Amco 87 Mk I

by Adrian Duncan

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The Amco 87 was introduced in August of 1947 and was the initial offering from the Anchor Motor Co. of Chester, Cheshire, England. With a bore and stroke of 0.375" and 0.481" respectively (9.52mm and 12.22mm), it was a typical long-stroke side-port design of its day. Displacement, as the name suggests, was 0.87cc (0.53 cuin).

The engine has a number of interesting features which can be seen in the attached photographs. The numbers on both the cylinder head and needle valve grip were intended to allow the user to record the best settings for starting and running. The compression stop pin could be set in any one of four positions once the correct setting had been found. The cut-out is unusual. It features a spring-loaded plunger which effectively blocks the intake tube on the engine side of the fuel jet, thus starving the engine of mixture as opposed to merely cutting off the fuel supply to the jet. The photos show it in the "stop" position. When the arm is pulled out and the lever turned at 90 degrees to rest on the outer end of the cut-out mounting, the intake is clear. When the timer pulls the lever into the visible slot, the plunger snaps home and stops the engine. Quite effective!

Another very interesting and unusual construction feature was the fact that the engine featured a screw-in cylinder despite the fact that there was only one "correct" position for the cylinder when tightened down. It appears that this was achieved by a combination of trial and error and careful trimming of the seating surface at the top of the crankcase casting, probably while the casting was set up for the screw-cutting operation for the cylinder threads. Once a cylinder-case combination had been established that resulted in the cylinder being in the correct position when tightened down, it was of course essential that the two components remain together for final assembly. To ensure this, the engine’s serial number (in this case, 86) was stamped on both the cylinder and the crankcase.

The engine had its cooling fins turned integral with the steel cylinder. The bypass passage was soldered onto the front of the cylinder rather like that of the ED Mk II, as was the induction boss at the rear. The bypass fed a single drilled transfer port of quite generous size which was angled up through the liner to almost completely overlap the exhaust. The induction tube was clamped into the induction boss by a small aluminum clamp which can be seen clearly in the photos. It was a simple matter to re-set the tank to accommodate different mounting positions. The rod was of hardened steel. The crankshaft looks a bit flimsy—the crank-web is rather thin, but it seems to hold up all right in this rather low-powered design.

Each engine was supplied with a special combination tool. This example still has that tool with it, although the box is long gone. The twin prongs at the left end align with two of the holes on the cylinder head to remove it if required. The rectangular "bump" in the middle fits the backplate cavity perfectly and keeps the tool centered when it's being used to shift the backplate. The C-wrench at the right end does not hook into an exhaust port—it hooks into a small cut-away on the lower cylinder flange—the one which sits directly on top of the case. This cut-away may be hard to see in the photos—it's directly beneath the intake at the rear of the flange. Its sole purpose is to allow this C-wrench to tighten or remove the cylinder.

This engine was among the first to be tested by Lawrence Sparey, the report being published in the August 1948 issue of Aeromodeller magazine as Engine Analysis Number Four. Sparey obtained 0.0456 BHP at 8,900 rpm, which he rightly characterised as a very good performance for an engine of this specification. He commented on the engine’s easy handling and relatively flat power curve. One quite insightful comment was the fact that the unusually tall fuel tank created a significant variation in fuel head throughout a run, making it difficult to establish a needle setting that would give a uniform run throughout.

An interesting note regarding Sparey’s test report is the fact that the stroke was given incorrectly! Sparey reported the bore and displacement accurately, but gave a stroke measurement of 0.412" (10.46mm) instead of the correct measurement of 0.481" (12.22mm). Direct measurement of my example has proved Sparey’s figure to be in error. He also perversely reported the engine’s weight as 1 ounce 15 drams!! My dictionary defines a dram as 1/16 of an ounce avoirdupois weight (the old British and US system of weights for commodities other than gems, precious metals and drugs) . Someone must have jumped on Sparey for using this rather obscure unit of measurement, since we find him from Engine Analysis Number 5 (the ETA 5) returning to the use of more conventional fractions or decimals! My example weighs in at 2 ounces (56 gm) exactly—near enough to Sparey’s figure. This is a very light weight indeed for a diesel of 0.87 cc displacement, and it was achieved through the elimination of any ancillary weight such as screws, nuts and bolts.

Sparey also had trouble with the cut-out, which he claimed to be ineffective above about 9,000 rpm. For my part, I simply cannot see how Sparey managed this—on my example the cut-out almost completely blocks the induction tube! If the engine was running at over 9,000 rpm before the cut-out was activated, it certainly would not be doing so after the cut-out was tripped! In fact, the cut-out on my example works fine at all speeds tested. It’s an odd fact that Sparey often had trouble with cut-outs refusing to operate at the higher speeds—this is by no means the only example of him reporting that difficulty.

I’ve run the example illustrated, and can confirm that it starts and runs very well indeed. It has been used but remains in near-perfect mechanical condition. An 8x4 prop seems to suit it just fine, and it turns an 8x4 Taipan glass-fibre nylon prop at a steady 6,300 rpm on the bench. The 9x4 recommended by the makers would have over-propped it unless a very "fast" prop was used. I can also confirm Sparey’s comment about the fuel tank head issue—to run the tank out at the same setting, you have to set the engine so that it runs at minimum lean near the end of the run, which means that the first part of the run is inevitably slightly rich. But this would be of little practical consequence in the kind of situation (small sport free-flight models) in which this engine would have generally been used. Furthermore, the tank simply screws into the metal tank top, and is thus easily removed if desired to allow the use of a separate tank. I’m very fortunate that my example still has its original tank!

Overall, a likeable little engine which would have given very good service. It is very well fitted, but is undeniably a bit crude in some areas. However, the Amco people were learning! The Mk I version of the Amco .87 was not long in production being replaced in 1948 by a Mk II. To complicate matters, there were two models of the Mk II known as the Series 1, and Series 2. The Mk II was essentially a completely new engine with a new crankcase featuring different, stronger webs. Externally, the fins previously cut directly into the steel cylinder were replaced by a screw-on cooling jacket, and the brazed-up venturi/cut-out assembly was replaced by a die casting. This example (from Bert Streigler's collection) shows the matched serial number feature on case and cylinder has been retained as the need to keep both as a pair still exists.

The Mk II Series 2 revision was advertised late in '48. According to the advertising material, this was virtually a new engine with internal transfer passages (note the plural and go figure how this may have been achieved given the opposed exhausts and rearward inlet port). The advertisement indicates significant changes were made to the crankshaft and other moving parts. A photograph of this engine appears in Clanford's, but we could not find a sample to provide an actual photograph for this review.

Numbers wise, the highest serial number we've seen for a Mk I is 3438; for the Mk II Series 1 is is 15089. The numbering appears to have been extended from one version to the next, but this does not mean that there were no gaps! Today, the little Amco is rather rare on the collectors' scene, implying that they were not made in the sort of volume implied by the serial numbers. The significance of this is not known. For more confusion, notice that the Mk I in this photograph (from Tim Dannels and the Model Museum) has a Mk II crankcase! This might be explained as a factory repair as the fragility of the Mk I case became apparent to the manufacturers. It is unlikely to be a "field" repair as the alignment of the venturi is correct which would have required very careful selection of a case with the right thread start position. Another curious anomoly is the lack of knurling on the Mk I prop driver.

As usual, we'd welcome information from readers out there giving the serial number and model of Amco 87's in your collections.


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