Ohrndorf 5 Cylinder Radial

Help Support HMEM:

petertha

Well-Known Member
HMEM Supporter
Joined
Jun 24, 2010
Messages
1,793
Reaction score
530
The plans had some internal pocket relief (milling) features & contouring around the lifter area which I stared at for a long time. It looked very nice & it yielded a slightly thinner case section in certain areas. But to my eye actually seemed a bit thin around the front mounting bolt head recesses. So, I opted to modify the section profile a bit so that all the internal surfaces could be done in the lathe in one setting as a series of steps. This provided a bit more meat around the bolt heads, but same annular thickness around the bushing radial holes. It cost some grams of aluminum which I didn’t really care about anyways but erred on the side of bit more strength since it was 6061 & not 2024. I also wasn’t entirely clear about how the pushrod tubes were being retained on the conical lifter bushings, but trusted the plans for now.
 

Attachments

petertha

Well-Known Member
HMEM Supporter
Joined
Jun 24, 2010
Messages
1,793
Reaction score
530
Next step was to chuck the part for the rear side internal cavity work. A spacer was positioned between the chuck face & nose as a mechanical stop. I wrapped some tape on the OD surface to protect it from jaw bite. The DTI said my 3-jaw was accurate, but in instances where required, an extra layer of tape under a jaw allows you to micro-shim. In hindsight the 4-jaw chuck would have been a better choice on 2 counts. You can dial it in exactly & also it provides one more gripping surface. This can be an issue on thin-walled parts where high initial gronk when the part is solid can result in slight deviations when the internal surface is machined out. Another thing I have subsequently learned is that not all tape adhesives play well with cutting fluids. They can dissolve, become unglued & I suppose risk the jaw grip.

I now favor an aluminum tape for protection applications like this. It also works well in shimming applications where its preferrable to pre-attach the material. I think this tape is used for furnace duct work.

So, after some material hogging, I just had to be careful as I approached the ID lip surface which becomes quite thin & must fit the gear plate OD properly c/w with its O-ring in place. It’s important to let the part cool to room temperature & take spring passes at different feed settings.
 

Attachments

petertha

Well-Known Member
HMEM Supporter
Joined
Jun 24, 2010
Messages
1,793
Reaction score
530
Next step was to make a fixture to hold the nose case by the ID lip so that the part could be gripped in a rotary table. It has a threaded hole for a retention stud. After some trial fit-ups that were a bit too tight & concerning moments where the parts were firmly stuck together, I put a smear of anti-seize on the surfaces for insurance.

The fixture/part assembly was gripped in a RT & 4-jaw chuck & 5 bolt holes drilled & counterbored. Now the RT was flipped upright. I used a parallel & DTI to register what is equivalent to horizontal reference off 2 bolt holes & then proceeded to drill & recess the holes for the lifter bushings. These are offset on either side of center nose case center & also offset fore & aft corresponding to the respective cam plates positions.
 

Attachments

Last edited:

josodl1953

Well-Known Member
Joined
Apr 24, 2016
Messages
151
Reaction score
118
Location
Rotterdam, the Netherlands
Peter,
I think cam timing isn't an issue at all. Just put the engine at TDC and the camring at the point amidst the intake and exhaust cam lobe. I did this with my Edwards ( as stated on the drawing, see sketch) and it works fine.
camring TDC.jpg


Apart from this, I think the profile of your cam lobes is rather odd compared to the Edwards ( and other radials, for that matter). The Ohrndorf cams seem to have a flat spot on top of the cam lobes whereas the Edwards lobes have a more hill-shape profile, causing a more smooth opening and closing sequence. Now I assume you fabricate the cams according to the drawings but I would like your opinion on this matter.

