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Thanks GreenTwin, very interesting comment on the CO2 exposure time. I started out worrying that the gas wouldn’t get into the cores enough and left them in the bag for hours. Then wondered why they fell apart under their own weight. Since then I’ve generally not been as concerned about the exposure time but was definitely using a wetter mix, so I‘ll scale the mixture back on the next batch and see how it goes with just a whiff of gas.

Anyway, thanks again, when I get the larger crucible I’ll put together a furnace as you suggest.
So just by way of an update to the Gypsy casting saga.
I have increased the amount of aluminum I can melt to about 6lbs which is more than enough for this casting.
At this point I‘ve had four goes at it.
In the first one, shown below, there were some areas the metal did not completely flow. The mold was pretty thin in those areas and possibly that plus moisture/ steam out of the cores stopped it flowing. I post it as the failures can be enlightening or entertaining at the very least.


This first one was cast in petrobond but the later attempts were done in homebrew green sand ( play sand sieved to through #50 mesh with about 15% bentonite clay by weight ), so pretty rough stuff in comparison to the petrobond. I figured there would be a number of trials to get things sorted out and green sand Is cheap and easy to re mull in comparison. If I could get my hands on some finer silica sand locally I’d stick to green sand. Petrobond does give a nice finish but it’s messy and expensive.
After this attempt I shrank the width of the cores by 2 mm on each side and didn’t have any further issues with run out in these areas.
The next two later castings were plagued by, as it turns out, core moisture. I don’t have photos of those but the top of the castings looked like Swiss cheese and internally there was evidence of gas pocket formation. I should have realized what was going on sooner, particularly from earlier comments…

On the fourth attempt I baked the cores at 400F for an hour just prior to inserting them into the mold.


That did the trick, I just wasn’t getting the cores dry enough. This was something Green Twin mentioned earlier so thanks for that tip. The weather has also been extremely humid lately so leaving the cores on a shelf for days before a casting session probably wasn’t a wise move.
After some light skimming of the casting the metal appears to be generally good porosity wise. I used a 160 mm length sprue, 8mm in dia at the bottom and 16mm at the top, with a pouring basin. This arrangement was easy to keep filled throughout the pour.
The metal itself came from eBay ingots so who knows what it actually is.
The surface finish is quite rough, but it’s homebrew sand so not unexpected. Also the pour temp was higher than usual. It was at 1475F when the crucible came out of furnace so probably 1450 by the time it got transferred to the pouring holder and actually poured. That’s higher than recommended.
I added a riser shown below at the front of the casting as there is a lot of solid metal in the front main bearing support. The original professional casting had bad shrinkage issues in this area.
In future castings I‘ll also add a riser to the rear end of the casting as there is a lot of metal in the cam gear housing and there was shrinkage evident there, though not critical it looks like a bottom rear corner of the casting got pulled up slightly, shown below.
The plan is to continue machining this casting, drill the crank and cam bearings, the cylinder bores etc. and just make sure that the internal dimensions are basically OK and things line up.
I think the issues of the original casting with the crank supports, oil ways, and the flange for the bolts to hold the sump are addressed but I want to make sure there isn’t anything else lurking.
If everything checks out on this one I’ll do a casting in oil sand to improve the surface finish.

Best Regards,


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You have more patience than I could muster. Just my experience with the original castings. When i bored for the cam shaft it came through at the front. Looking at it I think there was not enough metal there. A guy I know also had the same experience so maybe check it out
Patience indeed, but very interesting. It reminds me of die casting, something my family was heavily involved in... I'm curious what type of aluminum you are using. I know we used 356 as one of the dominant alloys in die casting, but I'd be hard pressed to say what benefits it has over other types. And bear in mind that those benefits may only be applicable to the aluminum being forced into a steel cavity at great speed through the use of a high pressure ram. Investment casting is a different beast.
From this source: and Aluminum Alloys Davis.pdf
Casting compositions are described by a three-digit system followed by a
decimal value. The decimal .0 in all cases pertains to casting alloy limits.
Decimals .1, and .2 concern ingot compositions, which after melting and pro-
cessing should result in chemistries conforming to casting specification
requirements. Alloy families for casting compositions include the f

1 xx.x: Controlled unalloyed (pure) compositions, especially for rotor
• 2 xx.x: Alloys in which copper is the principal alloying element. Other
alloying elements may be specified.

