Cox Surestart 049 vibration

Good thoughts so far. I swapped between two bottom ends. One with Davis crankshaft & the other with regular Cox crankshaft. Both were about the same for vibration, except that the engine came good with the Davis crankshaft when running at higher speeds - 21,000RPM plus. I'd never tried that before

The easiest next step will be remounting the engine in another way. I'll try that.

Maris

MarisD wrote:Good thoughts so far. I swapped between two bottom ends. One with Davis crankshaft & the other with regular Cox crankshaft. Both were about the same for vibration, except that the engine came good with the Davis crankshaft when running at higher speeds - 21,000RPM plus. I'd never tried that before

The easiest next step will be remounting the engine in another way. I'll try that.

Maris

A few weeks ago I mounted a junk box .049 Black Widow built from the best parts of at least 7 engines with a PT-19 prop hub and stud and rubber spinner to hold a stone stock 5X3 Rubber Ducky Safety Tip prop on a Sig Dewey Bird. For the first half of a tank of fuel it sounded like a real strong BW then it went into another octave. Didn't have a tach but ran two tanks thru it screaming... A few days later with a friend I ran it again and tached 21,000 rpms with a brand new tach, Went thru 2 more tanks of fuel with no vibrations
whatsoever with no balancing anything while still on the Dewey Bird.I had no out of balance issues at all with basically stock used BW parts.
I guess the engine just fell together unusually well or I'm just fortunate. However when I torn it down just to check everything before I ran it again, the stock BW rod was about to fall out of the piston! That would been fatal vibration I suppose......Steve

I had a tiny speck of gasket trapped on one engine between the fuel tank and the crankcase once that caused all kinds of headache. The head looked like it was wobbling 1/4-1/2 inch and it would not come up to full RPM. After a quick tear down and cleaning of surfaces I put it back together and the engine acted properly.

Phil

Problem solved by bolting the engine to a solid piece of hardwood clamped in a large vise. I put my Black widow tih Surestart back door through a range of test props all the way to 25,600 with the tiny Cox 4.5x2 prop. Smooth running throughout & it didn't seem at all fussed by those high RPM. I didn't know Cox reed valve 049's were capable of that!

Maris

They aren't meant to with the smallest recommended prop, a 5x3. The splined tip of the crankshaft will break off at those RPMs. If you want to run it at high RPM with smaller props, you should probably put a Killer Bee Crankshaft in it. Same as a diesel crank. Watch out for ones that come from unknown sources as there is a batch out there machined with crooked screw holes. That'll get your vibes going. Ex Model Engines or Cox International will sell you the straight cranks.

Of course those very high RPM were only to check how BHP drops off past its peak around 22-24K RPM with this engine. An APC 5x3 spun at a bit under 20K RPM, so would be about right in the air. Obviously the crankshafts that you mention have a stronger crankweb, but the splined bit up front is the same as the regular one. So should not affect the strength up front?

Maris

MarisD wrote:Of course those very high RPM were only to check how BHP drops off past its peak around 22-24K RPM with this engine. An APC 5x3 spun at a bit under 20K RPM, so would be about right in the air. Obviously the crankshafts that you mention have a stronger crankweb, but the splined bit up front is the same as the regular one. So should not affect the strength up front?

Maris

The KB cranks being manufactured now aren't made of the same steel as the regular Bees. Bernie from Cox International has talked about it, but I can't give you the details. If you use the Google search box at the top left and search the forums for Killer Bee Crankshafts, you might find it. He's on vacation this week though. Matt from Ex Model Engines is around here every day, he should know too.

Jason _WI reported the same vibration problem in the Tapered car crankshaft thread. Seems my experience was not the only one with that engine test mount. IT workes a treat with my AME 049 & probably fime with the smaller Cox engines too.

Maris