Mills 1.3 diesel

[Jeremy Bright]

I have just acquired one of these little beauties and am anxious to run it. It is a Mark II and has a R/C throttle. However there is a problem with the spinner nut which has a damaged thread and will not screw all the way up to the prop. I want to use a nut instead but what is the crankshaft thread? I was able to screw on a M5 nut but it is a sloppy fit and cannot be right. Mills would not have chosen a metric thread in 1948.

[Simon Chaddock]

Jeremy

"Mills would not have chosen a metric thread in 1948"

Don't be so sure!

According to this article

The compression screw is M4x0.7, the needle valve is 1/8" 40 tpi Whitworth and the prop nut British Association (BA) but I will have to check what size.

[PatMc]

I'm pretty certain it's 2BA.

[Jeremy Bright]

Thank you Simon and PatMc. I have just tried a 2BA nut and it doesn't fit (too small).

I will keep listening out.

[David Davis]

Peter Rose, who is an expert with the Mills 75, may be able to help.

PM me for his address/phone number

Edited By Steve Hargreaves - Moderator on 19/11/2016 17:23:49

[brokenenglish]

Jeremy,

I can't provide a direct answer, but a little info.

The Mills 1.3 crankshaft thread is the same as the ED Bee and ED Hornet, I've just checked.

IMO, your initial assumption that Mills (and ED!) didn't use metric threads in the late forties is obviously correct.

The reference link posted by Simon contains quite a few errors, but it isn't worth wading through all that prose...

It's frustrating, as I also have a couple of hex. nuts that fit these engines perfectly and are obviously the right size, but I have no way of measuring the thread .

Finally, although we may call it an "RC carb", the original Mills throttle obviously wasn't intended for RC. Actually, they work very well. The original ads state that, using a timer, you could reduce the throttle setting for "cruising", after the initial climb, and also that, for control line, you could use a timer operated throttle, to allow you to walk to pick up the handle before the throttle opens...

Edited By brokenenglish on 19/11/2016 13:05:51

[Robin Etherton]

Just out of interest British Association threads are metric.

A good quiz question guaranteed to catch a few out.

Robin

[ken anderson.]

hello Jeremy-take it to your local car accessory shop...they'll have a nut to fit it-or wilco's have a fair few out on display in the hardware pic'n mix bit....

ken Anderson...n....e1..job's a goodin dept.

PS...mod's I would remove the lad's mob number from the post above.  

Edited By ken anderson. on 19/11/2016 14:11:40

[Simon Chaddock]

Having extracted my run but still boxed Mills 1.3 mk2 from the roof.

I can confirm that the prop nut thread is 2 BA. (4.7 mm dia 0.81 mm pitch)

A 2 BA 'full' nut is a proper fit crankshaft thread and so is a 2 BA bolt in the long prop nut.

Although BA threads are metric based (0 BA is 6 mm dia) the thread angle is different at 47.5 degrees rather than 60, so even if a 'metric' nut was the right diameter and pitch (which they are not) it would still damage the thread.

[Steve Hargreaves - Moderator]

Chaps, maybe not a brilliant idea to put someones address & phone number on the forum. I know it was a well intended attempt to help but they might not appreciate it....

Better to supply such details privately via a PM to those who request it.....

[Former Member]

[This posting has been removed]

[Jeremy Bright]

A big thank you to all you guys who have taken such trouble to answer my query about the Mills 1.3 shaft thread. Yes, Simon is right. It is 2BA. What foxed me was that I could not screw a 2BA full nut on to the threaded portion of the crankshaft and yet I could wind a 2BA screw in to the prop. nut. On close inspection I see that the end of the crankshaft thread has a slight distortion which is sufficient to stop the 2BA nut and the spinner nut from engaging to start turning. I shall have to get a 2BA die and run it over the threads to true them up.

Regarding the RC throttle I note what brokenenglish says and he is correct. The interesting thing about the throttle barrel is that as it turns the air hole decreases in size restricting the airflow but at the point where it closes completely a tiny breather hole is exposed sufficient to keep the engine ticking over. There is a similar arrangement on PAW diesels where the breather hole is drilled through the carb. body and keeps the engine running when the throttle is fully closed. Personally I prefer to blank it off so that if I hit the throttle cut button on the transmitter the engine stops.

