Ohrndorf 5 Cylinder Radial

Home Model Engine Machinist Forum

Help Support Home Model Engine Machinist Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
This is maybe a good spot to insert some miscellaneous items.

My instinct was that gaskets on specific mating surfaces they would be beneficial. Or at least that’s what I usually see present on commercial engines to seal air/gas, fluids or even help with fastener retention under vibration or heat cycles. The plans were a bit vague on this other than the nose case oil bath area.

I did some experimenting with typical squeeze tube type sealant/gasket products with mixed success. There are so many products out there & admittedly I don’t really understand the variations of metal-to-metal vs in conjunction with gaskets. Actually, of the goopy products I liked, the problem was usually they worked too well. Disassembling the engine parts was quite difficult because of their delicate size & cleaning off stuck residue was a chore. I expect to be in & out of the engine often so I wanted something that lent itself to that. I can produce CAD based export formats for a computerized cutter, I don’t have a machine & it seemed excessive to outsource it for the low parts count plus spares. I see there are some interesting cutting machines used by crafters for cutting stickers & such, but I suspect the software/import capability might be another rabbit hole & again hard for me to justify.

So, I went old school & just hand-made ‘acceptable accuracy’ templates from scrap MDF to act as a cutting guide. I laminated my paper shop drawings onto the MDF & cut them out on the scroll saw. The gasket material I found (actually copied from another builder on the forum) was Teflon sheet, I believe also known as PTFE. The nice thing is it comes in very thin sheet thicknesses, starting at 0.001” depending on the supplier. Its impervious to oil & fuel & even used as head gaskets that see significant heat. I sprayed a light mist coat of adhesive onto the material which tacks it into position on the template. Then cut the outline along the template with a sharp Xacto or scalpel on a cutting mat.

I first tried drilling the gasket clearance holes for, in my case, M3 fasteners to pass through but a drill seems to make a raggedy non-circular profile even with backing board behind. So, I made a simple tool from O1 so that I could harden it & preserve the cutting edge. I used a 4mm ball end mill which made a natural edge to the ~3mm shank. It cut the sheet with a slight twisting motion or using cordless drill. I think the slight give of the cutting mat helps & also preserves the cutting edge. A punch style template might make better holes but would involve another mated template. I also think a thinner, harder cutting template like 1mm aluminum or plastic might allow better access for the blade on internal holes, but the MDF will last for what I need it for. To release the finished gasket from the template, it just needs a spritz of acetone or thinner. The spray adhesive dissolves clean & the PTFE is impervious.
One thing to consider is that PTFE makes good seals for many instances, but it does not have much "elastic memory", so under clamping pressure it can slowly "flow", reducing the clamping pressure, so under pressure can eventually allow leaks. Rubber type gaskets that retain their elastic memory will retain the clamping pressure far better, and paper/composite gaskets once compressed also retain the clamping pressures much better. There are some PTFE compounds that have fillers that help a a lot.
Glow plug drivers have been discussed a few times on the forums so I’ll just try & summarize a few points as I understand them. A typical plug requires ~1.2-1.3 volts & draw 2-3 amps each depending on variables like the plug element, state of condition, how much fuel wetting, fuel composition variations etc. They can briefly take a bit more power but it shortens life. The wire glow elements come in various gauges & material flavors depending on the purpose. Most are exposed wire viewed from bottom, some plugs have an idle bar or shield.

For RC purposes the plugs are typically only energized for starting, they then sustain glow during running conditions every combustion stroke. 4-strokes engines usually benefit or require a heavier glow element vs. 2-strokes, presumably because of the extra dead time between combustion. FS (Four Stroke) plugs generally draw a bit more power but not much. In some installations glow plugs are switched on during idle for more reliability or for inverted cylinder arrangements which tend to run richer which would be the case for a radial. In that case a remote battery power must be carried by the model & of course weight may be a factor.

