Hey Presto...

...Two new valves and seats. I was able to use the existing valves and cut them dowm from 14mm dia. to 11mm dia. I couldn't resist keeping the inlet valves bigger, so they're only 1mm bigger than the exhaust.

Smaller inlet valves and new seats

Sorry about the poor quality of the photo, it was taken under poor lighting conditions with my iPhone.

Next is assembly and test. This will probably take place at the weekend.

Better, but with the same old same old.

I've fired it up again and its still not performing correctly. Its better, but not right. I fear the crank shaft is suffering again. I'm going to look at the cylinder heads next and perhaps make two new inlet valves and seats. I'm thinking it was a mistake to make the inlet valves so big, so I'm going to reduce their size. Too much mixture is getting into the combustion chamber and ignition is taking place far too advanced of TDC. Its OK lower down the rev range, but when the throttle is opened it tends to labour and get extremely hot.

Back together again.

It's good to see it all in one piece. Assembly is by no means straight forward, and takes a considerable amount of care and patience. One of the trickiest parts is fitting the gudgeon pins. There is a PTFE pad on each end of the pins and you have to be careful not to push the first pad out of the other side of the piston. This is especially tricky when fitting the second pad as there isn't much space for them to fit between the ends of the pin and the cylinder bore.

Reassembled and...

...rarein' to go!

Another issue is the valve timing. The gears are cross helical and you have to "aim" the camshaft as you fit it because the working position is a few degrees away from where you first insert it due to the helix in the gears. I think I've got it right, but only testing will tell. I may have to advance or retard it a notch.

All the machining is finished - I think!

It all just requires assembling. Easier said than done. Two new cylinders have been made with a thicker lip at the top to reduce the compression ratio. Made from Meehanite it is difficult to obtain a good finish in the bore so hand finishing with fine abrasive paper is required. 

Cylinder and Piston

Finished crankshaft

I finished the crankshaft today, faced off the front end and drilled and tapped the M6 thread. Next is to start the assembly. First the primary cross helical gear has to be pressed on the rear shaft. Then the inner bearings pressed on each end, and then offered into the crank case where the conrods will be fitted.

More later, I now have a job to do for a client.

A new angle

Just thought I'd take some more pictures while I had the crankshaft set up to machine the counterbalance weights - in case I need to jog my memory in the future.

Two done - One and a bit to go.

Angled faces are machined at 38 degrees.

Almost finished, all that's left to do is face off the front and drill and tap the M6 thread.

Finally, a new crankshaft.

I got some infoozyazm together this weekend and have made a new crankshaft. Its not finished yet but is well on its way. To my amazement it runs perfectly true and is better than the last one. I'm going to counterbalance it this time, not as much as a single cylinder engine would be, but a bit more than "balanced".

The new crankshaft

The other thing I have to do is make two new cylinder sleeves. The originals were creating too much compression and that is what I believe was contributary to the demise of the crankshaft. I've bought some Meehanite a dense cast iron which is wear and heat resistant. I need to increase the lip thickness to raise the cylinder heads and reduce the compression ratio. The only control I have over the point of ignition is by the compression ratio. Too much and pre-ignition reduces power, increases temperature and leads to component failure as I've experienced. Too little and the engine is difficult to start, runs poorly and wastes fuel. I'll increase the lip thickness by a millimetre, if it proves to be too much I can easily remove some of the material to increase compression again.

The best day for flying.

Wall to wall sunshine and light variable winds mainly from the North made today the best by far for flying this year.
Seen at Westonzoyland today:-

BAe Hawk - Nice!

We held two competitions today, the first in the morning was a fly for fun where we had to do various events, spot landing, consecutive loops, limbo circuits and touch and goes. Planned so that it can all be done in one flight. The second was a scale competition where anything resembling a scale aircraft could enter. We had to create a flight schedule which replicated the full size. I'm glad I took the Yak, but I found it a bit challenging yet enjoyable. There were two rounds and after the first round we were shown how its done by an expert accompanied by a judge/commentator who talked us through the schedule. Then we had the second round to see if there were any improvements over the first round. I failed miserably as my Yak had an engine cut for no apparent reason and I had to land prematurely. I was allowed to restart the engine, which it did instantly, (je ne se quoi?) and continue my flight. I know I didn't do so well in the second flight as I was trying too hard, my brain kept getting in the way. Won't know the results until the AGM in December.

I've made a start.

Here we go, a new piece of stock EN16T sawn off and a flat milled all down the side. This is the location face for aligning the three centre drilled holes in each end. Once that is done it can be set up between centres in the lathe. Held in a carrier, and driven by a face plate and dog the offset journals can be machined. Here's how:-

Flat and three centres detail

Shaft held between centres

First finished journal

Here's a video of one I made earlier.

Disaster has struck!

After making some improvements to the Ketro 120fst I tested it today and the one piece crankshaft is now a two piece crankshaft. I am amazed the whole engine didn't smash to pieces judging by the noise it made when it let go. Fortunately there is little other damage. However, I've had some thoughts. (Careful). When the throttle opens the revs increase as expected up to a point when there is a change in the sound in that the engine sounds like its suddenly working harder yet not increasing revs any further. Open the throttle more and the revs actually decrease, yet it sounds like its working even harder. Eventually the engine cuts and has become overheated. The problems are three fold:-

• (1) I think this problem lies mainly in the size of the inlet valve, I have designed it too big and too much mixture is getting into the cylinders and compression reaches the point of ignition too soon before top dead centre (TDC). If it were a spark ignition engine I could control the point at which ignition takes place, but this is a glow plug engine or semi-diesel and ignition is controlled by the amount of compression. I fitted restrictors in the cylinder heads to reduce the gas flow into the combustion chamber. This has helped but there are other things to be done. I may have to look at the valve timing.
• (2) I was hoping the crank case would breathe through the front bearing and to some extent it does, but its not enough so I've incorporated a breather nipple to replace one of the gearbox retaining screws. I had to drill the existing tapped hole in the crank case through to allow the pressure to release.
• (3) The new crankshaft I'm going to build will have a sliding fit on the rear bearings to allow for the differing rates at which aluminium (crank case) and steel (crank shaft) expand and contract with temperature change. I think the existing set-up was trying to keep things a bit too rigid and bending of the crankshaft was taking place under high temperature resulting in an annular crack in the front crank journal, and eventually leading to its failure.

The picture says it all.

Crack detail.

You can see where the crack started to form around the outside and worked its way to the centre. Click on the photo and it should enlarge.

My first job is to retrieve the gear and bearing from the rear part of the shaft. Its probably best to drill the shaft out of the gear and do the same for the bearing. It's a good thing I've got plenty of stock material left over.