Simon, if you have not yet put your Mills back in the roof can you have a look at the crankcase backplate and tell me if it is inscribed "Mills Bros." Mine is not and yet my Mills .75 is. I always understood that it was one way of distinguishing a genuine Mills from a copy or replica.

Once again thanks to one and all.

[Former Member]

[This posting has been removed]

[brokenenglish]

Posted by Jeremy Bright on 20/11/2016 17:37:57:

Simon, if you have not yet put your Mills back in the roof can you have a look at the crankcase backplate and tell me if it is inscribed "Mills Bros." Mine is not and yet my Mills .75 is. I always understood that it was one way of distinguishing a genuine Mills from a copy or replica.

Jeremy, I possess quite a few Mills engines and I was surprised by your statement, so I've checked a few:

I checked one new in box, like Simon's. The backplate is alloy, painted black, and is marked "England".

I checked a couple of Mills 75 Mk II, and both have "Mills Bros" and "England" in the backplate recess.

I checked a 1.3 with a throttle, like yours, and the backplate is plain alloy, with no marking.

I also checked a couple of 1.3 Mk I engines (with the alloy crankcase) and both have a plain alloy, unmarked backplate.

So, the conclusion would seem to be that, while some engines are marked in the way you suggest, many aren't. So it only confirms identification in one direction, i.e. if the backplate is marked "Mills Bros" and/or "England", then it's an original. However, if there are no markings at all, that doesn't mean that the engine isn't original!!!

[Jeremy Bright]

I am relieved. For a moment there I thought I might have a fake. Your sound advice is appreciated.

[thomas oliver 1]

I have just read the thread on the Mills 1.3cc . I would like to add my tuppenceworth. I have been using these engines since they were first introduced and have quite a few spare parts and engines. I have measured all the parts with proper thread guages. First the compression screw on the MK2 is 5/32 BSW. 5/32x32tpi BSW is close to but not identical to 4mmx0.75 pitch A 4mm screw will bind in the Mk2 cyl head. The carb needle on the MK1 is 1/8th in x 40 but the Mk2 is 6BA. However it appears that Mills did use at least one metric thread on the tank top fixing thread on the Mk1. A 4 mm nut will fit both 4mm and 5/32 BSW screws as nuts are always slightly loose on threads. Nuts are made dead to size with a tap, but screws made with adjustable dies are always made slightly below size to ensure a fit. Even small screws are usually a few thou.below the nominal size. The thread on crankshaft thread on the smaller 0.75cc is 4BA. As stated BA system was metric and of Swss origin and 0BA is 6mm x 1mm pitch but the angle is different. BA is short for "The British Association of Scientists" who adopted it for scientific instruments such as microscopes. .

[Jeremy Bright]

Thomas. Many thanks for the gen on threads. Most useful and I shall make a permanent note of it.

I have been unable to start my Mills 1.3 and came to the conclusion that there was some internal damage. Turning over the prop. I could feel a slight click as it went over top dead centre. I could get the engine to fire but not run. I had a job and a half to remove the cylinder head. It would not budge so I applied some heat by passing a gas torch gently over it to expand the metal. It then came off easily revealing the cylinder liner. I undid the four screws and off came the cylinder liner. I noticed that there are two thin rubber gaskets sealing it too the upper crankcase. The piston was now revealed and I was able to drift out the gudgeon pin. As it came out it fell apart in two pieces. A clear case of metal fatigue. I noticed that the piston had worn two grooves in the ends of the pin. I had to make a replacement. The holes in the piston could take a 3mm diameter drill snugly but it was a bit of a loose fit through the little end of the connecting rod the hole having worn slightly oval. Not to worry. I cut a new pin from 3mm diameter silver steel 10mm long then hardened it to about 700 degrees C. and quenched it in water. I fitted it and reassembled the engine.

After the Christmas festivities are over I will try to run it and wake up the neighbours.

[Mike T]

Picky, I know, but BA = British Association for the Advancement of Science, but since 2009 known as "British Science Association".

Best of luck running it up, Jeremy! I assume the con-rod little end was OK?