• 3 xx.x: Alloys in which silicon is the principal alloying element. The
other alloying elements such as copper and magnesium are specified.
The 3xx.x series comprises nearly 90% of all shaped castings pro-

• 4 xx.x: Alloys in which silicon is the principal alloying element.
• 5 xx.x: Alloys in which magnesium is the principal alloying element.
• 6 xx.x: Unused
• 7 xx.x: Alloys in which zinc is the principal alloying element. Other
alloying elements such as copper and magnesium may be specified.
• 8 xx.x: Alloys in which tin is the principal alloying element.
• 9 xx.x: Unused

Heat-treatable casting alloys include the 2xx, 3xx, and 7xx series.

I bored out the casting for the cylinders and right off the bat hit a snag. The new casting shrank in length by 0.075” overall with each of the cylinders about 0.025” closer together. So on a fixed 1.6 inch spacing on centers they become misaligned with the raised cylinder seat features on the top of the casting.
The rear cylinder only just fits on without breaking through the rear wall. Well this was the reason to keep going with this rough casting, to find this stuff out.

A bit of a cluge but I‘ll modify the existing pattern by adding 0.025” spacers between the cylinders for the next attempt.
I also made a new pattern for the bearing caps I need before I can drill for the crankshaft. For some reason I hadn’t realized the bolts are only 4BA ! (I’ll use 6-32 UNF) and I had made the bolt head seats far too large, they would have looked ridiculous, not that anyone would see them….
I find it hard to visualize this stuff until I start getting parts together. The drawings assume you have all the castings so there are very few dimensions to go on.
Yes, that’s the plan. I’d like to get the crankshaft bearings drilled as well as the camshaft. Then I can mock up the gearing at the rear of the engine and check the gear housing for clearances etc. Obviously the crankcase casting shrank in width as well as length, I measured it as about 0.035”, so that needs to be accounted for. I’m not intending to alter the pattern for this, I’m hoping it can be accommodated into the existing arrangement. Anyway we shall find out…

