.

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    RE: Honda GX31

    Several people on RCU have reported success in mounting and running the engine inverted. Carr Precision, the fellow who makes the conversion kit also stated it is no problem to run inverted. And out of the Honda manual "A 31cc mini 4-stroke engine capable of 360° inclinable operation."
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    RE: Honda GX31

    The explanation is kinda long, you can find it on the Stihl website....It works really well, I took the rocker arm cover off to check..There's oil all over the rockers and valve stems...
    RC Ignition

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    RE: Honda GX31

    First of all the Honda GX31 is designed to run in a 360 operation.

    It will run inverted.

    If you are not familar with the operation of the Honda GX31 , which has been in production for 8 years or so , I strongly suggest you review the basic operating principals of it before making statements of any kind.

    I have no idea. somewhat similar to a four stroke glow I guess.. excelt it doesnt rely on blowby.
    Yes it doesn't rely on blow by , a gasoline engine running on the blow by method utilized by some glow nitro four strokes with a gasoline and oil mixture will result in coke and combustion by products being blown into the lubrication system rendering the engine unusable in a short period of time.
    The higher combustion chamber temp in a gasoline fueled engine will burn oil present and turn it into nasty goo which is then forced into the crankcase and subsiquently the lubrication system.
    The alchohol based nitro engines get away with the blow by approach because the lower combustion temp due to the fuel.

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    RE: Honda GX31

    Hey to all,
    RCIGN1 how do you like the 4stroke stihl? What prop? Ken Lambert is doing one for me and is almost ready! I was thinking maybe 26x8 or 26x10 on a 28% Wildhare Extra.
    As for the Honda, I think someone elso did one and posted pics not too long ago. Do a search and I bet you'll find it. The Honda conversion looked very clean but I dont trust the Honda with crankcase oil in an inverted position. I've heard that when the engine is monted on a weed trimmer that the trimmer head or shaft rotates (to edge with) so the engine doesn't have to.

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    RE: Honda GX31

    The notion that a 4 stroke has more torque than a 2 stroke doesn't seem to apply with these...24-8 Menz prop, 6000 rpm....22-10 about 6400...
    You have to get creative with the mount, there's no flat place anywhere on either side...The first one was done with my ignition and used the tapered end of the shaft to mount the hub..It runs clockwise that way..The next two used the stock flywheel mag and mounted from the other side, so the prop will turn CCW.....The shaft on that side is larger diameter but no taper...I threaded an aluminum piece on it and used Loctite and a lock nut to keep it from moving...The stock carb is environmentally friendly and at about half throttle it wants to quit..I put a Walbro SDC 80 on it with good results...
    The carb is mounted on a rubber sleeve just like a Poulan 46, so I used some 3/8 G10 routed out to fit the oval, held on with 10-32 screws 180 degrees apart through the sides and into the casting...
    The sensor is mounted in a 1 1/2 inch aluminum ring under the hub, screwed to the crankcase with 2-56 button head screws...
    No pictures from those, maybe if I do another.....
    How's this for "information close to the vest" ?
    The BR550 blower cost $399.99 here in Flagstaff...
    RC Ignition

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    RE: Honda GX31

    Thanks RCIGN1, very helpful, I found it interesting that the BR550 is 399, and now the new BR600 is 479.00 Here is the funny part... same engine, different impellar

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    RE: Honda GX31

    My dealer didn't have a price on the 600 last week, and didn't have a 500...
    Check out this one, a 34cc Shindaiwa...3 lbs 3 oz as it sits..Not run yet...OHV, NO oil in the crankcase, can be mounted and run inverted...Don't know about the carb either...It's a WYL with no adjustments, thanks to the tree huggers of the world...
    Wait till Tower sees this one
    RC Ignition

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    RE: Honda GX31

    Where did you find that baby at? And where did you get the conversion parts at?
    Like classic video games and pinball, check out www.retrogamingroundup.com

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    RE: Honda GX31

    The Honda conversion looked very clean but I dont trust the Honda with crankcase oil in an inverted position. I've heard that when the engine is monted on a weed trimmer that the trimmer head or shaft rotates (to edge with) so the engine doesn't have to.
    You can trust it........ it doesn't have oil in the crank case , just in the oil tank which is seperated from the crankcase , the dippers sling the oil into a mist and there is a hole in the crankshaft that oil gets sucked into and into crankcase on the piston upstroke stroke , on the down stroke the mist is pushed past a reed valve in the bottom of the case and it works its way up to the rocker box and then a passage connects it back to the oil tank.
    All that is inside the crankcase is an oil film , not a puddle.

    We ran one inverted in the test stand for 30 min. without problems although AFTER you shut it off the oil film that is in engine will collect in the rocker box and some may drain out of breather tube. It is wise to store the engine upright so that the film drains to the bottom of the engine.

    I have never seen a rotating head on any of the trimmers.

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    RE: Honda GX31

    The Shindaiwa was sent to me by a customer in Hilo, Hawaii...He says there's an 80cc in the works and will send me one of those when available...The conversion parts are mine, I convert the G26 engines for BH Hanson and have been converting engines for 18 years..I'm trying to do only Zenoahs, but once in a while a new and different engine comes along..The Stihl 64.9 and this one were a new challenge..
    RC Ignition

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    RE: Honda GX31

    Haven't been on the forum much lately, but couldn't resist a quick reply on this one.

    CAPTAIN RAMIUS,
    No one but you and probably Ralph, seems to have much idea of how the unique Honda lubrication system works.
    You are perfectly correct in your description of it's operation, - it doesn't have oil in a crankcase sump, it has a seperate cylindrical oil tank behind the crankcase, where the oil is whipped into a mist before it ever gets to the engine.
    At rest, the oil is isolated from the engine, because as you can see, the oil mist only enters the engine through a rotary valve in the centre of the tank.

    AVIATOR, those "L" shaped things are rotating vanes which whip up the oil into a mist.

    ACEMAKER,
    The Honda does not have an oil pump and sump, it makes use of the main piston to circulate the oil mist from the tank instead.

  25. #25
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    RE: Honda GX31

    In the diagram that I posted, it doesn't show a one way trap valve in the drain back from the valve train area to the sump tank, but there must be one since Honda says it can be run inverted. That diagram is actually animated, but the animation didn't work here. It very nicely shows in the animation how it all works, but once again, it shows a trap door for the oil leaving the crankshaft area but not in the return area to the tank. Someone mentioned that with inverted storage the oil could drain into the valve train area, so apparently that particular one way valve does not seal perfectly. It would have to be lightly sprung to work effectively, hence the reason it wouldn't seal very tight to prevent oil draining into the valve train area during inverted storage.

    Anyway, if mine will fit the Eindecker upright without serious or really ugly mods, I think I'll mount it that way. If not, I'll mount it inverted, but I think the plane will be awesome with that engine and a straight stack!

    AV8TOR
    If it is not SCARY, it is NO WHERE NEAR powerful enough!!
    All R/C planes have expiration dates---> It's just not printed on them anywhere!


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