Crack Kills

[mulro767]

Well this crack just killed my high spirits and general good mood. Just found on my IO-360 at annual on the engine case. I guess it's better to discover in the shop than at 500' agl after takeoff.

1280 since overhaul in 1993. I guess it did ok for a 33 year old engine. And I'm looking forward to a factory reman and separate mags.

Now back to positive energy and happiness.

[MB65E]

Bummer! #2 cyl? Looks like it was welded long ago. I bet half of the 200hp guys have delt with the same issue.

Good luck!

-Matt

[N201MKTurbo]

I've had two cracked crankcases in 5000 hours of Mooney ownership.

 

They can send you into a severe depression.

 

I R&R the engines myself and then give them to an engine shop. Still costs 4 to 5 AMUs to fix.

 

Give the engine shop another AMU or two to overhaul the cylinders and call it an overhaul.

[RobertE]

What was your first indication of a problem? Or we're there no symptoms but an eagle-eyed A&P found it at annual?

[kortopates]

They're usually pretty obvious by the trail of leaking oil. Doesn't usually take long before the belly is covered from oil.

[DaV8or]

That sucks!! The first plane I did a pre buy inspection on, we found a crack like that. It was not a good day for either the seller, or me. The logs for my plane show that it happened to my engine too. It was welded and I'm flying on it now. Both were in the same place on the same cylinder. The top front of number 2. Many others have had cracks in the case. We even had a guy on here where it went real bad and the cylinder came off.

 

I never ever hear of the case by the back cylinders ever cracking, yet people swear up and down that shock cooling is a myth. It always seems to be the front. I don't really want to open that pointless debate, but I personally avoid rapid cooling of the cylinders whenever possible. It might be all bull crap, but IMO, there are way too many cracks to ignore the problem and just chalk it up to random bad castings.

[N177MC]

 

 

I never ever hear of the case by the back cylinders ever cracking, yet people swear up and down that shock cooling is a myth.

 

What makes you think that cracks in the crankcase would even remotely be associated with shock cooling - or not, for that matter ?

 

The fact that the Lycoming wide-deck, angle valve crank cased tend to crack near the fron cylinders has everything to do with the internal stresses and NOTHING to do with cylinder operating temps.

 

Michael - A&P/IA

[carusoam]

Michael,

Any idea what would cause the internal stresses to be causing cracks at the front, more often than the back?

David is indicating that it is not as random as one may believe.

Short of not using full power (full torque) on T/O, is there anything that can be done to minimize the risk of this type of crack?

Thinking out loud,

-a-

[drbob]

Vans [VAF Forums] 

            has some good discussion abt. this. search:

Lycoming smiley crack # 2 cyl. flange.

  apparently WD crack more than the ND even though their heavier & stronger.

Wiliamsport probably has a very good idea why this would be.

[DaV8or]

What makes you think that cracks in the crankcase would even remotely be associated with shock cooling - or not, for that matter ?

 

The fact that the Lycoming wide-deck, angle valve crank cased tend to crack near the fron cylinders has everything to do with the internal stresses and NOTHING to do with cylinder operating temps.

 

Michael - A&P/IA

 

Ummmm... gee, let's see, maybe I might think that because the front of the engine is where all the cold air is and the back is where all the warm air is? I see a lot people talk about cracks at the front and not so much at the back? I don't have any scientific evidence or proof of this theory being correct, but in light of the fact that I have never seen any proof to refute my theory, I'll stick with the intuitive OWT.

 

Please do tell all about your internal stresses. I can't say I've seen any data on this and would love to learn.

[MB65E]

My guess is that it's just a 200hp horizontally opposed engine trying to pull itself apart. Each cylinder is pulling 50hp. The impulses that do occurr are isolated in a very narrow location. Contributing to it could be the fatigue in the casting from the cycles. High time cases that have cracked will eventually crack again. Also, if a cylinder is replaced, the individual cylinder torquing procedure is not the same as it is when the case halves are put together the first time. This will add stress to the case too. I think there is more support at the rear of the engine with the accessory case and engine mounts supporting the crank case better. Hopefully, a fully balanced crankshaft rods and Pistons will lessen the cracking. Anyone with a 3 blades prop have cracks in the case? Im just wondering if the crank stress will be better distributed with the 3 blades or maybe a wood prop (MT).

-Matt

[drbob]

Search:

            LPM case cracks,

apparently both 4&6 cyl WD concentrates stresses around #2 more than ND.

Somewhere it was stated that of 10 cases cracked  7 = WD 3= ND.

     Would MT prop help with powerful L-R pression stresses?

   Good question

[N601RX]

I was at a lycoming seminar a couple of years ago where a discussion about case cracking came up. Lycoming said they had put considerable research into this and could say that much of the cracking was due to improper torquing. They said one of the contributing factors was removing more than one cylinder and then retorquing them using the single cylinder torque sequence instead of the multi cylinder sequence. They said the multi cylinder sequence was developed by mounting strain sensors in areas prone to crack and developing a sequence that minimized strain in these areas. It's also important to lub the threads before torquing, otherwise the clamping forces will not be what they should be for the given torque.

