Merlin Versus Allison Overhaul Time

[GregP]

In another thread I came across some interesting things in the USAAF Statistical Digest. In particular we have had some discussions in the past about the Allison V-1710 and the Merlin, more specifically the Packard Merlin V-1650. It so happens I came across Table 115 which details the average man hours expended per major engine overhauls in continental US-based maintenance depots from July 1943 through August 1945 on a monthly basis.

During the period the V-1650 required an average of 320.2 man hours per overhaul with a high of 592 hours and a low of 190 hours.

During the same period the V-1710 required and average of 191.5 man hours per overhaul with a high of 376 hours and a low of 117 hours.

Much has been said about the relative complexity, with the Allison having around 7,000 parts and the Merlin around 11,000 parts. It was said that the extra parts cannot have taken that much extra time. With the data above we can see that the Allison has 7,000 / 11,000, or 63.6% of the parts a Merlin has. Somewhat interestingly the average man hours required to overhaul an Allison amount to 191.5 / 320.2, or 59.8% of the average time required to overhaul a Merlin.

These numbers track so well that I think you can see the overhaul time is very proportional to the number of parts.

In Table 115, the average overhaul times, in order of increasing time, broke down as follows:

R-1340: 105.5 hours
R-1820: 129.1 hours
R-1830: 188.5 hours
V-1710: 191.5 hours
R-2600: 256.3 hours
V-1650: 320.2 hours
R-3350: 304.9 hours
R-2800: 329.2 hours

In the time it takes to overhaul 3 Merlins you can overhaul 5 Allisons. The data do NOT indicate anything about wearout since the Military overhauled their engines well before wearout could occur. Waiting for wearout would produce a lot of unnecessary crashes since none of the planes these engines powered flew very well in a power-off condition.

None of this indicates anything bad about the Merlin or infers anything good about the Allison, or vice versa. All it does is demonstrate that fewer parts are easier to overhaul.

We all probably knew that instinctively anyway, so this is just for information. I'd fly behind either one anytime.

 

[Shortround6]

I think part of the original argument was that the Merlin was less reliable or went shorter periods of time between overhauls because of the extra parts.

That would be hard to prove because the the number of "moving" parts of a Merlin or Allison are somewhat closer than the total number of parts.

Having to deal with more screws/ bolts/fasteners to get inside the engine and seal it back up are certainly going to affect the time it takes to perform the overhaul and not in the Merlins favor.

People used to joke that 1930s R-R cars were very reliable but it took 3 men, a boy and dog to perform the factory recommended routine maintenance.

 

[tomo pauk]

A question: what V-1710, and what V-1650?

 

[GregP]

Hi Shortround,

If I am not mistaken, the factory-recommended overhaul times were in the 250 - 500 hour range based on performance in service, with the earlier times being shorter than later times as experience was gained in the field. This was in military service, and almost none of the engines overhauled at that time would be overhauled today at the same time.

Today we do things like strain the oil into white cheesecloth and look for metal particles. We can also send our oil in for analysis in a lab. So we catch things when they happen and tend to avoid unnecessary overhaul due to expense.

I know some of the hours between overhauls for the Allisons that came out of Joe Yancey's shop, but only known a small handful of Merlins times to overhaul, so I have little to say about one or the other being more long-lived than the toher.

Another factor in time between shop visits is the work done. An example would be is Joe got in an Allison and the owner wanted the banks to be refreshed and no other work. In that circumstance, Jow would endeavor to do the work specified and would only split the crankcase if something was obviously wrong. So if he did that and the engine went into the shop again in 50 hours for main bearings, it was nothing Joe did ... not changing or even inspoecting them was the owner's choice.

Once he had an engine fail in flight and it was the cam drive gear inside the crankcase. Fortunately the plane was over an airfield and landed safely with no damage other than the engine. Joe didn't even build the crankcase ... he did the cylinder banks and accessory housing. So when the owner wanted himn to warrant the crankcase, he simply faxed over the work order from the same owner to Joe clearly stating to not touch the crankcase internals. No more arguments.

