brokenenglish

The Cox reed-valve configuration. No access to anything. No flexibility. Unpredictable and inconsistent running.

One point I thought of, but too late, is that your engine can't be a very early one because you mention removing the quickstart mechanism, and the early ones didn't have the quickstart gadget. I tried, unsuccessfully, to find the quickstart introduction date. In any case, this point doesn't change or invalidate any of the info you've already received.

Yes, there was the S-75, with the cutout, and the P-75, without the cutout.

The diesel is an early Oliver, sold as the Raylite Panther or Jaguar, through the Raylite Model Shop in Nottingham.

I ran a Mills 75 inverted for years, in a Mercury Tiger Moth with never the slightest problem (wish I still had the plane). People's opinions don't count for much. If you have doubts, cut a mount from a suitable piece of wood and try it for yourself. For me, there's no problem and I definitely disagree with the statement that Mills don't like running inverted. It just isn't true.

Great Planes "Ultimate" biplane. Here it is: https://outerzone.co.uk/plan_details.asp?ID=13254

Following my earlier post, a bit of Web browsing confirmed that the castings source I mentioned in Oundle was called "Woking Precision Models", and that their range was taken over by Hemingway around 2006.

I didn't dare mention it... We all have moments like that...

Here's the "other side" of my engine. Note the adapter, under the glow plug. I don't think the castings can be "Hemmingway" (whassat?), as this engine has been in my possession for more than 40 years. Your build threads have motivated me to run the Atom on sparks ASAP. I'm looking forward to it, but the weather is b-awful. I would expect the Atom to perform a bit better on sparks than on glow, but that's of no importance for me. "Nice running" is far more important than minor performance differences. I seem to remember that, decades ago, castings sets were marketed by a model engineering firm in Oundle, maybe my engine is from those.

Here's my Atom Minor running on glow, at a time when I didn't have an ignition circuit. I'll run it on sparks as soon as we get some decent weather (reasonable light for photos/video).

OK Here are the photos showing the engine id. Looking from the front, you can see E & D on the left and right, and Mk III below the shaft. Again, yours should be like this (it may be Mk II).

Again, I think there's misunderstanding. Re-read my post. The front bearing is the casting bolted onto the front of the crankcase. If you look between the bolt heads, on the front of the engine, you should see an "E" and a "D", in the top part of the casting (above the shaft), and "Mk II" or "Mk III" below the shaft. Are those markings not there? I'll attach a photo within the next few minutes (I hope!).

I've just noticed that you think your engine is "unmarked" (apart from the s/n). Surely it should be marked "ED" and "Mk II" or "Mk III" on the front bearing.

Good !

Sorry John, there's a misunderstanding. Your engine should have the dish-shaped prop driver like my photo. This driver has a square hole that locates on the crankshaft. On your engine, the prop driver has been lost and replaced by odd washers with square holes cut to match the shaft. Those washers aren't original! All Comp Specials have a dish-shaped prop driver with a square hole, no exceptions. And all prop washers, that go on the front of the propeller (only one per engine!) have normal round holes. Like I said, your engine's original prop driver and washer have been lost and replaced by several odd washers with square holes. In fact, I think the reason you have several washers is simply to cover the square length on the shaft. No Comp Special ever had a front end like that.