Quarter Scale Merlin V-12

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Just had a look on ebay Australia and there are plenty of Chinese spiral taps in metric at least - looks like they've caught on.

Terry - Another excellent build for me to follow. I love the Merlin and I once got to fly 'hands on stick' in a very rare twin seat trainer P51D. Cost $2000 for a 20 minute ride. Over 4G in the loops, 500+ KPH flyover the family with me at the controls and that beautiful noise. Love it.

I purchased a set of metric spiral taps from an Ebay seller here in Sydney the set has 3-4-5-6& 8 mm in it are TiN coated and cost $15 !
I thought for certain they would be junk but for that amount I thought i would buy1set and suck it and see, at least i could use them to clean out tapped holes if worse came to worse.
When they arrived they seemed to be of ok quality , made in japan (yamasomething brand) nice and sharp with no apparent faults i could see , i even took to them with a magnifying glass and could find no faults .
I used the 3mm one yesterday and was happy with its performance in cast iron and 6061 aluminium.
I went back to that site today and bought another 2 sets !

I purchased a set of metric spiral taps from an Ebay seller here in Sydney the set has 3-4-5-6& 8 mm in it are TiN coated and cost $15 !
I thought for certain they would be junk but for that amount I thought i would buy1set and suck it and see, at least i could use them to clean out tapped holes if worse came to worse.
When they arrived they seemed to be of ok quality , made in japan (yamasomething brand) nice and sharp with no apparent faults i could see , i even took to them with a magnifying glass and could find no faults .
I used the 3mm one yesterday and was happy with its performance in cast iron and 6061 aluminium.
I went back to that site today and bought another 2 sets !


Hi Ian,

Thats a good buy.Please advise website address. For last ten years been buying from the M.I.J. "Linc'' Spiral Taps from neigbourhood hardware shop.Its not cheap.
Hi Ian,

Thats a good buy.Please advise website address. For last ten years been buying from the M.I.J. "Linc'' Spiral Taps from neigbourhood hardware shop.Its not cheap.

Hi Gus,
They don't have a website address of their own as it was from EBay Australia , the seller is called themoove (one word ) but they won't ship to south east Asia and quite a few other places .
I'm not sure if ctc tools sell them (out of china) i will have a look around for you , i have purchased a fair few reamers from suppliers in Hong Kong and have found these to be ok also .
Shipping will probably be a killer from Australia but from HK it is usually free .

Hi Ian,

Thats a good buy.Please advise website address. For last ten years been buying from the M.I.J. "Linc'' Spiral Taps from neigbourhood hardware shop.Its not cheap.

I just had a quick look around on the CTC website and they do sell spiral flute taps ( the 8mm one is around $14 au +postage ) i can't vouch for the quality but everything else i have bought from them has been really good.
I had a look at Ebay and searched worldwide for spiral flute taps and there are a couple of sellers (out of Hk) that have the exact same set and brand as what i purchased around $10 au with free postage.
Hope this helps!