Jos
 

petertha

Well-Known Member
HMEM Supporter
Joined
Jun 24, 2010
Messages
1,793
Reaction score
530
Hi Jos. Yes, hard to know exactly what the rationale was behind designers thinking. Maybe construction simplicity was a dominating factor. All we can do is compare the lift & duration profile relative to TDC/BDC & compare engines on that rough preliminary basis. My chart only speaks to beginning & end of intake/exhaust events. But when it comes to actual valve lift, one has to carry on & figure out the rest of the motion down the line - the lift of the cam followers translated to the effective motion of pushrods, translated through the rockers assembly & finally to the valves themselves. There are 3D angles (thus 3D motion) to factor & potentially non 1:1 lever ratio of the rockers. The fact that the O5 intake & exhaust cam profiles are identical, but are pushing against lifters which occur at different fore/aft planes tells me that the resultant intake vs exhaust valve lift itself must be slightly different. Now is that by design, or doesn't really matter in grand scheme?

The Edwards, as you already know, has it's cam lifters aligned & coincident to cylinder center whereas O5 draws a direct line from crankshaft center radially outward. So the cam profiles would look even if the timing were the same. I also noticed Edwards has a cam ramp up/down & more momentary max opening whereas O5 opens & holds that max open position for sustained period. I can envision for example a differently shaped piston top + combustion bowl shape combination may limit valve stroke into the combustion chamber, so if that's a limiting factor then maybe duration is opened up to compensate? The Edwards is a 20-deg conical piston top inside a similar shaped combustion chamber. The O5 is flat top piston with hemispherical chamber. I know from some of the Jung radial plans I have, the cams look more like O5 (sustained period of valve open) than they do vs. Edwards. Yet among this group, they all have comparable compression ratio & all appear to run & idle & transition well enough, at least from videos. So maybe not as sensitive as we think or within the range of other factors? You can see in the other comparison chart of RC engines, the radials are arguably more similar to each other but can differ quite a bit to other commercial 4S engines. Another Edwards build had a neat idea where the cam plate holes were slotted so the relative timing could be altered (but you are still stuck with duration). I think I posted (what I assume to be correct) Edwards timing on another post, but attached below FWIW. Hopefully this aligns with your assessment.
 

Attachments

Bentwings

Well-Known Member
Joined
Jun 18, 2020
Messages
123
Reaction score
12
Location
Minnesota
Peter,
I think cam timing isn't an issue at all. Just put the engine at TDC and the camring at the point amidst the intake and exhaust cam lobe. I did this with my Edwards ( as stated on the drawing, see sketch) and it works fine.
View attachment 130250

Apart from this, I think the profile of your cam lobes is rather odd compared to the Edwards ( and other radials, for that matter). The Ohrndorf cams seem to have a flat spot on top of the cam lobes whereas the Edwards lobes have a more hill-shape profile, causing a more smooth opening and closing sequence. Now I assume you fabricate the cams according to the drawings but I would like your opinion on this matter.

Jos
Actually this is how we installed hot rod cams until specialty cams with much more radical timing and lift became available
 

josodl1953

Well-Known Member
Joined
Apr 24, 2016
Messages
151
Reaction score
118
Location
Rotterdam, the Netherlands
Hello Peter,
I wasn't aware of the fact that your cam followers were offset ,contrary to the Edwards. Having said this, the original radial made by Forest Edwards himself did also have offset cam followers.
30231029.Forest01_lg.jpeg

edwards op een kist 1.jpg

I suppose Robert Sigler put the cam followers in line on his CAD drawings to make fabrication and timing somewhat easier.

Jos
 

petertha

Well-Known Member
HMEM Supporter
Joined
Jun 24, 2010
Messages
1,793
Reaction score
530
Interesting Jos. I've seen those pictures before but never spotted the different lifter orientation. The pushrod angle on the current plans is relatively steep by comparison, but obviously its not a problem. They run!
 

petertha

Well-Known Member
HMEM Supporter
Joined
Jun 24, 2010
Messages
1,793
Reaction score
530
The lifters (or maybe they are called tappets, I’m never sure) are made from nominal 3mm O1 tool steel. The internal end is a dome shape which runs along the cam plate profile. The external end has a partial depth 2mm OD spherical socket seat which mates the ball ended pushrods. The lifters slide up & down within bronze valve guides. So, I made some test guides first so that the drilled & reamed bores could be used as guides for lapping the lifter stock OD.

