Homebrew boxer twin prototype

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Well, power was out on Saturday for half the day. Found myself in the workshop with a camping lantern cutting out head gaskets with an exacto knife. When I was done, they looked like they were made in the dark, but still somewhat functional. Then I botched the first attempt at making an intake manifold, broke a critical tap inside the block on the second attempt, recovered somewhat, but I am swearing off three flute taps for 4-40 sizes. A poor workman blames his tools, but its my poor choice of tools at fault here. I just dont believe there is sufficient room for chips and a third flute in a tap that small. One of those nice helical taps is on order. The hardest part has been machining the tube flanges. I plan on using them to crush an o-ring to seal the tubes and keep them in place. They are 1/8 2024 and they get HOT really quickly when finishing on the sander.

The prop is too big to swing on my lathe (13x10 prop) so I removed it and replaced with spacer from scrap laying around. Ran it up on the lathe and found the cam timing seemed retarded a bit, so the minimum duration cam follower design is a little too minimum. :) Next up is a degree wheel and some mushroom style followers. Here are a couple of pics of her all dressed up before I took her apart again.


Boy its been a tough month folks, apologies for the lack of updates. Havent had a whole lot of time to work in the shop.


First thing was sit down and see how the cam timing actually came out. Cam timing isnt always as simple as the degrees of lobe you put on the camshaft. First set of cam followers I made were round ended where they rode on the camshaft. This has the effect of giving the minimum duration available, almost a knife edge riding on the camshaft on one tiny spot. The duration was too short, with wide gaps instead of overlap, I then made a set of "mushroom" style tappets, which significantly increased duration towards the design goal. This is one of those times whern I wish I had the mathematical wherewithall to do things on paper first. :)

Next I had to make a better prop drive washer. I couldnt figgure out how to bore the internal taper, and when I tried it was waay to far off. So I tried my hand at a reamer, which seems satisfactory for now. The proper way, I believe, would be to make a conical insert, slit it, and use a straight bore on the prop drive.

Which leads me to where we are today. After much fiddling with the valvetrain, shimmed some rocker stands, ball ending the pushrods multiple times, I have what seems to be decent compression. I fixxed the intake leaks with some plates squeezing o-rings to the tubes. I have pretty good draw out of the carb. What I cant seem to do is light off these 4 cycle glow plugs for more then a minute at a time. I usually use a AA style battery plug adaptor but these glow plugs stop glowing as soon as I run the tops off the batteries charge. I have a plan for that, Plan D to be exact, as plans A-C all involved too much time or money. :) But here she sits for now.
It`s coming along nicely!
I`m eager to hear it!

Lakc said:
What I cant seem to do is light off these 4 cycle glow plugs for more then a minute at a time. I usually use a AA style battery plug adaptor but these glow plugs stop glowing as soon as I run the tops off the batteries charge. I have a plan for that, Plan D to be exact, as plans A-C all involved too much time or money. :) But here she sits for now.

A while back I made some 4 channel glow drivers that you power from an automotive battery (12V). PM your address and i'll send one over.

Well, I managed to cobble up a commercial glow driver I had running the two plugs in series, fixxed a problem with the fuel tank pickup, and gave it another go this evening. We blew fuel out the exhaust ports and heard the happy sizzle of frying glow plugs, but no fire just yet. My current theory is not enough cylinder filling due to cam timing, effectively cutting the compression ratio down. I had to double head gasket the #1 cylinder to help a valve clearance problem and that didnt help compression much in itself. I made an 1.5" diameter degree wheel but cannot find it at the moment, I will recheck the cam timing, make a good pointer for that, and possibly thread a 1/4-40 thread adaptor for my airbrush compressor to act as a leakdown checker. I will have to sleep on it, and give it another go soon. The old saying about being too close to the problem to see the fix is a very real one. :(

Thanks for the interest guys, will keep you all updated.
Sorry for the lack of updates. Somewhat discouraged by failure to start and recent real life events have conspired to put this project on the back burner for the past two weeks.

When we last left this adventure, we had discovered some leaky valves, as they were never formerly lapped, this was not unexpected. What was unexpected, was how this tiny motor sprawled across the whole of every flat surface available. :) Actually quite common, it always seems to take me by suprise.

Also unexpected, was the rough surface of the valve face under magnification. A dedicated valve grinder project has to go on my list sooner or later.

Once we have the valves lapped and ready to reinstall, we added some heavier springs. These are quite stiff, and hope the additional seat pressure will help in the valve sealing.

Reassembled and without the rockers we tested again. Now the weak point is the head gaskets, simple fiber ones, which I had not held out much hope of working for any length of time. Next up is some fine copper wire o-rings which I hope will help enough to get it started. If not, there will be a series on making dies to fold an multi layer thin copper gasket sooner rather then later.
Ok, so I go over to Mcmaster and look up teflon sheet. I pick a piece of .032 and finalize my order. Now, they have an instant delivery option, and I call them up to check on it. I am in luck, as they just repaired the Heisenberg compensator on their new transporter system. I add my coordinates, subtracting 6' on the Z axis for basement delivery, and click the button. Instantly, $392 charge appears on the visa, and a few moments later my shop is bathed in an eerie blue light as my order is beamed right to my bridgeport. ;D

Just kidding :) Steve, thanks for the idea! I had a piece of teflon laying around for 3 years for another project and didnt even think about using it as a head gasket.

The round piece in the picture is the stub of material left in the chuck from when I made the cylinders. Gouging out the center and leaving a knife edge along the inner and outer surfaces, makes a handy rule die for cutting out head gaskets.

With a scrap wooden board for a backer, the die is pressed into the sheet in the vise.

Next, we put the punch out in the scrap cylinder head. Clamped in place the head bolt pattern was transferred onto the gasket as tiny dimples with a standard transfer punch.