Milling the crankshaft channel through the crankcase, fitting the main bearing caps, and line boring the assembly are important foundational operations for this engine. For these operations the crankcase needs to be rigidly supported with its bottom (reference) surface facing up. Although I was able to hold the crankcase in the mill vise earlier for light surfacing of the pan mounting surface, I didn't feel comfortable using the same set-up for these operations. The complex shape of the crankcase makes it difficult to safely support in a vise, and accurately aligning it to all three axes while being supported that way would be very difficult.
I decided to begin using the crankcase motor mounts for work-holding, and so I machined their (top) surfaces parallel to the bottom (reference) surface of the crankcase. Their narrow widths and unequal heights added some complexity, but they gave me full machining access to the bottom of the crankcase.
I've been waiting for an excuse to build a milling table for the cross slide on my Enco lathe, and it looks like the line boring operation that's coming up is a good excuse. I had a half inch ground steel plate in my scrap collection that was just wide enough to hold the Merlin, and so I spent a few days machining it into a universal fixture for my lathe. I attached the crankcase to this plate so it could be mounted on either my lathe or mill table. I set the height of the crankcase so the center of the crank bore was slightly below the center of my lathe's spindle. Later, when it's moved to the lathe, the height will be tweaked with shims.
After another day's effort the crankcase was finally under the mill's spindle and indicated along all three axes. One of Delrin plugs that was used earlier to help position the gear case cover was re-machined to snugly fit the bored opening in the lower gear case where it was used to indicate the crankshaft's centerline.
After milling the channel for the bearing caps, I machined the crankshaft thrust bearing surfaces on either side of the center bearing. Since the Merlin uses an offset prop shaft, the crankshaft doesn't bear a significant thrust component of the prop load. The midpoint of this bearing is the engine's forward/aft zero reference, and so it was used to surface the rear of the crankcase and oil pan to their finished lengths.
In addition to a pair of conventional cap bolts the Merlin also uses a pair of cross-bolts to tie each bearing cap to the sides of the crankcase. These were probably necessary in the 2500 hp version of this engine, but they greatly complicate the machining of the bearing caps in this scale model. The cross-bolts are made of 3-3/4" lengths of .098" diameter drill rod threaded 3-48 on each end. Fourteen .102" holes must be drilled completely through the 3-1/2" wide crankcase where they will actually intersect the cap bolts. The cross-bolt holes can't be moved without offsetting them in their external cast bosses since this would spoil the engine's appearance. The 8-32 cap bolts must be necked down where the cross-bolts would otherwise contact them. The total interference ultimately depends upon how well the trajectory of the cross hole bit is controlled, but it will be a minimum .010" if all goes perfectly.
This interference might be a consequence of the incompatibility of the scaling in this particular area of the engine with the use of standard size model fasteners. I've looked ahead in the drawing package, and I'm afraid this may be a continually repeated story. There's another nasty scale-related issue coming up involving the heads and cylinder liners.
The fixture'd crankcase was reoriented on the mill with one of its sides facing up for the cross-bolt drilling. A Guhring parabolic bit was used to drill the cross-bolt holes without the bearing caps in place. Drilling the holes with the caps in place may seem like a better idea, but I couldn't come up with a way of clamping them securely in place. I didn't want to drill the cross-bolt holes with the caps already bolted in place because even with necked down bolts the drill would have wandered after breaking into the cap bolt holes. Plenty of WD-40 was used as a lubricant since any aluminum stuck to the bit would also cause the bit to wander off course.
The major issue I ran into was indicating the starting positions for the cross-drilled holes. The bosses for the cross-bolts cast into the sides of the crankcase are dimpled to help locate the drill since there are no drawings referencing their positions. I used them for the first set of holes in the rear bearing, but they ended up offset .010" from its center. The starting locations for the rest of the holes were determined by indicating the centers of the cast bearing webs, but these still ended up with errors as large as .005". The errors that accumulate during the drilling of the cross-bolt holes will increase the amount of material that has to be machined from the cap bolts for clearance, and this may weaken them.
The bearing caps, themselves, plus a few spares were CNC machined +.002"/-.000 in 'cookie sheet' fashion. They were engraved with numbers indicating their bearing positions, and they were individually fitted.
With the snugly-fitted caps temporarily in place, the locations of the cross-bolt holes were marked on each side using the Guhring drill held in a pin vise. The caps were then moved to the mill vise where each hole location was indicated using a spindle microscope. The holes were spotted and drilled to half depth from either side which resulted in 12 operations per cap. A .102" reamer was finally run through both holes to smooth any discontinuities at the intersections.
The crankcase was repositioned on the mill with the bottom of the crankcase facing up. Each cap was temporarily held in place with four .100" diameter drill bits inserted in the cross-bolt holes. The cap bolt holes were drilled through to the crankcase using an 8-32 tap drill with the four drill bits being moved as required. After removing the caps the crankcase was threaded for the bolts, and a reamer was run through the cap bolt holes to provide clearances for the bolts.
The diameters of the cap bolts were smoothly necked down to .125" around the point of contact with the cross-bolts. This was sufficient to clear the worst-case cross-bolt, and since it matches the minor diameter of the 8-32 thread there should minimum impact on the bolt's strength. I'm currently using stainless SHCS's for the cap bolts since I had them on hand, but I'll likely replace them with stronger steel versions later. I had only 3-48 lock nuts on hand for the cross-bolts, but since I don't like their looks I plan to replace them with plain hex nuts. The other three holes machined into the tops of the caps will eventually be used to mount injectors that will supply pressurized oil to the bearings.- Terry















Hi Terry

looking good !

