Not much left to do

Two holes to be drilled and tapped M1.6 in the bottom of each conrod and then the shells have to be made. Like the small ends, the big end shells are made from Phosphor Bronze. Two flat pieces are soft soldered together, held in a four jaw chuck and drilled (under size at this stage) on the joint. Then the outside diameter is turned to size and parted off. Warm gently to melt the solder and split the shells. Fit them in the big ends and tighten with the M1.6 screws, then set up on the mill and bore the big ends to size. Job done. Then its a matter of fitting it all together and seeing if the engine runs.

To give you an idea of the size, the distance between centres is 29.0mm.

Conrods with small end bearings fitted

A drawing error - oops! conrods

Its about time I blogged some engineering, so I've done the following:-
Some how I cocked up. Its not the first time, nor will it be the last. Getting down to it, I designed, drew and made the conrods too short. The piston was unable to pass bottom dead centre. Oh! dear, how sad, never mind. But now I have two new conrods of the correct (longer) length. This also means I'll have to remove some more metal from inside the cylinder heads to maintain the correct compression ratio.

Ahh! Bottom dead centre

Conrods increased in length by 0.5mm

I went flying today, and it was a sad day. I lost an old friend. Yes the red Sunbeam met its match or rather terra firma. It was pilot error, I heaved in too much elevator and the wings folded and it went down into the bean field. Engine and radio are OK but the airframe has had it. At least I know it was my own fault.

Conrods and a spinner nut.

Far too cold and windy to go flying today, so more on the KT120fst.

Getting there with the conrods, just the small end bushes to make and fit. After press fitting they'll need to be faced to length and reamered to size.

Another little project I made for my mate Geoff - a spinner nut tapped 1/4" UNF.

Conrods

The fixture I made earlier for machining the conrods is very successful.

Just the thickness and central section left to machine. To give you an idea of the actual size, the bores are 28.5mm between centres. Next is to turn up the phosphor bronze bearings and press fit them into the bores. I'll warm up the aluminium first to ensure a tight fit. Then I'll finish bore the holes to 6mm dia. to suit the crankshaft and the other end to 5mm dia. to fit the gudgeon pin.

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.