Certified engine crankshaft repair

The mechanics scratched their heads for a while then they pulled the engine off the airplane.

 

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I have several hundred hrs behind the 320 HAD ( 700/ 800, I don't remember) and never did like the dual mag with a single drive. Always in the back of my mind when IFR. Then there was the cam problem, but that was taken care of with the additive. I flew a 77 and 79.
https://cessnaowner.org/h2ad-engine/

 

There is an STC to put a roller cam in one and totally eliminate cam problems. The biggest problem today is there are no big item parts that can be had new. Everything has to be refurbished. It’s more like dealing with a vintage engine now.

I have to deal with the duel mag a lot in some of the higher hp oddball engines so I’m use to setting them up.

Cowling a H2AD is a little harder as it is taller in the spine. Lycoming probably would have expanded the line to 360s if they did not screw up as bad as they did. I think you can turn one in as a core for just about any engine back to Lycoming. They really want to destroy them all.

 

Been several years since I went on the factory and museum tour and I wonder if they ever included the 320-H2AD in the museum. If I was guessing, I would say not.
Friend of mine put one in the Pitts he built, don't know why except for the cheap price. I don't know the weight difference between the 320 and the 320-H2AD but the H2AD looks a lot heaver.

 

Thanks! I have seen those used before but never knew what they were called. When a surface is ground by a hard grinding wheel, even though the surface is visually smooth, and even smooth to a surface checking machine, there are directional peaks on the surface. The final polishing with the clamshell basically is the same as a quick polish with a belt sanding attachment used to just knock the crown off the peaks.

 

With the clamshell, pressure can be put more evenly around the journal over the belt just riding up and down. For the right crank they will be taking off less than a .001. It’s going to be about the skill of the machinist to get it right or it goes straight to .010. I have seen chatter in crank journals where the feed was too fast. Good enough to pass but polishing does not take them out all the way. Depends on how close everything is measured for that up or down decision.

 

When you watch the video, what you see is a cursory quick few turns of the crankshaft with abrasive paper pressed against the journal. There is no substantial removal (.001) of metal. I can tell you from experience that grinding with an abrasive wheel will produce the truist and most consistant surface. Moving to abrasive paper only attempts to remove microscopic surface abnormalities, not chatter. There is no control over shape when abrasive paper is applied with something like a clam shell holder. It has to rely on the existing surface already being true. If the surface of the journal is not true, continued use of a clam shell with abrasive paper will not make a true surface. It may improve the surface by making it smoother than it was and possibly remove some high spots if used long enough. If the journal is tapered, it will most likely be smoother, but still tapered. If barrel shaped or concave, it will stay that way. In the case of the chatter you mentioned, the speed of the belt sander and the abrasive grade along with the speed of the rotating part will affect the finish. If chatter marks are still present after belt sanding/clamshell sanding, the crankshaft should be turned to regain smoothness and trueness. To me it would never be acceptable to reinstall a crankshaft with any chatter mark present. You mention that the clam shell will be taking off less than .001 (one thousandth). That is a true statement, but actually it probably will be .0001 (1 tenth of a thousandth) and most likely less than that if done as shown in the video. If someone makes a concerted effort to remove a .001, it can be done. It will take a lot of time and abrasive paper though. On the attachment below they mention the "microscopic burrs" and their direction caused by the grinding operation. They also mention just how small they are and that when removing them with a belt that its best to turn the crank in the opposite direction than it will turn in the engine. If you look closely at your Lycoming video, you will notice that the crankshaft is being rotated in the opposite direction from when its installed in the engine. I realize that out in the field, mechanics will do what it takes to repair engines, and some of them have the older tools to work with. I have used a belt sander to polish an aircraft crankshaft. It worked fine as far as I know, but have no idea how long it worked. While I think something like that will work, and possibly for an extended time....I also feel that given the correct precision that grinding provides for bearing/oil/journal interaction, a precision ground journal will last longer and be more reliable.I know you have a good grasp of how things work together, but most people don't have a good perception of the smallness of the tolerances and how little variation it takes to upset proper interaction between the components.


