ML Midge Cylinder Head

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Well-Known Member
Jan 6, 2013
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Melbourne, Australia
Hi All,
my first post (other than in the welcome forum). I am building my first model engines- a pair of 0.8cc ML Midge "diesels".

Today I started on the cylinder heads (see drawing). I'm using 2011 T6. I ground up a 1.5mm wide HSS fin cutting tool and cut the fins. I also tapped the M4 hole for the compression screw and drilled the cylinder to 10.5mm ready for boring to 12mm. I am very happy with the result so far (see photos).

However it occurred to me that maybe I should have drilled the four 2.5mm screw holes first. I'm now worried that when I drill the holes I will end up with a burr on every fin. Is this an issue?

Regardless of the answer to that question, what is the best way to mark out and drill the holes? As you can see, the holes are centred on a 16mm diameter circle. How do I mark this circle? It's too late to use a set of dividers as I've drilled the 10.5mm hole.

Should I drill the holes offset in the 4 jaw chuck or on the pedestal drill?

I don't have any construction notes, so I'm fumbling in the dark a bit. Appreciate any help.



tapping for compession screw.jpg

cylinder head.png
Hi Chris,

drilling the holes on a 16mm dia circle, I recommend that you produce a drilling aid that utilises the centre hole as location. It's basically a disc with a spigot of the same size as the centre hole. Since in producing the drilling aid you have the centre to mark and drill the holes. I recommend using the pedestal drill since it is simpler to set up than a 4 jaw.

Hi Chris, I am allso on with my first IC engines a pair of BollAero 18s. A free paln thanks to Chris Boll and Ron Chernich. Can you please tell me where did you get your plans from for the Midge? If the two engines I am on with are OK the Midge might be a good one for me.
Thanks Marsh.
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Somehow that happens to me sometimes. Enough times that I now have a container labelled "compass centers". They are simply short pieces of round stock in the most common bore sizes I use, with a small divot in the center, big enough to hold a compass point. (I generally do this in my lathe with a small center drill.)
the compass centres are a good idea. I can see myself ending up with a tin full of them too.

I got the Midge plans from Ron Chernich. They are Oz$15. He has them in metric and imperial.
Hi Chris,
Assuming from how you describe it you do not have a milling machine then making a small jig as suggested by Brian is really your best option. If you make it 'double ended' it can then position the holes for the crankcase too.

Should you decide to do another engine - and I'm sure you will when these burst into life :) - then I have found it is definitely better to drill the holes first before putting the fins in.

With 'fins first' the burrs from drilling are on the underneath of each fin and can be difficult to remove. 'Holes first' however puts the burrs from fin cutting into the holes and these are much easier to remove by running a drill down by hand. I have found no problems with the interupted cuts as the tool passes through the holes.

Good luck with your build - I've had a long distraction but am about to start some new diesels withing a week or two

Regards - Ramon
Thanks Ramon,
I will do the second head 'holes first'. In an effort to avoid ruining the first head, should I perhaps drill first and then ream to 2.5mm?
Hi Chris, that will help reducing the burrs but you shouldn't need to ream them - drill say 2.1/2.2 first then open up to 2.5 - these are clearance holes after all.

With a nice sharp drill and breaking through each fin slowly you can reduce the burr to a minimum which can then be scaped off from the side - I have a half round needle file the tip of which is ground as a scraper for such a situation.

I wish you every success with your engines - it's a great feeling when they burst into life. Even when you've made one before it still brings a big grin :)

Regards - Ramon
I finished the second head today. The one on the left is 20mm diameter with parallel sides, as per the plans. I think it looks a bit scrawny so I made the second one 22mm diameter at the top tapering to 20mm at the base. I think it looks much better. The second one was drilled prior to cutting the fins. This gave a much better result. Thanks to those who posted advice.

original and modified heads.jpg