The male dome profile was formed by a ball turner accessory. Then the part was flipped in the collet, trimmed to length & the female radius profile made with a ball end mill. These lifters were sent at the same time to the same heat treat guy who did the cams so I wanted to get the finish as good as possible now. The lifters would have been easy enough to heat treat with a torch to ‘some’ level of hardness but I wanted them to be a few points softer than the resultant cam plate hardness so as to preferentially wear the (easier to replicate) lifters. I didn’t trust my repurposed toaster oven to deliver accurate temperature for tempering. The male end was polished by gripping in my Dremel chuck & lightly running in a shallow well drilled in MDF wood with a smear of compound.
 

Attachments

petertha

Well-Known Member
HMEM Supporter
Joined
Jun 24, 2010
Messages
1,793
Reaction score
530
The lifter guides are made from bronze as per plans. The turning profile is pretty straightforward. I used my 5C collet chuck & sharp, uncoated insert like I use on aluminum. The holes were drilled & reamed. Its interesting when you make a bunch of the same parts, you gain an appreciation for variation. Some lifters slid nicely in the guides as planned, others felt a bit scratchy on entry or exit. The bore looked good. Turns out my handheld hole chamfer gizmo was leaving a micro burr, so I used a small polishing point to dress the edge.

The lifter guides will eventually get bonded into counterbored holes in the crankcase with Loctite. The conical shape is intended to accommodate the ID of the pushrod cover tubes which meet the guides at a mild 3D angle relative to the lifter axis. At this point I’m not really crazy about the metal on metal contact & mitered end. I would prefer some kind of rubberized or silicone material between tube & cone or somehow making a union. The plans call for the tubes being retained in the rocker perch with a small lateral set screw. I have some hopefully better ideas to test but don’t yet have a clear game plan. So, I have deferred this for now & maybe some divine inspiration will occur closer to final assembly. If I have to re-make the guides m, it’s not a big deal.
 

Attachments

petertha

Well-Known Member
HMEM Supporter
Joined
Jun 24, 2010
Messages
1,793
Reaction score
530
I will discuss the cylinders next. Spoiler alert – in my case it wasn’t quite a straightforward path making 5 cylinders, liners & heads as per drawings in batch mode. I first make some tester parts as per design to get an idea of what was in front of me. This highlighted a few issues where I thought some modifications might be a better way to go. But these 3 components in particular closely integrate with one another, so a design change to one part for whatever reason very likely has a direct knock-on effect to the other parts they mate. I guess we will ultimately see if my decisions were right, wrong or somewhere in between. Overall, I tried to stick to the critical dimensions.

So why the departure from the plans? The cylinder head (to be discussed later) is really the most critical part to have nailed down first because it encompasses many features (read lots of invested machining time). The inlet & exhaust ports are comprised of a smaller diameter gas passage drilled through into the valve cage. And a larger ID counterbore segment, threaded for a steel screw-in fitting which retains the tubing into the head against the counterbore ledge. This threaded style is used in other model engines, including commercial RC engines. Because the port axis is orientated to the head at an oddball angle in top view, the threads of the retention fitting are not initially fully engaged the way they would be like a bolt enters a nut. They must first hook up to a portion of the head thread for a few turns before becoming fully engaged around the port ID. At that point, it’s only a few more turns before the fitting bottoms out, sandwiching the tubing flared end to the counterbore step. No mention was made in the plans of a seal washer at the end of the pipe which I wanted to use particularly for induction pipe, but this would further reducing the engaged thread length. In other words, the design counterbore length is kind of short IMHO. To complicate matters, the heads also have a series of radial cooling fins cut through the port area which further reduces thread contact area. I figured with heat cycles & fuel mung & vibration, it might be asking a lot of these threads. I could select a finer pitch bottoming tap, but I was trying to avoid turning oddball metric threads in my imperial lathe for the matching fittings if possible. As it turns out, some imperial threads might be better candidates. This is a very longwinded way of saying that I really wanted the head to be slightly larger diameter to get more threaded meat in port area.