The simplest driver is a single NIMH cell (or NICD back in the day) typically integrated to a plug clip. These cells have output voltage quite close to plug requirements. It just needs to be of sufficient C-rating & maH capacity. Some excerpts from OS 5-cyl documentation


  • SNAG-27-08-2023 9.45.23 PM.jpg
    SNAG-27-08-2023 9.45.23 PM.jpg
    425.6 KB · Views: 4
  • SNAG-27-08-2023 9.45.11 PM.jpg
    SNAG-27-08-2023 9.45.11 PM.jpg
    445.5 KB · Views: 5
Next are commercial or shop designed circuit-based drivers which may offer added features like regulated/adjustable voltage, self-compensating current to help if the engine becomes fuel loaded or degrade, responding to element resistance change, I guess. Many are trending to more modern lithium-based batteries especially where weight is a factor. They are quite prevalent, come in many mAH capacities, have much higher current rating & higher energy density. The downside is lithium packs are nominally 3-4 v/cell, so a 1S (single) does not make a good match to 1.2-1.3V plug voltage, hence some kind of circuit is required. There are also commercial, multi-cylinder airborne driver modules with more TX flight control features because they are plugged into the RX. For now, I am just in test stand mode. Weight is no concern, just portability, ease of use etc.

Five Cylinder Glow Driver


E-commerce: Search

The wire harness to each of my O5 radial plugs works out to 6-10” length depending on where they would come together behind the firewall to meet the power supply. I had some inventory 2200 maH sub-C NIMH cells kicking around so as a test I soldered 5 in parallel (one per cylinder). A single cell ignited the dry plug sufficiently & confirmed the voltage/amperage range, so that was a viable option. But as the cells deplete, the glow intensity does diminish. There is also voltage drop along the harness wire & the clip, so a lower resistance multistrand wire is preferrable as is keeping oil off the clip so there is good contact.

I’ve seen various harness clips for RC use. Some are like an e-clip that snap to the stem groove. Some are like a push & rotate engagement that lock on the hex body. My plugs are recessed relatively deep in the head so I needed something physically longer to reach. I found some low- profile ‘remote’ RC harness / plug clips on AliExpress & ordered some to evaluate. The clip body is basically a brass cylinder with a spring captured inside which snugly grips the plug stem making electrical contact. Note that 2 sizes are offered, one for thinner (2S) and one for thicker (4S) plug stem diameters. The system looked as good or better than I could make & seemed like it would hold under vibration. One could also bend or resolder the wire more at a right angle & re heat shrink, making it look like a faux boot.


  • 1.jpg
    41.9 KB · Views: 6
  • 2.jpg
    202.3 KB · Views: 5
  • 3.jpg
    57.8 KB · Views: 5
  • 4.jpg
    201.5 KB · Views: 5
While poking around AliExpress I also came across some on-board RC igniter modules. They are purpose designed to output the correct plug voltage. They also allow a wide range of input voltage which was desirable because I have spare LiPo cells & charging equipment. I bought one to try & it seemed to function as advertised with a bit stronger glow than on my NIMH pack. I’m not sure if the modules have features beyond voltage regulation since I am electrically challenged. I decided for the net cost to sandwich 5 together operating in parallel, one for each cylinder to common battery source. I also wanted to switch them on/off independently both to start & also to evaluate how idle/transition behaved on dedicated upright/inverted cylinders out of curiosity. I snipped off the pretty anodized & chromed clips for some other project.


  • 1.jpg
    194.8 KB · Views: 5
  • 2.jpg
    101.5 KB · Views: 3
  • 3.jpg
    181.4 KB · Views: 5
  • 5.jpg
    181.3 KB · Views: 5
  • 4.jpg
    187.3 KB · Views: 8
I found a plastic project box to house the modules & switches & commenced my Neanderthal electrical construction. I needed something like a bus bar to tie the leads in common, but I didn’t see the point in a bunch of screw terminals & the wires were pretty small gauge. So, I cut some copper from scrap 1/16” sheet & just soldered the wires directly. The modules were sandwiched together with heat shrink & everything sits on a G10 plate with standoffs. With the switches it’s a bit of rat’s nest but hopefully functional.