[DaV8or]

I was at a lycoming seminar a couple of years ago where a discussion about case cracking came up. Lycoming said they had put considerable research into this and could say that much of the cracking was due to improper torquing. They said one of the contributing factors was removing more than one cylinder and then retorquing them using the single cylinder torque sequence instead of the multi cylinder sequence. They said the multi cylinder sequence was developed by mounting strain sensors in areas prone to crack and developing a sequence that minimized strain in these areas. It's also important to lub the threads before torquing, otherwise the clamping forces will not be what they should be for the given torque.

 

Sounds reasonable, but why are there failures at the front of the engine and not the back? I'm guessing that mechanics out there are also using improper technique in torquing the rear cylinders as well.

 

BTW, just curious, does Lycoming then recommend that if you pull one jug off for service, that when you put it back, you loosen and re-torque the nuts on all cylinders? Are there any mechanics out there doing this?

[DaV8or]

Search:

            LPM case cracks,

apparently both 4&6 cyl WD concentrates stresses around #2 more than ND.

Somewhere it was stated that of 10 cases cracked  7 = WD 3= ND.

     Would MT prop help with powerful L-R pression stresses?

   Good question

 

So if the WD cases crack a lot at number 2 and it is well known, why doesn't Lycoming, or and aftermarket company just beef up the case around number 2?

[cujet]

The truth about Lycoming case cracks: 

 

Some of the cylinders studs pass completely through the crankcase. They are foot long studs with a nut on each side. They are stretched in tension and are very fatigue tolerant. Any Lycoming mechanic knows how to torque these. Because they don't rely on case strength for cylinder retention, these are trouble free. 

 

Some of the cylinder studs are very short (about 2 inches) and simply thread into the case under the cylinder. These do not pass through the case, due to potential interference issues. They simply rely on case strength for structural integrity. This is where the case cracks form. It's a terrible design from an engineering point of view. 

 

Look at the lower left stud, vs. the lower right "through" stud. The stress placed on the case is non symmetrical. Interestingly, engineers knew about short cylinder stud failures just after WW-1. As there is not enough stretch in the studs to prevent fatigue failure, no matter how large they were made. In this example, the case simply flexes until failure. 

 

[drbob]

They did beef up the cases,

           the Lyc WD is a beefed up ND,

   so, for their effort, more cracking--statistically,

Lycoming GPU's IIRC didn't have this problem, but then again,

     they didn't have large rotating masses creating huge precession forces in

      THE FRONT OF THE ENGINE.    Just my opinion, see disclamer-- 

                                                                                       drbob 

   

         please note that past performance is no guarantee of future performance & or results, so your on your own!

[KSMooniac]

Sometimes adding more mass to an area just attracts more load, and creates more problems.  It is a complicated issue, made even more complicated by variable cylinder torquing practices as N601RX pointed out.  I believe the heavy and not-precisely-balanced rotating mass on the front of the engine puts the biggest stress on the area in front of the cylinders that supports that load... the back end is constrained nicely by the engine mount, and the inertial loads from the prop are likely damped-out by the time they get to the back of the engine case.

 

To minimize the potential, read and follow the manual when putting one together, and get your rotating parts balanced precisely.  Getting your cylinders flow-matched to ensure equal power output is also a good thing.  A balanced engine is a smooth engine, and  a smooth engine is a happy engine.    

[N601RX]

Sounds reasonable, but why are there failures at the front of the engine and not the back? I'm guessing that mechanics out there are also using improper technique in torquing the rear cylinders as well.

 

BTW, just curious, does Lycoming then recommend that if you pull one jug off for service, that when you put it back, you loosen and re-torque the nuts on all cylinders? Are there any mechanics out there doing this?

They provide a separate procedure for torquing a single cylinder vs multiple cylinders. They also have torque plates that may be installed when a cylinder is removed or when pulling the case halves together. 

 

Another thing that hasn't been mentioned is the starter.  It mounts on same side of the case and same general area of the case. If the engine ever kicks back, it puts a lot of extra stress in this area.

[N177MC]

Sometimes adding more mass to an area just attracts more load, and creates more problems.  It is a complicated issue, made even more complicated by variable cylinder torquing practices as N601RX pointed out.  I believe the heavy and not-precisely-balanced rotating mass on the front of the engine puts the biggest stress on the area in front of the cylinders that supports that load... the back end is constrained nicely by the engine mount, and the inertial loads from the prop are likely damped-out by the time they get to the back of the engine case.

 

To minimize the potential, read and follow the manual when putting one together, and get your rotating parts balanced precisely.  Getting your cylinders flow-matched to ensure equal power output is also a good thing.  A balanced engine is a smooth engine, and  a smooth engine is a happy engine.  

 

+1

 

Thanks Scott, saves me from having to write same ! 

 

Michael

A&P/IA