Yes, that is the one I was thinking about by Canardlover in Sweden.

I have increased the compression on my OS FS-26S today.

Stock compression: 7.8:1, now +/- 13.5:1 (haven't measured after the mod, will do tomorrow).
Took the liner to the machinist and he skimmed off 1mm the top lip (2mm thick).

First problem: he skimmed just a tad too much (0.10mm) and the piston was that much above it (no problem with the head), so I got a fine sandpaper and passed it on the piston.
Second problem: the intake valve strikes the piston on intake stroke, so i had to increase the rocker clearance for the intake valve.
After final assembly, I put it on my test bench and tryed with the only fuel I have for now, Model Technics Formula Irvine 5% nitro (18% oil, with 4% being castor and 14% klotz). The engine was extremely hard to start, unlike before! I had to resort to my trusty electric starter.
Observations: won't idle as low as before (over 3200rpm or quits), HS needle valve opened a quarter turn. Max rpm with a Aviomodelli/Graupner 10x4: 10300rpm and making some bubbles from the head gasket (needs a new one) due to high pressure. Couldn't hear any knocking.

Disassembled it and every part looks perfect, no signs of undue contact between parts, everything well lubed up!
Will make some 0 nitro fuel and test it again as soon as I can. In the worst case, I just have to buy a new liner and reset the valves.

See ya

Nice experiment. Do you think it would be possible to obtain a better idle from adjusting the mixtures?

I still need more patience to tune the carb properly, 'cos it's an airbleed carb and i simply hate it.... But i only managed a better idle (still high) with the airbleed screw almost out of the carb body!
I went to my workshop an hour ago and picked up the engine, just for picking it up sake. Then I noticed that a notch on the engine block is touching the head lip, and that's for sure why it doesn't seal up properly. I'll try adding a second gasket and report.
It's always interesting fiddling with different engines to learn and for others to have a "base" if one wants to "mess around" It's part of our hobby!

Here a video of the engine before the mods, with the same prop and fuel taken 5 days ago:

http://www.youtube.com/watch?v=TXlmI9mXZyw

Definitely take a video of it running once you get the leak solved.

Solved the leak - sort of. I improvised a gasket from a beer can, but i need to get a decent one, or - better yet - one made from paper, like the ones used in big carbs, because it still leaks a bit with the two gaskets (both were needed).
I had to remove the lip from the head (seen on the attached pics) with my dremel.

Measured geometric compression: 12.6:1
See the youtube video for more info.

I will try 0% nitro this weekend, as I'm waiting for a fresh batch of fuel (82% methanol, 18% castor).


http://www.youtube.com/watch?v=imxKsiBj-Cg

Can the materials and methods be explained more to find out the displacements? I haven't a clue what to use or how to use it. I can measure anything with my mic's but what instruments to use using fluid measurements is beyond me.

The only thing I can think of to measure the head button volume would be a digital scale, but I dont think that would be the preferred method.

Using a calibrated syringe its as simple as filling the combustion chamber and noting how much fluid, I use motor oil, was displaced from the syringe.

They can read down to 0.01cc?

To measure the stroke I am assuming you use a bright light to determine when the port opens to determine where to start your depth measurement to the piston?

My 1cc syringe is calibrated 80 units or .0125cc increments.

If you want to measure effective compression ratio, you would do as you state above.

Thank you Greg, I am unfamiliar with the common medical "tools and units". I assumed a syringe but that increment seemed very minute, and led me to think of using liquid weight.

I'm going to have to try to find one of those syringes but maybe this is a good time to explain the method I've been using.

I use a length of clear plastic tubing with a bore of about 1/16" (much the same as fuel tubing) and around a metre long or shorter for small engines. I calibrated it by filling to the 1 metre length then pouring it into a medicine glass (a calibrated syringe would be even more accurate though). I kept filling and pouring until I had 50cc in the medicine glass. Then I divided that 50cc by the number of times I had to pour in 1 metre of fuel and that gave a volume in the tubing of 2.564cc per metre or .02564cc per cm. It's easy enough to regulate the drips out of the tube (a finger blocking the open end) to at least 5mm which is near enough .01cc.

In use I fill the tube with more than enough to fill a combustion chamber and then measure the length of fuel against a steel rule down to the nearest mm. Then I start dripping the fuel into the combustion chamber until it's dead level with the edge of the squish band (this of course is with the head off ). Then remeasure the length of fuel remaining in the tubing and the difference is how much was poured in. This difference is then multiplied by the 2.564cc/metre to give the answer.

It's not necessary to do this with the head off (if you do then you have to calculate the volume of the squish clearance as well) and with some engines, particularly older baffled piston types, it has to be done with the head on and then it gets filled to the bottom thread of the plug hole. But you have to be certain the piston is at exact TDC.

I still want one of those syringes though..