Center main bearing turn during a top overhaul

[Skybrd]

I've been thinking about replacing all 4 cylinders on my aging Lycoming engine with a top overhaul. I found out today that there is a possibility that the center main bearing can get turned when the center crankcase bolts are relieved of stress by taking out the bolts. Is this something that you know of a procedure that can keep this from happening. I was told if the bearing gets turned then the oil pressure journal to the crank won't work and the engine will fail quickly. That would sure ruin your day with a catastropic stoppage of the engine in flight. Another option to a top overhaul is a complete engine exchange. I got a quote today: 29,000 for an overhauled and about 32,000 for a rebuilt, plus about 15,000 for core. 

[danb35]

Is there something wrong with each of the four cylinders?  Or is the engine telling you it needs an overhaul?  A third option is that if one or two cylinders is having a problem, repair or replace the cylinder(s) with a problem.

[N601RX]

Lycoming makes a set of torque plates to put on as you remove each cylinder to prevent this from happening. They are expensive, but if you have a set of old cylinders you can make a set by cutting them off a couple of inches above the base nuts. I used a set when joining my 2 case halves back together at overhaul. The torque pattern is also different for installing 4 cylinders or a single cylinder.

A talked to Lycoming about the bearing spinning and they said that would not happen with the dowel and tang holding it in place. They did say they were concerned about the bearing halves shifting slightly and changing the wear pattern that was already established. 

[jetdriven]

Yes, Cirrus have crashed up to 200 hours after doing a "top" becasue of bearing shift. Mike Busch talked about this in one of his columns. He considers one cylinder replacement a job, but a whole top an invasive procedure. I would replace the offending cylinders only.   Busch said replace one at a time to prevent main bearing torque loosening. IDK if it applies the same to Lycoming engines.

 

Also, there is no core difference charge from Lycoming for a two-mag A3B6 engine and the cost for it is 27,500$ (1700$ cheaper).   Also, a free roller cam.  We went with factory for that reason, rather than a field overhaul.

[Skybrd]

Quote: jetdriven

Yes, Cirrus have crashed up to 200 hours after doing a "top" becasue of bearing shift. Mike Busch talked about this in one of his columns. He considers one cylinder replacement a job, but a whole top an invasive procedure. I would replace the offending cylinders only.   Busch said replace one at a time to prevent main bearing torque loosening. IDK if it applies the same to Lycoming engines.

 

Also, there is no core difference charge from Lycoming for a two-mag A3B6 engine and the cost for it is 27,500$ (1700$ cheaper).   Also, a free roller cam.  We went with factory for that reason, rather than a field overhaul.

[jetdriven]

I would want more proof before spending 10-12K on a set of cylinders.  If the compressions are in the 70s and the oil does not get tar black in ten hours there is not excessive blowby.

 

Regarding the rolller cam, I am still trying to find evidence of ONE SINGLE failure of a roller cam or lifters.  I called tech support at Lycoming and they have had two, back in 2005, and the lifter was redesigned.  However, there are 4 M20Js on this board right now with spalled cams.  Lycoming is the source for the information, and some engines such as the Duke engine or the O-235 were never converted to the roller. But none are going back.  Who told you that?

 

What about the oil filter assembly?  I will say that Quality Aircraft Accessories will send your dual mag back unserviceable if the housing is cracked or the breaker point cam is work.  The last one they managed to find after a six week search in Thailand. The cost of a dual mag 500-hr inspection was 500$ a year ago.  Now, 1200$.

[Skybrd]

Quote: jetdriven

I would want more proof before spending 10-12K on a set of cylinders.  If the compressions are in the 70s and the oil does not get tar black in ten hours there is not excessive blowby.

 

Regarding the rolller cam, I am still trying to find evidence of ONE SINGLE failure of a roller cam or lifters.  I called tech support at Lycoming and they have had two, back in 2005, and the lifter was redesigned.  However, there are 4 M20Js on this board right now with spalled cams.  Lycoming is the source for the information, and some engines such as the Duke engine or the O-235 were never converted to the roller. But none are going back.  Who told you that?

 

What about the oil filter assembly?  I will say that Quality Aircraft Accessories will send your dual mag back unserviceable if the housing is cracked or the breaker point cam is work.  The last one they managed to find after a six week search in Thailand. The cost of a dual mag 500-hr inspection was 500$ a year ago.  Now, 1200$.

[jetdriven]

It sits between them. see attached photos.  I am not recommending an overhaul based on a hunch, but the only folks badmouthing the roller cam dont overhaul them and havent seen one fail. I heard it too.

[N601RX]

The longer 6 cylinders are probably more prone to bearing shifting when all the cylinders are removed.  Here is a picture of the torque plates we used to pull my engine halves back together at overhaul.  We removed these 1 at a time and replaced with a cylinder. My case was converted by Divco from press fit through bolts to dowels and the dowels are an extremely tight fit. They also have to all be seated at once where with the through bolts you can put them in one at a time.  The through bolts are now a slip fit through the hollow dowels.

There are a couple of other wear patterns that Lycoming likes to keep the same.  The rods are all installed with the numbers down and the pistons are installed with the writing inside the domes up at the factory. These parts are installed this way at the factory so there is a standard and can be reinstalled in the same orientation in the field when removed for work.

Lycoming tech support and engineering was very helpful durring this process. The one thing they did caution about was using a stack of washers to keep the tension on the throughbolts.  They said there had been a few cases of people doing this and the washer spun on the aluminum deck and damaged it as the nut was tightened up.

[danb35]

I'd agree completely with Byron--high oil temps, by themselves, are no reason to remove one cylinder, much less replace all four, much less overhaul the engine.

How high are your oil temps?  What's the OAT?  What speed do you climb at?  I normally climb at 120 mph, and in hot weather (95+ on the ground), sometimes my oil temps are uncomfortably high for me (220+).  Trimming nose down a bit to climb at 125-130 does quite a lot to bring down the temps.  It slows down the climb, of course, but it helps keep things cooler.

[MARZ]

Quote: jetdriven

It sits between them. see attached photos.  I am not recommending an overhaul based on a hunch, but the only folks badmouthing the roller cam dont overhaul them and havent seen one fail. I heard it too.