  • 0.jpg
    181 KB · Views: 5
  • 1.jpg
    301.6 KB · Views: 4
  • 2.jpg
    194 KB · Views: 3
  • 3.jpg
    250.2 KB · Views: 8
  • 4.jpg
    235 KB · Views: 5
  • 5.jpg
    231.3 KB · Views: 8
  • 6.jpg
    177.3 KB · Views: 7
  • 8.jpg
    297.7 KB · Views: 5
  • 7.jpg
    421.2 KB · Views: 5
  • 9.jpg
    298.8 KB · Views: 3
I made a test bar from aluminum with 5 plugs threaded in. I wasn’t sure if this would replicate the engine itself but the plugs did glow & no magic smoke escaped, so score1 point for the Neanderthal Electrician. The glow looks erratic across the plugs in this pic because I selected used plugs for the test in case, they went poof. OS-F plugs are getting very spendy & harder to find these days.


  • 1.jpg
    84.1 KB · Views: 7
  • 2.jpg
    167.9 KB · Views: 5
  • 3.jpg
    165 KB · Views: 8
  • 4.jpg
    169.6 KB · Views: 6
Great work of precision, where can you buy the light centering device in the photo that you used?
Thank you

(Blubbering in my water - - - - if only I had the $10 to 15k usd for a copy of solid works (with the options that I think I want) - - - - but then I'd have to run M$ Win and then my mood has to improve.) Fortunate man to have such.

Thanks for sharing - - - - hopefully I'm not too grumpy!!! (LOL)
Only over 60s are allowed to be this grumpy. It takes that long to develop the mental attitude, based on historical experience, to determine the short-comings of most things in life. (like the $15,000 you have not got).
I am still an apprentice Grump. - Being 37... (have been for over 30 years). Thanks for the lesson ...
I made a test stand from 3/8” birch plywood. The joints were glued & screwed together for rigidity & a couple coats of 2K clear so it can be wiped down. My flying field has a very beefy wood table that was used for engine runups back in the day, so for now I will just screw the base down with deck screws which will save me making a dedicated sawhorse or buying a portable work table. The exhaust pipes will blow outboard of the firewall, thus providing copious lubrication to the operator, but I can rotate them within the head port so at least they don’t blast directly at the firewall face as they are quite close right now. One day I would like to make an exhaust collector ring, but that’s a project for another day. The engine is mounted on 5 aluminum standoffs which have M4 clearance holes, so basically bolt passing through the standoff into the threaded manifold mount plate part. The vertical ears give the firewall rigidity to the base & somewhat house the glow driver box. I routed the plug harness through a single hole to avoid the carb opening & then to individual cylinders. A LiPo battery is hooked up to the input power.


  • a1.jpg
    190.4 KB · Views: 5
  • a2.jpg
    286.3 KB · Views: 5
  • a3.jpg
    232 KB · Views: 5
  • a4.jpg
    226.9 KB · Views: 5
  • a5.jpg
    385.8 KB · Views: 5
  • b1.jpg
    407.8 KB · Views: 5
  • b2.jpg
    360.6 KB · Views: 6
The fuel tank is just a regular RC style held in a cradle, but I can locate it up or down if that affects running. Generally, tank center should be about in-line with carb & it should pull full to empty without going lean. If its positioned high it can provide a bit of head but also risks fuel free flowing into carb & flooding. I made a makeshift throttle arm with a pinch clamp. I’m not sure what carb type will end up on the engine, so keeping things simple for now.


