Engine problem on Allison, cracked liner

[Bill Greenwood]

On one of the Glacier Girl sites someone asked about a cracked liner. I am not fully familiar with the Allison, but believe it is similar design to a Merlin. You can't just change a clyinder like on a radial, each side has 6 cylinders arranged in a bank, in 2 V shapes, rather than individually. The basic engine is aluminum alloy, the cylinder bores have steel liners inserted in them, changing the liner would invlove a dissasembly of the engine from the crankcase up. The coolant mixture of water and glycol circulates around the outside of the liner, and a crack would probably make a leak, maybe some further problems. There are lots of folks who know more about this, but they are probably busy fixing some engine rather than on a computer.

[Shay]

These are different Allisons than the original ones that GG was restored with? I thought I heard that they were changed out for this trip?

Anybody have the correct info?

Shay
_____________
Semper Fortis

[Forgotten Field]

Bill,
Thanks for the better explanation. I agree with you that the experts are probably out there building engines instead of gabbing here.

Any insight into how the cracks get started? I am guessing differential heating and cooling rates in the water flow scheme. Does this happen to Merlin's? My only hands-on with water-cooled aircraft engines was with an OX-5 I had in my possession two years ago. I've never looked clinically at a cut-away Merlin or Allison, and am just curious. Is there one at KOSH that we could look at and make wholly amateur guesses at causes of this cracking process? Thanks again. See you at KOSH, KYIP, and maybe Geneseo.

[Dan K]

Here's your Merlin:

http://www.rrocwa.com/RRTour/merlin.jpg

As far as I can remember (which isn't very far), the museum at OSH has a few real, err, radial engines on display, but no V's.

[bdk]

Any insight into how the cracks get started?

The last Allison parts were built when- maybe some 50 years ago? The manufacturing, inspection, and processing (heat treating) methods are far more repeatable now than they were then.

Also, these engines were esentially designed and built during a time of war for limited life applications. When was the Allison used commercially? Never? Engines frequently didn't reach TBO (if there was even one published) because the airplane wasn't expected to last that long. The Merlin was redesigned post-war to create the transport version for commercial applications.

Many of the radials had long post-war commercial lives and were well refined in commercial service.

[Jesse C.]

We run a top fuel block in my buddy's dragster with steel liners and the biggest culprit in them failing is when oil slips between the block and the liner. The heat difference produces different expansion rates that can cause cracks or total failure, such as water entering the cylinder and causing the piston to hydraulic, lock up and throw a rod and bend valves.

Modern day aluminum engines use special sealing at the base of the sleeve to prevent leaks. Maybe its time to retrofit those old engines a bit here and there. Would not be cheap, but well worth it.

[SparrowV12]

The last Allison parts were built when- maybe some 50 years ago? The manufacturing, inspection, and processing (heat treating) methods are far more repeatable now than they were then.

That statement is not quite true BDK, the manufacturing processes during that time frame was VERY accurate. I've had people tell me they could beyond a doubt make a better G6 connecting rod with todays high tech metalurgy and processes not to mention the "new and improved" space age metals. So I said OK to one company and gave them a damaged rod as a sample to analyse. Guess what-- they COULD NOT improve upon the original part. There is also the story about having replacement sleeves made for the Centaurus radials. The critieria was given to the company and they said there was No Way they could hold tolerances like that. Funny, they could do it 50+ years ago but it can't be done today?!?! The liners on an Allison are a lot more difficult to remove than on a Merlin. The Allison liners are shrunk fit into the cylinder head and have a gland nut at the base of each liner that has a very high torque value. It requires special tooling and fixture's to pull Allison liners. Not to mention, an Allison liner is way thicker and heavier than it's counterpart Merlin liner. If the Allison liner is indeed cracked, it probably originated from an external flaw like a corosion pit or something of that nature.

Sparrow

[Forgotten Field]

Sparrow,
I don't want you to give up trade secrets, but can you explain why they didn't pull a cylinder on the Allison and return it to service rather than put a QEC on? My generic engine knowledge, probably more from hangar talk than anything else, is to leave a good bottom end in if it is running well, and change cylinders. Why is this not true for the Allison, or is this a professional choice on the part of the caretaker of the airplane? If it's one of those Captain Ron, "Nobody knows, boss, that's just the way it is..." answers, that's fine too.

[Pat]

Sparrow,
I don't want you to give up trade secrets, but can you explain why they didn't pull a cylinder on the Allison and return it to service rather than put a QEC on? My generic engine knowledge, probably more from hangar talk than anything else, is to leave a good bottom end in if it is running well, and change cylinders. Why is this not true for the Allison, or is this a professional choice on the part of the caretaker of the airplane? If it's one of those Captain Ron, "Nobody knows, boss, that's just the way it is..." answers, that's fine too.

