Ohrndorf 5 Cylinder Radial

The heads at this stage were cleaned & the valve cages installed using Loctite 680 HT retainer. Permanent installation is required at this point because the next step is to cross drill the port passages through the aluminum head & into the bronze cage. As you can see, the valve seats are cut. I will discuss this later on.

Fixtures were machined to hold the head in the correct orientation for making the intake & exhaust port passage holes in the lathe. This took some CAD work to figure out the correct orientation & placement. Two 2 separate fixtures were required for intake & exhaust because the offset was different. The fixture was worth it though, my earlier attempts in the mill failed miserably.

Once the head was mounted to the fixture, the gas passage hole was spotted & drilled through the side of valve cage. It broke through with no drama, the Loctite joint held. This hole was then opened up with enlarged counterbore & threaded for the tubing retention nut. At this point I already had a prototype coupler nut & flanged tubing prepared to test the fit.

Tapping the 7/16-28 threads was a bit concerning because it enters the head at an angle where only a portion of the hole is getting threaded, maybe 3-4 threads being introduced on one side until the tap starts cutting the entire hole. The threads also encounter interruptions from cooling fin grooves on one site. I held the tap in the tailstock & basically pushed it in along the bed by feel, turning the spindle chuck with the other hand, using only partial chip breaking reversals & lots of cutting fluid. Because the threaded hole is so shallow it required a bottoming tap. I sacrificed another plug tap by grinding the leading threads off & carefully repeating the operation while in the same setup. Single point threading was not a viable option to me.

Overall, I hate these threaded holes even though they are predominant on commercial model engines. They are fussy. The problem is one of scale, it’s more challenging to make typical flanged tubing fittings with smaller yet flange bolts into the head. Especially at some of the entry angles somewhat required with radial engines & their manifolds, not intersecting other holes or features etc. Anyways, I mentioned earlier that I increased the diameter of head slightly from original plans & this was one of the reasons why. Catching just a few full threads on a coupler nut without eventually striping or losing seal or crudding up just didn’t seem appealing, at least from my own RC experience.

This shows the tester head with port completed, exhaust tubing stack & retention nut

Sneak peek showing heads essentially done.

Valve Cages. As mentioned, the O5 plans originally called for wider valve seats than what I was prepared to adopt. After some forum reading by experienced builders & some experiments of my own with prototype parts, I came to the conclusion that a seat cut face of about 0.010” would be a good target. It would also allow me a bit of wiggle room for installation variation or seat alteration once the cage was permanent in the head. BTW this (thin band) seat dimension was further reinforced by what I saw of model RC engines. This changed the wall thickness of the cage & where the lip occurs within the head in order to keep the valve face somewhat flush & not affect CR too much. The valve length & spring retention features required a bit of modification too. So, some CAD work there.

I used 544 Bronze for the cages. It machines well with sharp tools. I tried a few different ways to make them but eventually landed on this procedure. I turned the outer body OD & stem boss in one setting, parted off & finished to length. I have a Set-Tru 5C collet chuck that grips reliably within about 0.0003-0.0004”. I re-gripped the stock end for end & did all the remaining machining with this setup. Spot, rough drill, re-spot to drill valve stem hole. Then bore the main body ID with a mini carbide boring bar. The edge that would eventually become the valve seat was left at its 90-deg corner.

One issue to note. Bronze can be kind of a grabby material like brass. You want a decent sliding fit on the valve stem, but I think if the hole wanders off track even a tiny bit, that may be a reason for sealing problems that were more difficult to correct. If the valve stem hole is off axis, the valve face will not be square to the cage seat & is dimensionally exaggerated. Seems like it doesn’t take much to make a leaker.

Initially I was drilling the valve stem hole on the first operation before flipping & re-chucking the part. I think that may have been the main culprit. But it could also be related to unequal drill point facets, or maybe reaming the hole, or maybe the operator. Eventually I used a short, stub length carbide parabolic drill on a 120-deg carbide spot drill & results were more consistent. I also eliminated the reaming & just used the appropriate drill size to match the fit of valve stem. In hindsight I also think some of my sliding fits were a bit too close. Perhaps what going on is a slightly looser annular gap allows the axially valve to float just enough to find its way to seating (within limits). We’re talking tenths here but more on this subject later regarding valves.

After following some engine builds, primarily by Terry Mayhugh on the forum, I purchased the same 45-deg multi-flute (chambering?) tool from Brownell in USA. I machined a new center pin. One end snugly fits inside the cutter body, the other outboard end acts as a guide, sliding within the valve stem hole of the cage. I practiced making seats on (many) dummy cages when I was getting familiar with vacuum testing & trying different cage designs.