While an engine may have say .002 of clearance, the important thing is the fourth illustration where the forces in the engine push the oil
out and only a minute film of oil remains. This oil film is so thin that the variations in the crankjournal finish may begin to contact the rod
or main bearing. Surface trueness is important as well as finish.

https://www.enginebuildermag.com/19...nals-on-the-crankshaft-are-properly-polished/

 

A grinding wheel is not going to guarantee round at .003. Although my crank was technically standard bearings, it was .002 out across all mains. You need more material to square up if you are going to hit it with a wheel. .003 is going to take a special crank for them to guarantee at .003. It’s going to be close to perfect off the bat.

 

The tech is running a piece of crocus cloth on the journal for 6 seconds. Nothing is being removed. Only polished.

 

I'm a little surprised that all four mains were consistantly out .002 . Usually there is some variation from end to end.

If a crank is reading .002 out of alignment, that means its .001 off center. First thing I would do is lap the center and see if that helps. Even if it doesn't help, the grinding wheel will have .002 on one side and .001 on the other side. If it was perfectly true you would only have .0015 on each side. It is technically possible to true a .002 crankshaft. (but not always as readings are not always
perfect when measuring......)

What you are left with is a choice between grinding the crankshaft true and hoping the surface irregularities clean up OR polishing the journal until .003 is removed but still not true.

If you grind and the surface doesn't clean up, you can polish a couple more tenths (.0002/.0003) and not hurt anything. If it doesn't clean up, then you have to go to the next undersize.
As you said, its not guaranteed.

If you simply polish the existing journal .003 undersize you retain the out of alignment and possibly incorporate some more irregularity along the surface. .003 is a lot to polish because the surface is hardened and you are using something that isn't ridgid to remove the metal. The belt itself can be irregular with high and low areas. It can't be dressed like a wheel. I wouldn't think the clam shell would be used to remove that much metal. More likely a belt would be used and then finished with a clam shell in the opposite direction. Notice that the belt in the video is not as wide as the journal and must be moved from side to side. (about 1:30 mark)

What you have is a conundrum and cost is the deciding factor. Not the cost of grinding .003 vs .010, but the loss of value of the crankshaft if you go to .010 and ever have another problem.
Thats long been one of my pet peeves with aviation engines. Cost overides other important considerations when performing engine repairs. You have a lot of experience and you can make a reasonable decision, but many others make bad decisions about reusing some parts because they don't have your experience. Will the next guy decide because his crank is just .003 (vs .002) out of alignment also decide to polish .003 instead of grind .010 . At that point the internal clearance between the eccentric journal and the bearing begins to be a problem.

 

Most of what you posted here was spot on, I just wanted to correct/add a bit here:

With new belt on the polisher, I can take .001 off a journal in about... idk, but less than 10 seconds. Wouldn't take long. With a used/ older belt (my preference to use for actual polishing work - I polish a junk crankshaft for 30 seconds with new belts before use) you can't really take metal off. The belt I use for polishing I could probably spend 5 minutes on one journal and not take off even .0001"

That said - I'd use a crank grinder if I had to remove more than .001" of metal, and I have no idea why I'd even need to remove .001". I either send cranks to the grinder if they're out of spec or polish them if they are in spec.

As far as belt polished cranks, well, they're in every engine I've ever built, over a hundred at this point. Some making triple their factory horsepower rating for many thousands of miles. No reason not to do it unless you aren't allowed for paperwork reasons, or because it's the wrong tool for the job.

 

Is there some sort of truing hone that would make it round?
Decades ago I hand honed several thousands on cylinder barrels with a Sunnen hone and a drill motor.

 

A hone is used internally, while a crankshaft grinder is for an external surface. In machining, making an external surface round is basically done two ways when using a grinding wheel.

First there is a "centerless grinder" which basically rolls a round stud shape between two wheels. The stud is pushed thru the machine and rubs against the two wheels. It isn't used for crankshaft journals but would work great for a pistons "wristpin". Generally these are parts that have no need for a "center" location. They only need to be a certain size and a very fine finish. Usually the machinist is working with a tolerance inside .0002 variation. Normally a lathe gets the part close (maybe .001) and the grinder makes it smooth and the right size.



A "cylindrical grinder" holds a part between two centers for grinding and is used for straight shafts where the outer diameter needs to be concentric to an inner diameter.



A crankshaft grinder must deal with not only main bearing journals which are in a straight line like the cylindrical grinder, but must also deal with rod journals which rotate about the main journal. Notice in the video that when he starts making sparks, they are intermittant which means out of round or out of concentricty (or both). Watch the guage reading at the end of the video.



and if you want to spend the time, here are the journals being ground. Just the first 2:00 minutes

Last is an "internal grinder" that can grind cylinders and move the centerline by mounting it off center.