Would it be possible in your next post to include a six inch scale for reference ? I think I have the scale , but a reference would be nice.

Man that really is a bunch of holes so far

Hi Terry,
Gus watching the Grand Master at work. Now dreaming of building this engine. I know my limits------machine tool ,space,skill and space. Just dreaming. Meanwhile still bashing the V-2. After the V-2 ,maybe the V-4 2016.
Hi Terry,
The methods of assembly, machining and design is captivating. The cross drilling with the Guhring would have had a breath holding journey across the crankcase hoping that no miss-alignment would be amplified on its arrival on the other side.
I wonder why that method of main bearing cap retaining was employed, my thoughts are that the engine would have had to be manufactured with serious weight contingencies in mind (it has to fly). The bearing caps would have to be as small and light as possible therefore a means of reinforcing this primary load bearing area makes an interesting view of the ideas, technology and materials available during those times. Its actually good to see this stuff is being replicated in a scale build when it would be easy to avoid this complication.
Great stuff Terry, I think I would have thrown the towel in at stage one, straightening distorted crankcases!
My next step was to line bore the seven main bearings in the crankcase. I've never done an actual line boring operation before since the engines I've made up to this point have all used built-up crankshafts, and their crankcases were short enough for a simple boring bar or reamer. The Merlin's crankcase is almost 12" long with a target bore of .938". My original plan was to support a 27" shop-made boring bar between centers on my lathe, and with the crankcase bolted to the carriage I hoped to take advantage of my lathe's power feed. My 36" Enco lathe was theoretically capable of the job, but I ran into a couple of problems.
The first issue involved the new shop-made lathe milling table that I had put so much effort into just a few weeks ago. I designed it to replace the cross slide on my lathe's carriage so it swivels and can be locked down in exactly the same way as the cross slide. Because of clearance limitations I mounted the crankcase directly to it instead of using a sub-plate. This put the mounting screws that I needed to get access to for shimming in difficult-to-reach locations. After wasting a lot of time moving the mounted crankcase on and off the lathe in order to shim it, I realized I had yet another problem.
Line boring the Merlin crankcase requires most of the usable length of my lathe. All the wear on its 20 year-old ways is up close to the spindle since I seldom turn workpieces longer than 7 or 8 inches. I found what appeared to be a slight three dimensional bow in the ways when measured over the distance I was now planning to use. While I was trying to figure out if I could live with these errors, reality began to set in about just how flexible a 27" length of 5/8" drill rod supported between centers really is.
In order to eliminate the uncertainties of the lathe errors and to stiffen the boring bar I made a pair of temporary bearing plates that I mounted to each end of the crankcase. These were placed on the crankshaft centerline and bored to exactly the diameter of the drill rod. These end-plates isolated the line boring operation from any errors outside the crankcase; and, in fact, the whole operation could now be done using an electric drill with the crankcase laying in my lap. I still wanted to use my lathe's power feed, though, since it could produce a superior surface finish. With the crankcase back on the lathe's carriage, I inserted a double U-joint between the boring bar and lathe chuck to absorb any misalignment between the lathe's spindle and the centerline of the crankshaft. As an alternative, I probably could have just hung the crankcase from the boring bar, but I already had a machinable U-joint left over from a custom car steering linkage from a previous life. Testing showed this combination ran with negligible vibration and essentially no run-out where it mattered.
I made a cutter for the boring bar from a broken carbide end mill. I ground a left-hand profile to cut on the boring bar's centerline with lots of rake. A flat ground on its top allowed it to be secured in the drill rod with a set-screw. A second axial set-screw against the rear of the cutter also acted as a crude vernier to adjust the depth of cut.
Because of the lack of a real vernier, I wasn't expecting to obtain an exact bore diameter with this set-up. An exact diameter isn't really important because the diameters of the bearing inserts and even the crankshaft journals can be adjusted later to fit. But luck was with me, and I was able to come within a thousandth of the finished value.
I calculated the expected bore diameter before each pass by measuring the cutter height. This was done using a depth gage referenced to the oil pan surface and the formula: Bore Diameter = Boring Bar Diameter + 2 (Height of Cutter Above Boring Bar). The surface finish was monitored during a few of the initial passes as well as the next-to-the-last pass by temporarily removing the bearing plate on the head-stock end and pulling the bar. Final measurements showed all bearing bores to be within a tenth or so and no measurable circularity errors. - Terry