Then an interesting thing happened. I neglected to consider that the protruding lip on underside of head (which fits into the top of cylinder) in combination with the valve angle, would have prevented me from actually cutting the valve seat once the cage was installed. The tool body OD was a bit too big. So, despite my initial plan of cutting the seats after the cages were installed, I was back to cutting the seats before installation, vacuum testing & hoping they would not distort & compromise the seal.

So, my procedure was to paint the valve cage edge with a Sharpie marker along the virgin 90-degree corner, insert the cutting tool into the stem hole. I found it best to hold the assembly upright so only the weight of the cutter is acting on the seat. With a feather light touch, rotate the cutter by hand, kind of backing off at the beginning end of each rotation. There are more than a few ways to mess this up, but pressing too hard is the biggest no-no. If the cut ever starts looking like a superimposed mini sine wave ‘chatter’ pattern, that’s usually a bad thing. The cutter will then start feeding on this pattern & get progressively worse. I have not found a good way to restore it at this point other than using something like the lapping tool to try & flatten the hilltops & start again. The trick is to catch it early, but more ideally, try hard not to do it.

I made a master testing valve that would be used to validate the new cages, at least as much as possible. It is a regular good valve but with a thin flat relief ground down the length of the stem. The reason for this is to allow bypass of air when vacuum testing the seat once the silicone tube is placed over the top end of cage. The notion here is that if you have a close annular gap between valve stem and hole and maybe lubrication oil, this could yield a false sense of acceptable vacuum leak off time because the annulus itself is acting as a restriction whereas the test is trying to ascertain condition of the valve seat area.

One by one I cut the valve seat on each cage, tested it against the same master valve looking for a lengthy drawdown (ideally a permanent hold) on the vacuum gauge. I also made a valve lapping tool for insurance as shown. The body is made from brass with a steel pin attached with Loctite. The pin is gripped in a collet & 45-deg profile turned. It has some radial slits cut for excess lapping compound. If it starts to show a wear ring, it goes back in the lathe on same setup & re-cut the 45-deg face for fresh geometry. I played with it in test mode and it does work. But it’s not a magic cure for bad geometry.

With the valve cages now complete with seats cut, the parts were cleaned spotless with acetone & Q-tips. The cages were installed into the holes with high temp Loctite 680 retaining compound. As mentioned, be aware that bronze/aluminum sets off the cure significantly faster than steel, so bad things can happen if you are not prepared & the part sticks prematurely before landing into final position. Clean up any excess Loctite especially any that gets on the valve seat. After curing the cage seal was retested with the test valve just as a quality check. Then the seats were covered with tape to protect them against more machining & swarf.

The valves are made from 7/16" diameter 303 stainless. I made some prior testers from 416 which possibly machines a bit better, but I started to have reservations about corrosion which is bad enough on these methanol burners. Turns out 303 isn't bad at all once you have the right tools, speeds & feeds figured out. My regular go to inserts were ‘just OK’. I wasn’t necessarily after a after a stunning finish but I did want to hit close dimensions & my prior experience was SS does not like to be crept up on. I was ready to try HSS again but landed on this particular DCMT ‘for stainless’ insert. It came from eBay or AliExpress which is always a bit of adventure. Anyways they weren’t expensive & they seem to work quite well.

The stems are 3mm nominal (0.118") for sense of scale. The stock is initially held in 5C chuck & supported on end with my newly acquired Shars slender nose live center.

I turned the valve stems aiming for 0.002” oversize. After this I initially tried prepping them with an abrasive block made from MDF & wet/dry paper sized for the full span of the stem. It conditioned it nicely but the paper wears out fast. Subsequently I found with a decent finish & consistent diameter, one can go straight to these simple lapping tools.

The laps are just blocks of steel or aluminum made my sandwiching 2 halves in the vise & drilled through on the centerline with +3mm drills, which ends up a bit oversize nominal & about right for finish lapping. I’ve made a few other homebrew lapping contraptions but these clam shell styles seem to be about as easy as it gets & seem to do the job. Consider them as a disposable item. As you can see, I make a series of lap holes in the tool so as they wear out, I proceed to the next. You can also use the worn/enlarged ones for slightly oversize stock. The split, clamshell feature means it can be applied to the part while in the lathe. I wanted to keep the valve supported in the tailstock for as long as possible which means the waste lump neat the live center is still attached. Whereas other typical lapping tools would require the stem only.

