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HelicopterEngines Plugs Mufflers Fuel › Is crank web mod causing excess oil blow-by?
11-22-2006 01:29 PM  11 years agoPost 1
JerryLu

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Gretton, Northants - England

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A while back my OS50 SX-H rear bearing started grumbling so replacement of both bearings with ceramics was done. At the same time I did the crank web mod but did not notice any improvement in vibration.

The original front bearing, though good was replaced and I noticed on the new ceramic one that I could see right through. There was a gap between the shield and the inner race which was not apparent on the original bearing.

On dialing in the start shaft (Sceadu) it seemed there was axial play in the bearings as I could move the shaft tip up and down and feel some movement at the crank pin

Well, I set up the shaft allowing for this and there did not seem to be excess vibration, but pretty soon I engaged in un-planned arrival at terra firma

Re-built the heli and been flying again but the engine, fan, frame and exhaust after a couple of flights are drenched in oil

I replaced the original front bearing but I still suffer lots of oil blow-by. After 2 flights, the exhaust was covered in burnt residue though it had been clean to start with.

Any ideas??

Jerry

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11-22-2006 01:36 PM  11 years agoPost 2
JuanRodriguez

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The Villages, Florida

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Jerry,

There are thousands of 50's out there running the "crank mod" without any oil "blowby" problems (including myself).

I would look elsewhere such as the muffler to exhaust port seal, or perhaps a damaged muffler pressure line due to the crash ?? I would replace it with a new line to be on the safe side...

I would also look for any damage to the other lines, as well as the fuel tank and stopper.... any leakage to those components will surely get fuel everywhere giving you the impression it's coming from someplace else....

Good luck...

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11-22-2006 01:44 PM  11 years agoPost 3
JerryLu

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Gretton, Northants - England

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You could be right there as I do not remember any excess oil before the prang. I think the exhaust joint is good as I seal with epoxy but the tank may be a possibility. Will check it out.

Thanks

Jerry

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11-22-2006 01:50 PM  11 years agoPost 4
Doug

rrElite Veteran

Port Saint Luice Florida....

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Even if the "shield" was touching it would not seal. The sealing is accomplished by a timed bleed (using the crank shaft rotary valve to insure negative pressure on the front bearing. If the engine has a damage in the crankcase behind the front bearing that could be the cause. All engine weep a bit out the front , some worse than others.

First member of Member of Bearings Anonymous

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11-22-2006 02:13 PM  11 years agoPost 5
JerryLu

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Gretton, Northants - England

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I am sure there is no engine damage and the carb is seated on the seals OK.

If it cannot blow oil out of the front bearing then I guess there must be another source such as the tank bung or pressure pipe. The last is OK I think and the tank bung is on the dry side of the heli so I remain baffled until I can check those possibilities.

Thanks for the ideas.

Jerry

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11-22-2006 05:26 PM  11 years agoPost 6
Doug

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Port Saint Luice Florida....

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If the crank or the case is damaged in the front sealing areas it can leak quite profusely out the front.

First member of Member of Bearings Anonymous

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11-22-2006 05:49 PM  11 years agoPost 7
DS 8717

rrProfessor

Here wishing i was somewhere else

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If you run the engine too long with a bad rear bearing it will let the crankshaft rub the inside of the crankacse and wear it down causing excessive clearance between the crank and case. This will cause the engine to blow fuel out the front bearing.DOUG

YOU ONLY LIVE ONCE..IF YOU LIVE IT RIGHT THATS ALL YOU NEED

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11-22-2006 07:34 PM  11 years agoPost 8
JerryLu

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Gretton, Northants - England

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Doug x 2 ??

Definitely neither is the cause. There is no damage to the crankcase either from prang or excessive wear in rear bearing. If the bearing had worn that much (which I agree could well cause the problem) then I think the big end would be shot too and I am still using that OK. I think the wear would be obvious from marks on the crank which is not the case.

Now I'm home, I will go look at the tank.

Done. Another check all round reveals nothing.

Tank bung and pressure nipple are tight. Blowing up the pressure pipe provides no evidence of leaks. Plug is tight, head and backplate are tight. There is a fillet of epoxy all around the muffler joint.

The fan and all inside the cooling duct is soaked with oil which is dripping (practically) off the throttle leaver. The muffler has been liberally sprayed with fuel leaving burnt on castor. This may be symptomatic of my change of fuel from Prosynth 2000 (whichn is 100% synthetic) to Model Techniks which has some castor.

Still no clue other than bearing blow-by

Jerry

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