These tiny little dimples are used to center the hand punch, and clearance holes are punched precisely in place.


The result is now oodles of compression. Thanks Steve!
Guys, I would be interested to hear how well the Teflon gasket method holds up to high temps. I have never seen this method used on small commercial engines, but it certainly looks like an easy method for making the gaskets.
LongRat said:
Guys, I would be interested to hear how well the Teflon gasket method holds up to high temps. I have never seen this method used on small commercial engines, but it certainly looks like an easy method for making the gaskets.

I wouldn't use it on 40,000RPM nitro engine but for a gas powered display engine they will last for many years.
Since this motor is definately in between the two extremes we will see how well it works. For now, it only needs to seal at room temps until I get it running. ;D If it eventually melts, it will already have exceeded the design goals for a prototype.
When we last left this project, we were dotting some I's and crossing T's. Only two pics for this update, they are the counterbored flanges for the intake tubes, and worked well keeping the o-ring in place enough to seal properly.


The head gaskets worked great, perhaps too good, as the compression ratio is too high for the rotating mass. We got a few pops out of it but no running. We did break the temporary motor mounts as the oversquare pistons fought back hard at tdc. I went out and borrowed a piece of steel from a friend, but nearly two months later I have not gotten back to it. That should change soon though, as I am almost caught up wearing the various hats of husband, father, homeowner, and master, which have dominated my time in this very wet springtime. We should have some more updates soon, because I am itching to make this thing fly.
Dancing in place
Got some devoted shop time in this week, got a few things done but in the end, it feel a bit like I got nowhere.

We had to stop trying to start the motor when one of the mounts broke. Here is a pic of where we left off, you can see the fatigue crack across the mount at the bottom two mount bolts.

So, mounts are rather boring. :) I also wanted to install a crankcase vent. This should have been obvious, but when both pistons move towards TDC or BDC in unison there is a tremendous pumping action generated. In boxxer motors you cant forget the vents. This necessitated a total teardown to drill and tap a 1/16npt hole.

This hole looks like it was firing. ;D

One of the things I thought would help overcome the compression would be a little more flywheel effect. I found an old kent moore transmission tool which was 1 3/4" diameter, not ideal, but it was the biggest thing I had that didnt already have the wrong size hole in it. I bored the hole tight for the crankshaft, but had no luck whatsoever trying to use the lathe shaping trick to put a keyway in this particular piece of mystery metal. I eventually milled the keyway with a 1/8 carbide endmill in a plunge cut. A quick moment with the belt sander made a brass half round half square key. I just hope it clears the oil pump cover or it may need a little extra work.


Realizing there was no good place for this large vent, which was the smallest available and under 2 bucks, I settled on a spot that hopefully will miss the intake manifolds.

Then came the big decision. I wasnt totally happy with the first camshaft I made. It was functional, and should start and run the motor, but I wanted to make a cam that actually hit the numbers. The bearing surfaces hit the numbers, but the block turned out a little oversied, so I also wanted to add a couple of thou back on the journals. Aside from that, the base circle was a little wavy and that made lash adjustment nearly impossible. Finally, not the least of the reasons, was that I wanted to use my new dividing head I picked up at NAMES. :D I didnt have a good height guage either back when I made the first cam, so layout was a breeze this time around.


This cam was made with a slight radius to the lobes, instead of the flat ramp design like the first one. It was made by the milling method, using the calculator right off Ron Chernich's website. I had already made a proof test of a lobe this way several years ago. It works well if you can maintain your concentration and enter all the right data into his online calculator.
This is what happens when you enter the wrong data. :mad:

It seems if you have a .25 base circle on a piece of .375 your lift is only .062 not .125 :eek:
All was not lost however, only a 2.25 inch piece of 41L40. I corrected the calculations, transferred it to a spreadsheet, added a bunch of nmenonic notations to put the bumps in the right places and practiced on the wasted cam. That lobe came out perfect, taught me the virtues of developing a routine, and I quickly went to work on another cam blank.

Armed with this newfound knowledge from the school of hard knocks, the remaining 210 step cuts and 24 turns +16 holes for each base circle went rather uneventfully. At the very end I had something that looked like this.

So I accomplished quite a bit, but never got around to the mount problem I set off to tackle in the first place. I have some nice shiny new pieces, but the engine is in different Mcmaster bags awaiting reassembly. Maybe sometime this week we will get around to that mount work.
Quick update, we stole a few moments away to work today. The oil drain return problem was slightly improved by drilling the drain nipple out to a 34 size. I needed to make a small spacer for the flywheel to clear the oil pump cover bolts but otherwise it fit fine.

Then I dusted off a nearly 8 year old scratchbuilt plane to determine how to mount the engine.

Thats a 6 point rubber mount system, from Mcmaster parts, where else, and if you look really closely you will see parts of that are riveted like a real airplane. Next we figgure out just what and how to incorporate this engine into the airframe best.
Looking nice!
I would love to see it flying! :bow:

Just read the thread and it looks like a really nice engine.

Cautionary note about the teflon gasket. The reason they are not used is that when it burns it gives off a highly toxic gases :-


it's interesting to note paragraph 3 and the dead birds.

I remember when using teflon in a high voltage test oven (only to 80C) there was a lot of extra paperwork for Health and Safety.

Look forward to seeing it running - hope you will post a video.

Best Regards

Thanks for the comments guys. Indeed, one has to be careful not to burn teflon. This is rated to 500 degrees only, but for now running on alcohol I suspect there wont be any problems. Later on, with spark ignition and a different fuel, we may have to rethink the head gasket composition. Teflon coated multi layered steel is a common head gasket material for full size applications these days.
I have some really nice pictures of the engine mount work I did last weekend, but I will just leave this here instead for now.
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