I modelled the new head in CAD. Everything looked good except aesthetically the head diameter was now extending over the original cylinder top diameter, not as pretty as when they were the same diameter. I’ve seen full size radial examples of both, but I preferred the original look & it solved other issues. So, I changed the cylinder crown diameter, which then meant a different taper angle to end up the same base skirt area dimensions. I also changed the cooling fin thickness & pattern to match my grooving inserts & a more nominal inch spacing pattern. The net change helped provide a bit of cylinder meat for what were shaping up to be slightly thicker liners. More on that later.
 

Attachments

petertha

Well-Known Member
HMEM Supporter
Joined
Jun 24, 2010
Messages
1,793
Reaction score
530
So, with a new plan in place, on with actual cylinder making. They start out as drops of 6061-T6. I rough drilled them 7/8" in a batch mode. Then each is chucked & machined with most all features to preserve the setup. A boring bar was used to open ID to ~0.005" undersize. Then a 1.0625" reamer passed through so the ID would all be consistent diameter & finish in preparation for the liners.
 

Attachments

petertha

Well-Known Member
HMEM Supporter
Joined
Jun 24, 2010
Messages
1,793
Reaction score
530
Next was cutting the cooling fin grooves. I used a 0.043" wide Nikcole insert. They cut like a dream, just keep a bit of cutting fluid on it. Some groove depths are different. The top 3 are a bit shallower to stay clear of the threaded head bolt holes. The next 3 are limited by maximum DOC of the insert ~0.220". Then the remainder grooves are one constant base diameter which then & matches the diameter occurring above the skirt flange. All the edges were lightly chamfered using one of those HSS triangular scraper tools & cleaned up with fine 3M pad. Then parted off.
 

Attachments

petertha

Well-Known Member
HMEM Supporter
Joined
Jun 24, 2010
Messages
1,793
Reaction score
530
I had one of those machinable expansion arbor blanks handy, so turned the main portion to match the cylinder ID with the bolt lightly engaged & also a raised step datum surface for the cylinder top to rest against. With the cylinder lightly gripped I could face the bottom flange & bring the cylinder to final length.
 

Attachments

petertha

Well-Known Member
HMEM Supporter
Joined
Jun 24, 2010
Messages
1,793
Reaction score
530
Next, I turned a spacer collar so the chuck jaws could grip the head portion, because at this point the skirt flange is a larger diameter. Using a rotary table, drill & tap the 5 x M3 holes for the head bolts. These holes were then utilized to attach a rectangular fixture plate. The assembly was held in a vise so the flange could be drilled for clearance bolt holes & milled to the rectangular profile. I have a choice to use SHCS, or threaded studs in the crankcase with topside nuts on the flange.
 

Attachments

petertha

Well-Known Member
HMEM Supporter
Joined
Jun 24, 2010
Messages
1,793
Reaction score
530
Partial assembly pics. BTW, the liners will extend through the cylinder bottoms and that portion is what mates the matching hole in the crankcase
 

Attachments

Last edited:

petertha

Well-Known Member
HMEM Supporter
Joined
Jun 24, 2010
Messages
1,793
Reaction score
530
Next up are the liners. I should mention upfront that the path I took detoured a bit as things progressed, so the pictures might seem a bit out of sequence without some elaboration. When the CI liners were lapped to final bore & heat shrunk into the aluminum cylinders, the cylinders squeezed back under cooling. The bores reduced to the extent that they needed to be lapped all over again to return them to target dimension. I figured any shrinkage would be quite small, like within a few tenths, thus only requiring a quick lapping correction tweak. But they reduced more like 0.0005-0.0015” & also non-linearly down length of liner. Likely a function of the cylinder’s tapered shape squeezing it differently on the top vs. bottom of liner. Anyways the bottom line is my careful bore finishing work before mating into the cylinders kind of went out the window. I could have opted for something more like a slip fit, but I started to read articles suggesting some liner interference is required to achieve proper heat transfer. And as the engine warms up under running condition, a sliding fit will only get looser yet as the aluminum cylinder expands more than CI liner.

