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Mills 0.75 compression screw

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Martin McIntosh10/04/2020 18:43:41
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Since like most of you I now seem to have excess time on my hands I am going through some of my old motors and getting them back up to scratch.

A 4BA compression screw for a late Mk1 0.75 eludes me having tried everywhere.

Anyone have one or can steer me in the right direction please? I have some 4BA screws but getting the necessary dished end similar to a grub screw is beyond me.

bert baker10/04/2020 19:45:53
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Weston uk could probably make you one

brokenenglish10/04/2020 20:02:06
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Martin, unfortunately, I don't have a spare, but what do you mean by a "late Mk I"...
The Mk I Mills 75 was only made for a few weeks and, in any case, I think the Mk I and Mk II compression screws are the same.

If you want an exact replica, I think it will have to be specifically turned. You won't find a commercial screw with a round head as deep as the original, and the screw head then has to be drilled and tapped for the screw-in tommy bar...

As mentioned above, it would be too difficult to make from scratch, but I don't think you could modify an existing, standard 4BA screw.

Edited By brokenenglish on 10/04/2020 20:03:17

bert baker10/04/2020 20:10:30
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Weston Uk is still a ED agent as he owns ED,

Plus he has the nessesary skills and equipment to make one

Martin McIntosh10/04/2020 20:38:34
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Thanks for the replies. I have a couple of clubmates who could make one; it does not need to be an exact replica but there must be quite a lot out there somewhere.

I may give Weston a try.

By late Mk1, many years ago as a youngster I inherited a grey crankcase one, 1946 at a guess. This is of identical design but the case is black, S/No. 449-35.

Mike T10/04/2020 20:57:20
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Or try Tony Eifflaender at PAW. I got a 4BA comp screw and a small size NVA off him last year to get a DC Merlin going.

brokenenglish10/04/2020 21:08:23
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Posted by Martin McIntosh on 10/04/2020 20:38:34:

By late Mk1, many years ago as a youngster I inherited a grey crankcase one, 1946 at a guess. This is of identical design but the case is black, S/No. 449-35.

OK Martin, just FYI, I think the Mk I Mills 75 was released in 1948, and the Mk II in 1949. Only the machined case 1.3 was released in 1946.
All original Mills 75s have a black case but, if you think you had a grey one, probably some of the black coating had worn off...

I'll have a look in my box of bits. Good luck anyway!

Martin McIntosh10/04/2020 21:32:55
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Not so. I have a grey case 1.3, 1949 Mk2 series 2, the same as a 1946 one I got from the same source as the original 0.75. The black certainly does not wear off.

Mike, I have looked at the PAW site already but under `spares` they list nothing, only new motors. I shall give Tony a call.

Thanks both.

brokenenglish10/04/2020 22:02:17
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Posted by Martin McIntosh on 10/04/2020 21:32:55:

Not so. I have a grey case 1.3, 1949 Mk2 series 2, the same as a 1946 one I got from the same source as the original 0.75. The black certainly does not wear off.

Sorry Martin, either you're a bit confused or you've been given wrong information. Here's my first model Mills 75, released in 1948. It's in perfect condition but, as you can see, the black is starting to get a bit grey.

mills 75 mk i.jpg

The only Mills that existed in 1946 was like this one below, except with a parallel fin profile. This photo is a 1947 engine, I haven't got a photo handy of the 1946 Series 1, but they're identical except for the fin profile.

mills 005.jpg

Finally, here's a 0.75 Mk II, made from 1948 to 1963 (I think!), which really does have the black coating intact, but it was purchased new by me and it's still in the box!

mills 003.jpg

Anyway, the bottom line is that there is no such thing as a 1946 Mills 75.

If you still disagree, I'll dig out the original release adverts! (Sorry!).

bert baker10/04/2020 23:20:33
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c44b204c-e3c0-428e-90a7-ffb919b946ad.jpeg

bert baker10/04/2020 23:25:17
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The three at bottom are Indian ones as they are stamped in back cover

thee top left is a Irvine Mills as that’s what is Stamped in cover and the front transfer port is clearly visible

the top right I believe to be a original Mills,,,,, can’t see back cover but does have the same looking head as a Mills compared to the others, 

plus the front transfer port isn’t as bulky as the Irvine 

Edited By bert baker on 10/04/2020 23:34:52

bert baker10/04/2020 23:31:00
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The black coating is a bit mottled comaired to the Irvine that looks more like a anodised coating

brokenenglish11/04/2020 06:42:21
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Posted by bert baker on 10/04/2020 23:31:00:

The black coating is a bit mottled comaired to the Irvine that looks more like a anodised coating

Yes Bert, I think that's probably due to the fact that the original cases are magnesium, with a chemical coating, whereas the Irvines are a coated normal light alloy (I think, I don't possess an Irvine).