Of the engines Joe did in their entirety while I was there, and that was about 12 of them, two came back for warranty work. In both cases, the oil pressure was within spec but was about 5 - 8 pounds lower than the owner wanted. In one of those cases, the owner had flown an airshow aerobatic routine in the warbird with only 2.5 hours on the engine since overhaul! We watched the routine and shook our heads the whole time. We changed the mains and the pressure was back to nominal, and told the owner that if he did it again, he was on hs own ... the warranty would not ever cover an aeroibatic routine again until proper break-in hours were flown at break-in power settings. In the second case it turned out to be a bad oil pressure gauge in the aircraft as our gauges showed normal presure when we installed it on the test stand and ran it.

I have one good friend who owns and flies a P-51D Mustang. His Merlin showed bearing particles in the oil filter at only 50 hours of operation. The shop that did the Merlin fixed it and he had been operating it almost every weekend for more than 3 years since that time without a major problem. I suspect the issue wasn't the Merlin but was something the shop did, and so does the owner.

The Museum flies two P-51D's and their engines have been very relaible for many years.

In fact, we usually have 25 - 35 WWII warbirds at our airshow every year. They fly in on Thursday, practice on Friday, and the airshow is on Saturday and Sunday. We probably fly something like 80 sorties a day on the weekends and maybe 40 on Friday. That is 200 sorties or so per airshow, give or take a bit. In the last 8 years, we have had a total of somewhere around 10 aborts out of those 1,600 sorties ... and a few of those were for flat tires or low batteries.

That says a lot about the reliability of 70+ year old engines that were built for WWII! We've had more troule from our Korean War F-86 engine than from the WWII piston engines!

 

[syscom3]

I'm surprised that the R3350 had less time than the R2800.

 

[GregP]

I was surprised until I saw them side by side on engine stands.

The R-3350 is bigger and has mor space for accessability than the compac and powerful R-2800. There are MANY places on an R-2800 that are hard to get to. Most of the R-3350 is quite accessable in comparison. Obviously not THAT much more so since the times are close. I am in NO way inferring teh R-335 is easy to work on. Just saying my hands fit more places on an R-3350 than on an R-2800.

There are places on the Allison that God didn't intend for nuts to be ... but they are there anyway. Some are only possible with small hands and special wrenches.

I suspect the Merlin is the same only a bit moreso. I have done minor work on a Merlin to help a friend but have never disassembled one down to pieces. I have done that to several Allisons and many components on the Allison. There are at least two nuts I can think of on an Allison that I have never figured out a way to torque. You have to estimate it so you turn it down similar to one you CAN torque.

The R-1340 looks like the best bet for a warbird on a budget. Let's go fly a T-6!

 

[KiwiBiggles]

That says a lot about the reliability of 70+ year old engines that were built for WWII! We've had more troule from our Korean War F-86 engine than from the WWII piston engines!

Henry Ford, who famously discovered that the kingpins in Model T engine never failed and so had them made to a lower spec, would say that the V-1710 and V-1650 were therefore over-engineered, and that the J47 which wears out when it reaches its designed lifespan is a better design.

 

[GregP]

He probably would have said that! I disagree, but he was the more successful of us by far.

 

[tomo pauk]

Ahem:

A question: what V-1710, and what V-1650?

 

[GregP]

Sorry Tomo, I wasn't ignoring you., I just saw your first query when I opened the thread this time. I must have glossed over it the last time , probably due it's brevity.

All of them taken as a whole in the inclusive timeframe. It is a month by month accounting of the average overhaul man hours for engines that were completed that month regardless of variant. The average man hours get less with time. The averages in 1945 were significantly less than in early 1943.

Familiarity helps the process considerably. I know from personal experience that it is a LOT easier when you are working on disassembly or reasembly of your 8th engine (or more) than it is your first one or two of them. After a few times you already KNOW what to expect and you kNOW what beeds to be done, even if you are just following a process in the tech order.

You are suppposed to have the checklist open to the correct page when you are working on the engine. But after 10+ of them you KNOW what comes next and you also know where the problems lie. Ity's probably something like an 8 - 12% or better learning curve.

 

[Shortround6]

You also KNOW which combination of extensions are needed on a ratchet drive for which bolts/nuts and you may have acquired a few "cheaters". My Father worked for Sikorsky and Chance Vought for several years in WW II and his tool box in the 1950s still had a few wrenches that were cut off short and a few that were little more than the end of wrench with a long rod welded to the side in order to get into certain confined areas. Anybody doing a LOT of overhauls might acquire a few "extra" tools over and above what the manual calls for that save time or skin on the knuckles

 

[vikingBerserker]

That is an interesting thread, thanks gents.