I cast the crankshaft bearing end caps today from the new pattern. My first attempt at this resulted in some shrinkage along the top of the casting so I added a 1” dia. riser about half way along the pattern and that fixed the issue.
It’s amazing to see the metal collapse down the riser after the pour.
l worked on the ‘shrunk‘ casting a little more and drilled through for the cam shaft.
All seemed well but I realized the foundry that produced the original casting I am now using as a pattern made the gear housing dimensions exactly to the drawing dimensions. This means I need to enlarge them, widen it essentially, to compensate for the shrinkage.
I decided to just make the cam gear housing wide enough to machine down as required to match my oil pump housing assembly ( another saga pages back) rather than attempt to ‘size’ it to the drawing dimensions. I think Les Chenery may have done something similar on his original design but not sure.
So with the length of the casting extended overall by 75 thou and some general enlargement of the cam gear housing I had another shot at it.
This shows the latest sprue , gate and riser arrangement. On all previous casting attempts there was quite severe shrinkage in the cam gear housing at the rear of the engine ( left side of photo) . I found that gating the riser only to the bottom of the casting was not effective enough, so after the cope was rammed up I broke the sand out between the riser and pattern almost up to the height of the pattern to allow the riser metal to reach further up. That seemed to work better though there was still a little shrinkage in that region. Maybe a bigger riser is needed.
After quite a bit of machining of this casting I have seen very little sign of porosity so I think the sprue arrangement is working well.
After boring out the resulting casting for the cylinders I was relieved to find the modification to lengthen the pattern was pretty much spot on with the cylinder bores on 1.6 inch centers.
The flanges for the cam gear cover / oil pump assembly are large enough now.
Milling out the crankshaft bearing supports here. The webs line up well with the oil feeds on the side of the engine and most importantly it is possible to drill the oil feed to the front bearing. This was a big problem with the original casting.
Fitting the crank bearing caps.
Well l‘m keeping fingers crossed but I’m hopeful this is the final iteration. My small workspace is beginning to fill up with prototype Gypsy castings !E7B31B0C-F944-4CE3-912A-983746B416EE.jpeg
As it turned out there was an issue with my ‘final‘ casting which meant another go at it. The tenth try.
The bore for the crank shaft is 0.625” and is counter bored to 0.875” at both ends. At the front bearing the counter bore extends 0.375” into the casting. I had not allowed enough depth in the casting here, it’s hollow above the main bearing for the oil breather or filler, and the counter bore broke through into the cavity.
But number ten is progressing.
I started by machining the lower surface with a fly cutter. The goal is to get the height of the casting to 2”, so once I have the lower surface machined flat I flip the casting over and machine the top where the cylinders mount. Lots of checking on the surface table that the top and bottom surfaces are parallel. Then keep removing metal off the underside until the height dimension is achieved.
My next operation was the bores for the cylinders. The casting pattern was made longer to accommodate shrinkage and its close but actually a little too long. It’s barely noticeable but to keep it that way I bored the number 2 cylinder first and then used that as the datum for the other three, they being 1.6” apart.
So then the the front of the casting needs to be 1“ forward of the rim of the front cylinder bore. Front of casting milled down accordingly.
This front surface and it’s relationship to the cylinders is very important because it’s the datum used for the position of all the crank bearing supports.
The crank bearing mounts were milled to required widths. The rear of the casting was also machined down to the required overall length at this point (which is what the oil pump assembly attaches to).
At this point I made up the crank bearing caps from a previously shown casting and drilled them for the bolts.
The drawings spec the bolts as 4BA which I translated to 6-40 UNF And made a bunch of them from SS.
Drilled and tapped the casting for the bearing caps…. and disaster!. I was sooo careful. Had bought new taps etc. but the tap wrench I used managed to jam in the vertical axis and I stripped the thread on one hole.
I thought about just re threading a size up but in the end opted to try one of these low temp aluminum brazing rods. Fill and re drill.
The trick to success with using this stuff on castings is to get the casting hot enough overall so that you can melt the rod by playing a flame from a gas torch over the area. These rods melt at about 650F so well below the melting point of the casting but the casting will wick away heat very effectively. In fact the instructions that came with rods warn that an oxy acetylene torch is required on castings. Instead I heated the casting to 500F and then used a propane torch to get the localized area up to the melting point of the rod. I had drilled out the original threaded hole to something larger. You need to be sure the rod is melting into the bottom of the hole properly.
The operation went well and I re drilled and tapped the hole. Back in business! The caps were bolted down with 0.02“ shims in place.
At this point I was ready to drill for the crankshaft. On the mill I bored the front and back of the casting 0.875” dia to required depths. This is where things went wrong on the last casting.
I considered drilling the crank bore on the mill but in trying to set it up it just felt too awkward so I opted for the lathe.
I made up a bracket to support the casting on the saddle out of 3/16 angle iron. The holes were all slotted so it could be adjusted for height etc. The casting is secured to the bracket using three
M6 bolts through the cylinder apertures. There are packing pieces made up to fit in the casting for the bolts to screw down on.
I made up some 0.875” dia blanks to fit the end counter bores that were held in the chuck and tailstock to assist with alignment. Also a dial indicator to ensure the casting was horizontal.
I had made a 1/2 “ on center boring bar intending to get to final dia with it, but chickened out and went with a reamer. The bar needed to be at least 15 inches long and it just felt too flexible to get reliable diameters with it. I might practice using it on something less critical at some point. Reamers at this size are $$$.
The hole for the cam shaft was done in a similar manner. I made some slotted extension plates to raise the casting up. In some ways this is a trickier operation than the crank because the drill goes quite deep into the side of the casting so it is always wanting to veer off with the asymmetric loading.
The internal side walls of this casting need an extensive amount of machining out to allow for the swing of the con rods around the crank and in retrospect it would have been better to do this first as it would have eased the pressure on the drill. Oh well.


I should mention that the drilled holes were started with a center drill at each web, then drilled out and the next web center drilled progressively. Only the last drill and reamer really went all the way through non stop.
The results seem OK, that is the hole for the crank came out where it was supposed to!