My initial workflow was to first establish the bores of the aluminum cylinders to a consistent diameter & finish, which was brought about by a reamer. It was an imperial size next closest to the nominal metric size specified on the plans. I was already modifying the cylinder barrels as mentioned earlier, so this change was incorporated. This resulted in a slightly thicker liner wall which I thought was fine, maybe even desirable. With the cylinder ID established, I would finish the liner OD to whatever dimension was required for the correct slight interference fit. The interference amount was driven by being able to place the mated liner/cylinder assembly into an oven at moderate soak temperature so that they would release from one another based on the different thermal expansion of the two materials. I’ve done this operation many times on RC engines with my small toaster oven to replace liners.

But let me back up a step. This is my first engine & I was intimidated by making good quality piston rings. This topic has been beaten to death in many other posts, so let’s just say I found myself at the same fork in the road I suspect others have arrived. I was aware of the Trimble ring method documented in Strictly IC magazine. There are also some excellent build posts on this forum where others followed Trimble’s procedures with great results. I find Terry Mayhugh’s (Mayhugh1) build posts to be particularly informative. Making the ring fixtures represents some work, but didn’t seem too onerous. But I didn’t have access to heat treating equipment or related experience which seemed pretty important to success. I wanted this engine to run & rings are crucial to success. So, to my thinking, there are 2 main paths:

(A) Bring all liner bores to ‘whatever’ diameter they arrive at, as long as each are identical to one another & appropriate final finish. Using that resultant measured bore as an input value, all of the dimensions to make the ring blanks & heat set fixtures can be determined using Trimble’s equations. The advantage here is that all the liners can proceed along together somewhat as a group. They receive the same tool setup treatment one after another, especially up to the latter stage of finishing where it counts most.

(B) Purchase commercial rings, assuming they can be reliably sourced. This solves the ring making issue. But you need to make the bores exactly the same as the liners they were intended to run in. The O5 is a nominal 24mm bore which happens to be the same as an OS-56 4 stroke engine. Therefore, it seemed like a good idea at the time in my case to purchase 5 rings, including spares for unforeseen replacement. So, I somewhat naively, went down this path. Although it seems like a good plan, in reality it’s actually more work & higher potential for mess up. At least for multi-cylinder engines where the count increases. The issue is the dimensional target – trying to stay within say 0.0001” bore target and simultaneously arriving at that target with the appropriate finish. If the bore is inadvertently exceeded for whatever reason, that’s kind of the end of the trail as it will no longer match the commercial ring. Next engine I will likely go the Trimble route.

When the rings arrived, I measured the cross section against the Trimble values & they were quite closely which is assuring. I also got a new OS-56 liner to closely examine for fit, finish & use as a dedicated glorified bore gage. And added a piston to obtaining corresponding dimensions like ring groove, crown & skirt OD’s etc. to replicate for my pistons. Thus, the shopping list expanded but I figured I could sell the piston & liner as spares one day & recoup some costs. As of today, several years later, they are still sitting in my box. :/
 

petertha

Well-Known Member
HMEM Supporter
Joined
Jun 24, 2010
Messages
1,793
Reaction score
530
So, onto the liners. They start out as drops of 1.25" nominal diameter Class 40 grey cast iron bought from Speedy Metal in USA. They arrive ~1.35" OD I think so you arrive at the good stuff under the skin. Previous to the real liners I also made some testers out of 12L14 & 1144 Stressproof. The 12L14 finishes beautifully but I was a bit concerned about corrosion in methanol fuel environment. The Stressproof machined well, likely a bit stronger & probably a good choice too according to others experience. But sourcing the appropriate diameter was more difficult at the time & sadly 90% of material core ends up in the swarf bin. CI seems to have a reputable track record in conjunction with CI rings. I’m sure wear will be just fine for my occasional running. CI is relatively inexpensive & available in progressive sizes for expected mess ups, so CI it was!

I took a skim cut to get through the crust, faced the end, then pilot drilled 0.375” to 0.875. On my prior testers I experienced a bit of harmonic ringing & minor chatter which I assumed was because I turned the OD to size first & then the bore work. This time I reversed & did the boring first while there was more meat on the wall. Seems to have helped but could also be CI itself vs the prior steel alloys. I found I could hold dimensions quite well as long as one account for any heat buildup. CI is a bit messy so I cover the ways.
 

Attachments

Latest posts

Top