More Diesel

Up / Terug

Another example of a internet transaction: two1.5 cc engines picked out of a large batch that mainly consisted of larger R/C glow motors. The unscathed example is a German Webra Record, the battered one is a British Davies Charlton Sabre. These diesels date back to the 1950's and 60's.

The Sabre's scarred head is mounted on a power drill and rubbed with emery cloth to remove the nasty burrs. The red anodized finish, or what was left of it, is sacrificed in the process.

The Sabre could not be turned over because the initial compression setting was far too high. To fix this, the cylinder was dismantled and he contra piston driven back with a dowel. After a brief test run the engine ground to a halt and inspection showed that the crankshaft's crank pin had succumbed to over-compression.

The crankshaft cannot be repaired and will have to be replaced. The connecting rod will definitely have to be swapped in consequence and the piston and wrist pin will need careful inspection. Fortunately, I know someone with spare parts and hold a few myself.

The restored DC Sabre starts and runs but cannot compete with contemporary engines like the Webra Record and is simply not good enough to power a simple control line plane. So it ended up in a box, waiting for a good idea to come along.

3 Years later ...

After reading a few discussions about modifying DC Sabres I decided to give it a try and started off by improving the balance of the crankshaft. This change, and all others below, can be achieved by “kitchen table engineering”, i.e. No elaborate machine tools are required.

Using a simple grinding wheel (mine is hand-operated!) the crank web is changed from round to triangular.

The 0,5 gram reduction is small but at high rpm it becomes important.

The DC Sabre's piston is very massive at 7 grams. I have seen replacements that weigh only 2 grams! Using a Dremel tool and not taking too many risks, some excess cast iron was removed from this example.

Although it is possible to reduce the piston's mass even more than was done in this case, the operation is limited by the means at one's disposal – my grinding burr was rather large and did not have the ideal shape.

Without reducing gudgeon pin bearing area, the piston's mass came down from 7 to 6 grams.

A Australian correspondent recommended this timing improvement. Note that the intake hole is only enlarged to one side (right side in this image – see yellow arrowhead).

Looking at the transfer ports, you wonder how high speed gas movement is at all possible with the bends that need to be taken.

The transfer ports are simply cut into the cylinder wall with no effort made to smoothen the passage of the gas passing to and through them.

The outside cylinder wall is filed down at an angle below each transfer port, so as to create a streamlined approach for the fresh gas mixture.

Another notorious shortcoming often seen with DC Sabre engines is the gap between the cylinder head and the cylinder itself, which results in poor heat transfer i.e. poor cylinder cooling. This can lead to the engine slowing down and stopping. A shim cut from a drink can and folded around the cylinder before the head is screwed on, is a quick fix.

Although the upper crankcase is a loose scarf around the cylinder exhaust ports it appears to blank them off and so an additional opening was made to hopefully improve exhaust gas flow.

The lessons learned were that compression adjustment should not require excessive force and that the crankcase modifications cannot go too far!

Well, fortunately a spare crankcase was available! A less extensive change was made to this one.

No more washers between the spinner and the prop: the crankshaft is ground down 5 mm and the tommy bar now passes through the spinner hole. In order to fit into a Webra Record engine bay, the excess length of the crankcase bolts is filed away.

The mounting lugs were also adapted to the Webra Record swap. This may seem drastic, but the original holes would not even accept M3 bolts.

Bench runs are promising. The result of the various modifications is an engine that runs faster than the stock DC Sabre. I look forward to bolting it to my testbed CL plane to see how it performs in the air but first the needle valve needs shortening and a impact support.

Well, perhaps it is better not to balance the crankshaft after all!