The lapping compound is inexpensive AliExpress diamond paste which cuts very well & washes off readily with thinner. So basically, charge the groove with paste, apply it to the part & just finger clamp pressure go back & forth. It’s very controlled. I think I started with 600# and ended with 1000#. The steel laps were for rouging, the aluminum for finishing. You can get a bit more life out of them as they wear by dressing the flat surfaces on some abrasive paper. That closes the effective diameter, but technically it’s no longer a circle. It's important to have decent tenths reading micrometer & take measurements along the stem. Lapping is messy business so I recommend using gloves & clean often especially your test equipment.

I forgot to take a picture making the fillet / trumpet profile, but it was done with a 6mm diameter carbide insert tool taking about 0.010” stepover until it just kisses the stem diameter. The valve is then parted off, flipped & each face trimmed to a defined thickness. In hindsight at this point I should have taken a bit more care finishing the face with abrasive paper or something just for cosmetics.

Then I use a 5C collet stop and a split holding fixture to hold the stem. The fixture is slightly larger diameter than the valve head. The stems were trimmed to finished consistent length.

The 45-deg valve face is cut with a parting tool corner in shallow passes. I pre-blued the stock with Sharpie. The valve seat profile is complete when there is 0.010" blue remaining relative to valve face. I just held a vernier with jaws opened 0.010" using a magnifier. And now seeing these closeup pictures, my carbide looks chipped which probably didn’t help matters.

To finish the valve face, I eventually decided its better to just get it off the lathe. Lathe power isn’t necessary, the cross slife or compund is not being emplyed, RPM is limited & usually comes with vibration. So I just held the valve stem in my Dremel (Milwaukee Cordless actually) with a collet & spin it whatever RPM its happy. I figure at this stage, its all about finish albeit controlled by hand, so I’m less concerend by collet or chuck running absolutely true. After doing vacuum pull tests trying different methods, my conclusion was leakage was typically related to the ‘record player’ micro machining ridges on the valve face coming off the lathe. If it vacuum happens to seal first time, I think its more luck than anything else. A polished finish seat stands a much better chance IMO & usually requires no seat lapping usually.

So to achieve that finish, I first blue the seat face with Sharpie & then carefully take the surface down in progressive stages with nice smooth motion, until the machining ridges are completely gone & no more blue appears. Hopefully the pictures provide a sense of this. I start at 1000# - 1200# (blocks with wet-o-dry paper). For final finishing I found these foam backed nail file boards work well, they have 3000# on one side & you can do a lot of valves. Initially I was paranoid of freehanding the faces like this, worrying about roundover but its actually quite controlled. You can also re-blue & observe.

The vacuum seal success ratio improves dramatically & consistently. Now there may be something else going on & I’ll leave this as an opinion. Even though the polishing is using a backing board and probably only tenths are coming off, there may be some propesity to make the slightest of curvature on the face. This might not actually be a bad thing (within limits) if it means the valve profile can make a continous seat line even if the stem is off by a teeny degree. Who knows.

Last operation is making the groove for the valve spring retainer. I used a Nikole system insert tools which are really nice. Then just chamfer the edge and give it a bit of dressing. I hardened the rockers but the valves are left stock.

Glory shot of valves and a few spares. All the soldiers ready for action.

Not much to say about the valve spring retainers other that they are made from 303 stainless & rather fiddly. I have since what is probably a better machining sequence where it’s made in one setting with the cup side facing out vs 2-step flipping around like I did.

The intake & exhaust tubes are retained in the head ports with threaded hex nuts. The tubing end is flared & that is what the nut edge acts on. A Teflon seal washer will reside between the flared end & face of the head port.

The nuts were machined from 0.5" 303 stainless hex. Once the thread OD was brought to size, I made a thread relief groove with Nikcole radius insert of 1.5mm (0.059") diameter. There isn’t a lot of remaining wall thickness on the nut, so I thought this profile might be a bit better than using a square cornered parting type tool. The groove width doesn't offer much error in reaction time to disengage the lathe during threading. I also didn't consider until I got going that the thread point has to terminate in the middle of the circular groove, half the groove width. Not that I have a lot of threading experience but I am reasonably happy with these eBay / AliExpress threading inserts. I was concerned my lathe rpm might be too slow, violating most all guidelines for carbide, but they cut nicely and the insert did not chip.

The generous chamfer on the outboard side of hole is to accommodate the radius bend in the tubing. May as well toss in some more (preliminary) CAD images showing the general principle. In reality, flaring the tubing is not an exact science & I was fixated on having some kind of sealing element washer between the head face. Maybe not so much for exhaust, but for sure induction. All this takes up space in the port hole & robs precious few threads. So that’s another reason why early on I decided to just increase the head diameter a bit & solve all these issues simultaneously.