The bronze bushings were turned and a trial fitting of the crank shaft made. The shaft sits in the bearings and can be rotated freely without that telltale tight, loose, tight , loose feel of misalignments. I need to make some new shims for the bearing caps, l lost the originals, their probably in the shop vac with 10 thousand other shim looking bits of metal.

The first couple of attempts at making the bronze bushes were not successful. I did the usual slitting of a bronze bar then silver soldered them together. Drilled and reamed but the solder joint broke both times.

I had better luck with two flat bars of bronze half the dia thickness, held together in the four jaw.
Whilst drilling and reaming I also held the stick outs together with a C clamp (at slow rpm). So.. good news… no slit saw, heating the bronze or messing about with solder.
But… the technique is more expensive, not twice but getting there.



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I admire your persistence (& talent).

It looks like you're making some progress!

Keep up the good work!

A bit more work on the crankcase and spot facing for the front cam bearing. The casting is a healthy 0.25 in thick here.

Aside from the ends there are three internal bronze bearings for the shaft, essentially rings that sit in the crank webs. The drawing shows they are to be held in place by 8BA screws tapped through the side wall of the crankcase. Not a big fan of this.
Having studied a couple of completed examples of this engine I have not seen any screws in the crankcase wall and I’m wondering what was done to secure them. Maybe they were superglued or the screws were brought through the webs rather than the sidewall?
Rear cam bearing trial fit.
Having studied a couple of completed examples of this engine I have not seen any screws in the crankcase wall and I’m wondering what was done to secure them. Maybe they were superglued or the screws were brought through the webs rather than the sidewall?
Superglue will flow at engine operating temp. Dutching the edges to lock the bushing is kind of meh. You could counterbore the bearing wall and set it. Is this end going to be submerged or splashed in oil?
I might:
  • preheat the case to a safe 250F or higher than the expected operating temp of the crankcase (ask wife's permission lol)
  • machine bushing OD Ø.0015+ oversize. I'm just eyeballing the bore sizes. Make extras and lightly chamfer.
  • Set bushings in bowl with liquid nitrogen and install using a simple home made tool on the lathe that fits the ID when frozen. Dry ice may not be enough but an easier alternative. I've used both for different projects.
  • Set the crankcase vertically and have a plate on the backside to keep the bushing from falling through (or flange it). If you go too slow the heat from the crank might freeze it halfway.
  • The inner bore will shrink from compressive forces, you can line ream again or make test samples to calculate shrinkage.
Thanks for your thoughts on this. In fact the drawings call for securing the cam lobes in position on the cam shaft using loctite.
The cam shaft isn’t submerged in oil but is constantly splashed by oil from the crank throwing it about the casing.
I’ll try shrink fitting the bearings first. I’ll look into liquid nitrogen though the containers seem expensive, or maybe just using freezer spray and some kind of locating tool. If that doesn’t work then plan B is the loctite.

The crankcase cam follower positions milled to height and holes drilled and bored for the lifter sleeves. Also the holes drilled and tapped for the cylinder head bolts using UNF 4-48 in place of 6BA.
Well to be fair to Les Chenery although I didn’t spot any cam bearing fasteners in the side wall of the crankcase in any model engines, I did find a real example of a Gypsy mk1 that appears to have some type of visible fastening at the right places on the crankcase adjacent the cam.

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Well to be fair to Les Chenery although I didn’t spot any cam bearing fasteners in the side wall of the crankcase in any model engines, I did find a real example of a Gypsy mk1 that appears to have some type of visible fastening at the right places on the crankcase adjacent the cam.

View attachment 142792
Did the original have oil galleys (galleries) ported to the cam bearings? I could see ports being drilled then plugged.
Very nice. Yes that is what is indicated on the drawing using 8BA screws. I hadn’t seen the screws on the full size engine because most references to the Gipsy Moth show the later 120hp mk2 version which utilized a different arrangement.
I’m tempted to get a copy of a Gypsy 1 engine maintenance manual but I’d likely spend too much time perusing it at the expense of getting this engine built !
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