Note: All these grinding wheels are dressed true with a diamond tipped tool to insure smoothness and true.

Hope this adds some info for everyones gray matter bins........

 

At that point he maybe polishing. The tool was the question. If it had stone faces, it would grind just fine. It does not have to be that tool to get the information. Getting too literal. The tools are out there. Rare as hens teeth now. Like lifter facers. All shops use to have one. None now. My first shop I used was a third generation shop from the 10s. They still had available the belt driven tools with a big 8 ft motor winding that looked like a wagon wheel. They still used the cylinder grinder if you would pay for it. It really made round cylinders. It made engines run great. All that stuff was huge. They did a lot of pre war cars. Babbit and such. Third gen opened up a different business and junked it all but what second gen took home to do in retirement. The shop I go to now has been there since the 40s but most stuff is new inside.

 

I was referring to doing repairs without machines. I seem to recall a device to true a rod bearing in the car without removing the engine. Just drop the oil pan.
Couldn't find it, but this guy talks about hand repair with an oilstone http://www.enginemechanics.tpub.com/14076/css/Repairing-Crankshafts-And-Journal-Bearings-71.htm

Some sort of clamshell hone like the clamshell polisher could be made.

 

Grinding a crank without removing it from a motor might work fine in something with BIG clearances... but anything modern with 1-3 thou of oil clearance and you can kiss the other bearings goodbye in a hurry.

No idea how you'd spin it enough times without pumping oil through the rest and making a colossal mess, etc...

That seems like something where it would be easier to remove the engine and do it right than to try that.

I still use a "drill" style sunnen hone. Not as fast as modern stuff, but just as accurate if you are willing to spend your time with it.

Winginnitt mentioned moving bores - That can be done with my old school boring bar as well, did a mini for a customer to put extra big pistons in it that wouldn't fit otherwise.

Can also move crankshaft journals with the crank grinder if you know what you're doing, to get a little bit more stroke.

 

Yes,I believe there were machines that ground crankshafts in cars, but I have never seen one. There are lots of reasons why that is not a good practice to follow. I can see where third world countries like Cuba might have uses for them, but it's not something I would trust . Don't know how they would flush it with soluble oil to cool and remove the grind slurry. Don't know how they could dress the wheel true or align it square to the journal. Never saw one and don't know how they work, but just can't imagine them being very accurate. You couldn't use one on a Lyc.

I like the old tools that were used by many different tradesmen. The move to automation has provided a windfall of these precise but slow tools to home shops as well as many small AP field shops. Not just machine tools but electrical,welding, and sheetmetal. The old hand operated swedge tools still command high dollars. There is one on eBay right now for about $3k. I've got a Sunnen hone and a set of hones for use in a lathe. Got a Clausing 15" swing lathe, and a 10 ft leaf brake. Bought them all from industrial auctions. The old tools were just as precise as the N/C stuff but much slower. Also the N/C can cut irregular shapes and patterns much easier. For a home shop the old tools work just fine for many things. A friend of mine has one of the old lifter grinders and he let me grind some lifters with it. That's one reason I like alternative engines. Lifters and even crankshafts are cheap enough you don't have to use something questionable.

 

Aircraft aircooled engines are not very close fits. For example, the service limit on a O-200 main bearing is .0055" loose. That's the legal service limit which allows reassembly if desired. It can be looser. They will run with even looser parts for years.
A homebuilder could work with such loose fit numbers. It is not a modern car engine.
The A-40 doesn't even have a center main bearing at all! So the crank wallows and wears the bearings in less than 250 hours. Still considered to be the original engine that made private aviation possible.

 

There are things that can be done but probably shouldn't be done. One thing to remember is that resistance to flow creates oil pressure. When main bearings have excessive clearance the oil will flow with virtually no resistance and allow other areas of the engine to possibly starve for oil. If someone knows they have that much clearance, it would seem logical that they at least polish the crank and use an undersize bearing when reassembling.
https://www.melling.com/videos/oil-pump-pressure-vs-flow-melling-engine-parts/

 

Well sure. Say the crank is already worn .002". It might be simple to hone/polish off another .001" and fit the .003 thicker bearings.