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TDC Pinch getting "worse"?

Old 10-11-2011, 11:13 AM
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Default TDC Pinch getting "worse"?

I purchased a 25FX for Q25 racing (so I guess that voids the warranty). I broke the engine in using PowerMaster 10/22 and an APC 8/5 prop for 32oz (8 4oz tank runs). The engine performed well spinning an APC 8/6 at just over 16K. I had run another 6 4oz tanks of 10% PowerMaster flight trimming never letting the engine get hot or run too lean.

On to race day...

The CD supplied PowerMaster 15% nitro 18% oil for the race. The first tank of fuel (practice laps), the engine ran as it had before with good power. The first race, the engine seemed to be down on power slightly but I chalked it up to a fresh engine vs well run in engines used by the rest of the competitors. The engine continued to loose power race by race. I finally withdrew since I was no where near competitive. I tached my engine and was surprised that I could not get over 15K with the APC 8/6 and 15% fuel since I had been getting well over 1000 more RPM 3 tanks earlier.
Inspecting my engine, I noticed that it is very difficult to turn the prop over at TDC. The piston squeeks and clicks as it's brought past TDC. Like a good ABC engine is supposed to, but 5 times worse. It does this even without a glow plug. I took the engine apart to see if I could find anything wrong with it, but I can not see anything visually. The piston and sleeve look very good. The top of the piston has some light brown staining on the top toward the exhaust port as you would expect.
One interesting thing I noticed is if the head bolts are just slightly loosened, the engine turns over easily. Snug the bolts down and the engine is VERY difficult to turn over -with a prop and without a glow plug.

Sooo what made the "pinch" worse? I know this may be impossible to determine without physically inspecting the engine.
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Old 10-13-2011, 06:07 AM
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Default RE: TDC Pinch getting

If, when the engine was apart, the piston rode smoothly up and down the cylinder, but was tight when you had the engine reassembled, you likely have a bent rod.

It's possible that the engine got warped at some point. Warping can be caused by a high-temp run followed by rapid cooling. Running under a high load at full bore and then suddenly chopping the throttle to descend and land may do that.

You may have some buildup of oil residue in the engine. This is particularly true if a castor-based fuel is used. The piston and liner would need to be cleaned with "four-ought" (#0000) steel wool and hot, soapy water. Rinse in water as hot as you can stand with the bare hands, shake off any water and let dry. You can use a rubbing alcohol rinse after that to help drying. Oil and reassemble immediately after the parts are dry.
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