Please read Bill's first post! You CAN'T pull individual cylinders off an Allison V-12...either the left bank of six cylinders, or the right bank of six cylinders...you're thinking in terms of radials or flat opposed engines (or OX-5's)...TOTALLY different animals. Go to www.missthriftway.com on the left side of the page, click on "PHOTOS- Engine"; this Allison was used in an old unlimited class hydroplane. Look at ALL of the photos...you should get a pretty good idea of what makes an Allison tick.

-Pat

[Forgotten Field]

Pat,
Thanks, now I understand. My typical reading error- missing the important bit that I am looking for.

[bdk]

That statement is not quite true BDK, the manufacturing processes during that time frame was VERY accurate. I've had people tell me they could beyond a doubt make a better G6 connecting rod with todays high tech metalurgy and processes not to mention the "new and improved" space age metals. So I said OK to one company and gave them a damaged rod as a sample to analyse. Guess what-- they COULD NOT improve upon the original part. There is also the story about having replacement sleeves made for the Centaurus radials. The critieria was given to the company and they said there was No Way they could hold tolerances like that. Funny, they could do it 50+ years ago but it can't be done today?!?!

Interesting anecdotes. I am involved with aerospace suppliers on a daily basis. Maybe there is no raw material readily available that is better, but you cannot tell me that flaw detection has not improved any in 50+ years. No forging house is going to pay to develop and qualify (analyze material properties and guarantee material strength allowables) for your small lot of parts. You couldn't even afford the mill run for a new material. Not only that, they likely don't want to accept the liability for your limited production parts.

You can hold any tolerances you want. The supplier may no-bid the job because they don't want to bother or know that you won't want connecting rods at $25K each. Why should they waste their time bidding a job like that? They make their profits on building products to traditional aerospace tolerances. The market is competetive enough now that they don't want to be sidetracked by small complicated jobs, they'll take the low hanging fruit every day. Wait until the next aerospace downturn and try again.

[SparrowV12]

First off that wasn't my point at all. My point was, in regards to the rods anyhow, was that the "60 year old junk" couldn't be improved upon. In regards to better material, unless you could find unobtainiam, the material that the G6 rod was made of was as good as the best modern material they could find in regards to grain structure etc,etc. The cost to have the parts made was quite reasonable I thought. My next question is this, have you ever seen a sleeve for a Bristol Centaurus radial to know what it is like and be able to understand the tolerances that are required of it? As with a lot of this old aviation engine stuff, believe me, it's not as easy to make as one would think and product liability is a big part to consider as well.

John, speaking from my point of view, sometimes it's just as easy to change the whole engine as it is a bank or two. Probably the line of thinking in regards to Glacier Girl is the fact they would want to use an engine as a whole that has already been proven as a package on the test cell or in some other use rather that having to "start over: so to speak proving a repaired engine,

Sparrow

[warbird51]

I've heard that they will swap out the offending engine with a spare engine that has been built up for GG. I think the work is planned for after the Legends show.

[bdk]

First off that wasn't my point at all. My point was, in regards to the rods anyhow, was that the "60 year old junk" couldn't be improved upon. In regards to better material, unless you could find unobtainiam, the material that the G6 rod was made of was as good as the best modern material they could find in regards to grain structure etc,etc. The cost to have the parts made was quite reasonable I thought. My next question is this, have you ever seen a sleeve for a Bristol Centaurus radial to know what it is like and be able to understand the tolerances that are required of it? As with a lot of this old aviation engine stuff, believe me, it's not as easy to make as one would think and product liability is a big part to consider as well.

4130 and 4140 steels have existed since the 1930s, so those are nothing new and they are still widely used in similar applications as an example. There are different materials now, not necessarily better, that have been developed for different applications that are of far higher strength. Metallic materials are all compromises of strength vs. ductility. That is just a fact of metallurgy.

Yes I've seen a Centaurus sleeve and I've flown an a few different Sea Furies all with the Centaurus engine. I don't know offhand what the tolerances are on the sleeve, but if there is a drawing it can be built. Complexity factors just add to cost/scrap rate. The number of tools, the number of setups, and the number of opportunities to introduce error into the process all add up. Never said it was easy, only that it could be done if someone was willing to spend the money. I'm sure there were a few tricks of the trade used on these components. The question is are you willing to be the Guinea Pig on the learning curve (which was conquered years ago but has now been forgetten as tribal knowledge)?

[airnutz]

John, speaking from my point of view, sometimes it's just as easy to change the whole engine as it is a bank or two. Probably the line of thinking in regards to Glacier Girl is the fact they would want to use an engine as a whole that has already been proven as a package on the test cell or in some other use rather that having to "start over: so to speak proving a repaired engine,

Sparrow

That's pretty much how I thought of the situation, Sparrow. Repairing something
out in the field in unfamiliar surroundings can be risky, prolonged, or frustrating. Better
to R&R a tested unit and get a better of guarantee GG's safe flight.