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Hi Terry

Very very impressive, it's a pleasure to watch

A couple of the photos shows the set-up on my mill that I used to machine the crankcase decks. It's important that these final steps in the crankcase machining are carefully referenced to the centerline of the crankshaft.
I left the end plates used for the line boring operation in place on the crankcase through which I inserted another length of 5/8" drill rod so I could indicate the crankshaft centerline. In addition, I bolted a flat plate to the oil pan mounting surface to give me a reference plane for setting the 30 degree deck angle since the Merlin is a 60 degree V engine.
One end of the rod was supported in a 5C collet chuck, and the other end was clamped in a V-block. It was aligned to the mill's axes within .001" along its entire length in both the x-z and x-y planes. For convenience an initial alignment was done before the crankcase was slipped onto the rod. Keeping the cylinders normal to the crankshaft will prevent rod bind, and care taken now will also minimize fitting issues later between the intake manifold and the very tall cylinder blocks and heads that are coming later.
A sine plate was locked to exactly 30 degrees on a surface plate before being brought into the set-up on the mill. It was slid into position below the crankcase, and the crankcase was then rotated on the rod until the full length of its bottom reference plate fit perfectly up against the sine plate. These two were bolted together, and then the sine plate was bolted down to where it happened to be sitting on the mill table. The result was a rigid and precise fixture for machining the right-side deck. For the left-side deck machining, the sine plate was simply turned around and similarly repositioned.
The spindle was zero'd to the center of the drill rod along the y and z axes, and it was zero'd to the center of the number four main bearing along the x axis. After the deck was surfaced, the bores for the skirts of the cylinder liners were machined, and stud mounting holes were drilled, tapped, and counterbored with respect to the center of each cylinder bore. In this engine, cylinder liners will be installed in cylinder blocks which, in turn, will be sandwiched in an unusual manner between the heads and the crankcase decks. I'll describe the details of this rather unique arrangement in my next post. The skirts of the liners extend below the cylinder blocks where they will protrude into the crankcase. A pair of oil drain holes still need to be drilled through the bottoms of the outside stud counterbores. The reasons for these will also become apparent later.
The drawings specify the distances between the cylinder bores, but accompanying notes warn about issues with some batches of castings requiring a different spacing. My castings fell into this latter category, and so here I began to deviate from the drawing package in a very critical portion of the design. For the crankcase, this deviation isn't important because it matches a deviation I've already dealt with while machining the main bearings. It will become more important, though, when the cylinder blocks and heads are machined.
Notice the raised surfaces left on the decks for mounting the cylinder blocks. Very few gaskets were used in the original engine, maybe to facilitate wartime maintenance. These surfaces need to be smooth with minimum machining marks since they will seal the blocks to the crankcase.
These steps complete the major crankcase machining operations. The biggest reason for the extra care taken in the above set-up was actually because of its impact upon the next machining operations on the cylinder blocks and heads. These operations require the machining of some 28 fluid tube connections between the head and cylinder block on each deck in addition to the matched holes required for the 38 fasteners that hold them together. These operations would be challenging enough in billet parts, but they will made even more difficult by fabrication details of the head and block castings. I plan to diagram this portion of the design in my next post before doing any machining so I can draw on the experience of others about potentially better approaches. At this point I'm not feeling real confident about my ability to successfully machine the cylinder blocks and heads per the original drawings especially since the notes warn about unresolved issues with the design. - Terry





Hi Terry
Looking good. Nice setup.

How much deviation was there along the length of the cyl. bores ? The screw holes still look to be centered in their bosses.

Impeccable work Terry! I really enjoy looking at the setups you make. It takes a lot of ingenuity at times to figure out how to machine parts. Sometimes the setup take longer than the actual cuts.
Hi Terry
Looking good. Nice setup.

How much deviation was there along the length of the cyl. bores ? The screw holes still look to be centered in their bosses.


The distances between bore centers were amazingly uniform at about .005" to .007", and I think part of that was measurement error due to slight imperfections in the bore circularities. It was the whole length of the crankcase that was too short including the relative spacings of the stud bosses, and so I think that's why everything looked centered at the end. - Terry
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