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.

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.



Nearing completion, but its a lot of tedius work

The crank shaft is almost finished, its been hard work as only small cuts at a time can be taken so as not to strain the setting. Its only held between centres in the lathe and if a tool were to pick up it would be disaster. As it is I'm very pleased with it. I need to reduce the weight a little and I've been studying some pictures of the Saito twin cylinder crank shafts and the balance weights have been shaped as a semi diamond on both sides ie where the counter balance weight would be. So I'm going to do the same.

You can see what I mean by the diamond shape...

I need to cut the same on the other side.

There'll be no flying this weekend. Have you seen the weather forecast?

High winds and horizontal rain, sleet and hail. Just as well, I can get on with my engineering project. Today I've made a fixture to hold various sizes of conrod and I've set it up on the rotary table to machine one of the conrods for the KT120fst.

For the conrod I'm currently making, a 6mm dowell is required to locate one end on the centre line. For the other end a dowell with a 10mm diameter is used to locate in the slot of the fixture and a 7mm diameter to locate in the big end of the conrod. So now I can machine various sizes of small end/big end/hole centre combinations.

Fixture for machining conrods

The process begins

...And I've made a start on machining a conrod.

...And I've finished the crankshaft...

Alignment and concentricity end to end is a lot better than expected.

More next time.

No flying again today...

...but you wait, tomorrow will be glorious when I'm back at work - 'hit happens, aaargh!

So, into the machine shop for some more exciting swarf making. This engine I'm building is my own design, and I'm drawing it up on AutoCad before making the parts. Occasionally I have to go back to the drawing board to make alterations to the drawings because in practice, sometimes things don't quite turn out as you'd expect. Its called research and development or R & D. The engine I've named, the Ketro KT120fst. is 1.2 cu.in., 20cc swept volume and about 22cc full capacity. Its a horizontally opposed twin cylinder four stroke engine also known as a "Boxer".

Centre drilling position for the crank pin.


Having drilled the hole its now ready for reamering.



Finished, and its a good fit.

All the bearings arrived this week, so now I'll be able to finish the diameters on the crankshaft parts, as I will use each bearing as a gauge to test the fit.

All the parts together, nearly finished. Just the counter balance to mill and the holes for the clamping screws.