David Davis11/04/2020 08:15:17
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Are there such things as 4BA socket headed bolts? If so one of those might do at a pinch.

Martin McIntosh11/04/2020 09:42:45
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OK thanks for correcting me. I am starting to think that the original grey machined cased Mk1 may have been a pre- production one, or dare I say a rare as hen`s teeth 0.5. The original owner of them was a distant uncle in Scotland by the name of Douglas Paton and it is likely that he would have been in the position to test company products because I was told that he was a very prominent modeller in his day. The 1.3 sported a r/c carb. to boot. I also got a large selection of booklets on stuff like aerofoils, lots of graphs for calculating Renald`s numbers, prop. efficiency plus one on early model engines.

The models I got from him were very advanced for their day. The 1.3 was in a model with a second escapement driving the carb. on a quick blip arrangement and one with an Elfin 2.49 had LE slats and a hinged former carrying the actuator battery pack and a mercury switch, the idea being that in the event of a `stick on` it would swing forwards and break the actuator circuit. The mind boggles!

The smaller Mills was in a Southerner Mite type free flight model.

Martin McIntosh11/04/2020 09:55:19
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Thanks for the pics Bert.

David, tried that but without a cupped end they just unscrew in flight.

Engine Doctor11/04/2020 11:01:12
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Hi Martin you say that they ( the comp screw ?) Unscrew in flight ? If that's the case then the problem is more likely to b a loose contra piston . If compression can be adjusted very easily then a loose contra piston will be the culprit. Two possible fixes

1 Make a new contra piston

2 Remove contra piston an find a ball bearing that will sit in the recess without touching the sides. Lay contra piston on a smooth flat metal surface ,t something like a vice etc then hit the ball bearing with a hammer. The idea is to expand the contra piston slightly to restore the fit and resulting friction. Trial and error is the game. So hit it and try the fit , if it's still loose hit it a bit harder and try fit again. Repeat until a good tight fit in the top of the cylinder is obtained. If you go too far then fit the contra piston to a screw or bolt , and mount in a drill ( there is usually a thread in the top used to fit during manufacture) and using some fine art and dry paper abrade the outer surface until it fits tightly in the cylinder.

Do be careful as hitting the contra too hard can crack it.

brokenenglish11/04/2020 11:50:49
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Another point is that, during this discussion last evening, I examined 3 original Mills 75 compression screws, and they just have a plain blunt end (as normal), with no sign of any concave recess.

ED's remark about the contra-piston fit is obviously valid. Just about all model diesels have a plain blunt end on the compression screw... If compression "unscrews" during running, it's due to the c/p fit, not to the fact that a normal comp screw is being used!

bert baker11/04/2020 11:56:15
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Engine doctor,,, thanks the the tip

I have a old 2.49 that the contra piston is so slack it get pulled back down the linner when the piston goes down whilst flicking the engine over

I took it to the manufacturers stand at a show and one chap said send it in for repair as it’s a fine art to do,

whilst a younger lad in back said take contra out and hit it with a hammer,,,

Dont think the older chap was to happy for a secret to escape.

I do have a old AE 0.50 that has a stiff contra piston but it still tries to unwind the screw,,,,

some of the other diesels I have have a lock key fitted

bert baker11/04/2020 11:56:17
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1584 forum posts
326 photos

Engine doctor,,, thanks the the tip

I have a old 2.49 that the contra piston is so slack it get pulled back down the linner when the piston goes down whilst flicking the engine over

I took it to the manufacturers stand at a show and one chap said send it in for repair as it’s a fine art to do,

whilst a younger lad in back said take contra out and hit it with a hammer,,,

Dont think the older chap was to happy for a secret to escape.

I do have a old AE 0.50 that has a stiff contra piston but it still tries to unwind the screw,,,,

some of the other diesels I have have a lock key fitted

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