  • c1.jpg
    254.8 KB · Views: 3
  • c2.jpg
    222.6 KB · Views: 3
  • d1.jpg
    229.2 KB · Views: 6
  • d2.jpg
    235.5 KB · Views: 5
  • d3.jpg
    383.9 KB · Views: 6
Hi Peter
It is really taking shape, and looks absolutely beautiful !! Your attention to detail is very inspiring. I am very impressed :) I have been following along and always look forward to new posts. Thank you so much for sharing all of your set ups and tooling choices and for your very detailed write ups it is very interesting.
I have a request if you would be so kind. In one of your upcoming posts with the engine on the stand, would you include a common object for a scale reference? Maybe a 12oz. can or a common shop tool we all have.
Thanks again for sharing your journey!
It really is quite stunning.

Thanks for the nice compliments & I will put it beside some meaningful object. How about a celebratory beer? LOL.
In the interim rough reference the cylinders would be about 9" diameter including rocker covers
Apologies for my posting lapse again. Well, the day finally arrived with the realization that I somehow had come to the bottom of my seemingly endless To-Do list. There was nothing left to make. After <ahem> ‘a number of years’ it’s a rather weird feeling.

My preference was to attempt a run up in the back yard, fully expecting a multitude of unforeseen teething pains, requiring access to tools etc. But I don’t think my neighbors would appreciate the unmuffled noise (assuming there would be any noise). I decided to take the engine to my local flying club where we still have a heavy, oil-soaked run-up table from yesteryear. It doesn’t see much action these days, but it was a simple matter of screwing my test stand to it. I mentally prepared myself that this was probably going to be the first of many road trips over the summer to get it going.

I have collected a few different carbs & have machined adapter plates so they could be swapped to the back of the manifold. But once the engine was mounted to the firewall pillars & the ignition wires were routed & throttle control hooked up, swapping out carbs was not going to be as quick as I anticipated. So, I decided to go with the O.S. carb first. It has the smaller orifice size & what I consider nice mixture control features, so hopefully that would assist early running attempts which was the prime objective for now.

I felt that hand starting (propeller flipping) a new, shop made, relatively unknown engine was probably wishful thinking on my part. I blew the cobwebs off my old RC Sullivan starter, which has not seen an engine for a dozen years, but discovered the NiMH cells were in bad shape. I could not trickle charge them back to life & had no spares. I could have retrofitted a LiPo or wired it to my 12V deep cycle battery, but I figured probably just as well because the silicone spinner cone was not a good profile match to my smallish prop nut. But mostly I was apprehensive about its relatively high torque & RPM. Should the lower cylinders become flooded, or any internal parts became loose, a strong starter would probably inflict more damage. So, I made a makeshift drive dog accessory to use in my 18v cordless drill. Basically, a hub with short pegs to engage either side of the prop near the root & a center hole to allow the prop nut to protrude into. It wasn’t pretty but I thought good enough for now. The drill is a typical planetary reduction drive so lower RPM. It also has a variable clutch setting so I could utilize that as a bit of safety release. More on this later.

I turned it over by hand choking the carb until I could see fuel flow in the line. I put a few drops of fuel directly into the top cylinder glow plug hole, set the throttle to 1/3, turned all the ignition plugs on & commenced turning it over with the drill. Imagine my astonishment when it barked to life! I think I just stared at it for a while not quite believing what was happening. It sounded a bit rough & choppy but all 5 cylinders were firing. I lowered the throttle a bit to where it ran OK but not wanting to quit. The needle valve seemed insensitive but I could see the exhaust was oil wet & I was happy to let it run rich that way for a while. After about a minute I shut it down. Nothing seemed hot. No parts were sticking out of the side of the crankcase. I decided to grab my iPhone & record the happy occasion.

Now I must confess, this monumental day occurred earlier in the summer. I figured I would slowly figure out how to get a YouTube channel & then time commitments got the better of me. It just occurred to me there is a video upload button integrated into the site, so here you go. I have subsequently had the engine apart for inspection & modifications a few times, so will document a bit of that now. It is running better each session. I was just about to give it another go these past weeks but the winter white stuff appeared, so I will have to wait for a better day.
Last edited: