Hi Ramon/Jason

Thanks for clearing this up when Jason first posted the query I went back into my records and had a good head scratch, I'm not familiar with the working of this type of engine so failed to pick it up when I checked things through, so I guess I contributed to it somewhat.

Thanks Again

Stew

0.002mm? That's several wavelengths of light, you should be able to SEE the difference

Neil

Hi Jason, Paul et al,

Despite that best will in the world errors, ommissions and downright mistakes can and do get made. I put my hands up – I'm not infallible!

I have updated the crankhaft drawing and emailed it to Jason who will hopefully post it here later.

Regarding the tolerances on the crankshaft – Yes I agree they would be too much to expect to be able to turn to – they are after all more realistically a grinding allowance but they are on the drawing as a guide to the kind of fit required for the bearings. If you check the text you will see reference is made to 'polishing' the diameters down in the areas shown so as to give a light hand push fit. The bearings do not want to be over tight on the shaft as this can lead to brinelling and ruined races. I do not have access to cylindrical grinding – all the machining of the engines made so far has been carried out on a Super7 lathe and my quite old Linley milling machine and all this kind of work is done by hand using nothing more than wet and dry carborundum paper stuck to a flat surface (a parallel) with double sided sticky tape. By having the paper stuck to something flat a far greater degree of control is able to be exercised and using parafin as a lubricant prevents the paper loading up though it can lead to the sticky tape gradualy releasing.

Where measuring is concerned I use nothing more than a Mitutoyo 0-25mm mic which reads to 0.001mm on it's vernier scale. Whilst that is somewhat variable at such fine measurement it does never the less give you some indication as to where you are at such fine limits – the rest is then down to 'try and see'.

As Jason says these engines do require close limits particularly as above and as said on the piston and liner fit but all are achievable using basic kit with a degree of forethought and careful approach.

Once again, I hope that's useful, allays some fears and gives a degree of confidence for any prospective builder of these small type of engines if not specifically the Eta's. If I can be be of assistance to anyone then please do PM through this forum or MEM

Regards for now – Ramon

Edited By Ramon Wilson on 16/03/2014 19:22:07

Edited By Ramon Wilson on 16/03/2014 19:23:44

A late question, of a general nature…

I am ignorant of all aspects of model IC engine design, and would like some enlightenment.I have read the construction articles, and Mr. Wilson has fitted the piston to the liner by lapping to a close fit. In other IC articles, I have read that the piston should be fitted with a clearance, to allow for expansion at working temperature, with rings to take up the clearance. My question is, why is piston expansion in the hot engine not a problem for Mr Wilson's engine? Can someone tell me about when expansion has to be allowed and when not?

I don't hope to attempt to match the machining precision required here, I'm simply interested in the design aspects.

Thanks

Paul

The ETA 15D is a diesel engine which require a very high compression- maybe 15/1. This cannot be obtained with rings fitted due to leakage. The piston must be lapped to fit the cylinder. Glowplug and petrol engine work at a much lower compression and are made with ringed aluminium pistons or can be unringed. Lapping is a black art and you will find that many modellers have had to make two three attempts before achieving desired compresssion. Rings are not too difficult to make with fine grained cast iron but require heat treatment to remove brittleness and to give some springiness.

Hi Paul – I have just opened an email from Jason refering to your post – my apologies for any delay. My apologies to Mark Walker too as I have only just seen his query above. I do not visit this site very often these days my main posting being done now on the Model Engine Maker site. There is no other reason for this other than time – there simply isn't enough of it – even posting on MEM can take more than what's left of the day at times!

So firstly Mark, the milling machine I use is a quite old but in very sound condition Linley jig borer. That sounds better than it is – that was it's original function but it is definitely a milling machine now! It is a very sturdy machine but does have limitations – not much spindle clearance and does not have a tilting head. The Lathe is a basic Myford Super 7 – no gear box.

Paul – your question has been fundementally answered by Jason and Thomas but perhaps a little more ?

The 'diesel' or compression ignition to give it's correct term does require extremely good piston to liner fit relative to glow and spark ignition to acheive the kind of compression required to create the temperature for ignition. Any kind of clearance that would be acceptable to a degree in the latter two would give rather poor performance in the first. Aluminium pistons, as far as I am aware, have never been used in basic, run of the mill, 'diesel' engines for the reasons stated – mainly different expansion rates. They are used however in high performance diesels but then the liners are usually made of brass or ally which has been hard chromed and the aluminium used for the piston is a specialist grade – you would probably not get far with using HE30 or even HE15

Many early engines of my youth had very tight set ups with the piston runnning in basically parallel bores. These would take careful running in to achieve a good fit and getting the engine hot by over rev-ing or overloading (coarse pitch props) would soon lead to the engine becoming hard and eventually slowing to an abrupt stop. Long, slow and rich oily runs with larger than normal props but with a fine pitch was usually the initial approach. I can think of several well known engines that would take an age to 'settle down'. As Thomas states most were steel liners, many hardened though not all, and virtually all had CI pistons.

The 'tapered' bore came later and is now a well accepted way of providing better conditions – the bore is tight at the very top of the stroke where it needs to be, the liner expanding at that point where the heat is greatest to provide a much better running fit. Running in is much better carried out with short, lightly loaded but fast runs allowing the engine to cool between. Once the optimum bedding in has been achieved steady consistent runs are easily maintained in average weather conditions – really hot days can make a difference though.

This taper technique was/is used in the 'ABC' set ups – Aluminium piston – Brass liner – Chromed

These unringed pistons, are very tight approaching TDC but as soon as the temperature is up the running conditions are reached very quickly. To run an engine in in the traditional manner would soon wear the piston in the (relatively) cool upper cylinder so it is important to make those early runs much faster and leaner than previously carried out. This period is very short compared to standard bores – hence Thomas's remark on engines requiring very little running in.

Hope this helps a little more.

Regards – Ramon