 

[GregP]

Bingo Shortround!

The first thing a new B-29 crew chief did was acquire the dreaded "B-29 thumb." When you went to drain the oil in the R-3350, you had to reach into a hatch with a standard rachet and extension. It was usually a short extension that was issued in the kits and Boeing didn't exactly believe in deburring the sides of the hatch. So when the drain plug let go, you immediately cut the backside of your thumb, usually fairly deeply. While in trainiing, the planes they used there had been deburred by the maintenance people out of self protection. The new baby crew chiefs got a surprise intriduction to it when they reached the field.

So ... the guys in the field would order several extensions and weld several together with a bend to make a combination that could be used without injury.

That was only one of MANY such exercises and Shortround is quite correct about people coming up with "special" tools for certain jobs. They were guarded jealously by the owners.

I don't believe you could work on an Allison without some of the special tools Joe Yancey has made. The "dumbed down" socket sets you can buy today certainly won't do it. Some of the special tools are made using a standard socket of appropriate size and a 1/2" or smaller end mill on a rotary table. I'm thinking of the cam tower wrenches that are necessary but oh so scarce, and a few other "specials" such as the towers needed to pull the cylinder banks off and split the cases. There is a bizarre tool needed to change the cylinder bank studs and you also need a torch to heat the Aluminum case while you tighten or loosen the slightly oversize studs.

The set of specials is large for the Allison ... but nothing like the specials needed for the Merlin. My Merlin friends have entire tool cabinets just for the "specials." They might let you LOOK at one, but forget borrowing it. You have to go make your own.

 

[BiffF15]

GregP,

Do you have comparable data for the German, Russian or Japanese engines as well?

Cheers,
Biff

 

[gjs238]

That was only one of MANY such exercises and Shortround is quite correct about people coming up with "special" tools for certain jobs. They were guarded jealously by the owners.

Like this? Made this to clean the scale from our toilet bowl (water closet) water jets.

 

[Balljoint]

Phil Remington, a renowned race car fabricator and builder, was particularly known for his bag of original-design tools and gizmos. Perhaps this is, in part, the result of spending his early youth as a B-24 flight engineer.

 

[fastmongrel]

Any mechanic with time under his belt will have a compartment in his tool cabinet full of odd special tools he has made over the years. One day you will be looking at a seized woggle grommet shaft and think "aha I have the very left handed screwdriver I need". I have special tools I probably made 40 years ago when I was an apprentice I cant remember what they were for and they are probably for cars, motorbikes, lifts, cranes, railway locomotives and wagons that barely exist these days but you never know one day maybe one day.

 

[GregP]

Hi Biff,

I got these numbers from the USAAF Statistical Digest for WWII. I stress the data above was for maintenance depots inside the cointinental USA, and does not reflect times for field overhauls or for maintenance depots outside the continental USA.

I've never seen the same type data for German, Japanese, Russian, or even for British engines, though it might exist at least for the British. It COULD exist for all the rest and I've just bever seen it before ... I don't know. It WOULD be interesting.

It's sort of like the data on aerial victories. We HAVE the data for victories awarded to the USAAF and USN, but I've never seen the files for claims. I've seen the claims for the Germans, but have never seen a file of vistories actually awarded by the German Luftwaffe.

So ... it's fun to look at the data for engines, but there seems to be little with which to compare it.

I suspect the Daimler Benz engines were in line with the Allisons at around 200 hours or so for an average overhaul, but I have no data to back that up with. I could be off by 150 hours. I have no clue as to the Jumos since I have never seen one apart. The only onme I have seen in person was in the old Doug Champlin Fw 190D at the Champlin fighter museum in Arizona, It has since been sold to Paul Allen. I DID get to hear it run once in Arizona in the mid-1980's. The cowling was in place so I didn't even get to see the engine itself other than the exhaust ejectors.

 

[Elmas]

None of the studs, nuts and bolt were accessible- you couldn't even see them!-You were feeling around corners and you'd got weirdly contorted spanners to get at them, giving it half a turn then chosing another spanner and giving it another bit of turn! oh, that was shocking!
Peter Jago, RAF engineer, about the Napier Sabre engine.

 

[GregP]

Hi Elmas,

If that was the case with the Sabre, it would surely fall into the "very high man-hours per overhaul" group, wouldn't you say? The R-2800 was the highest, on average, of the AMerican engines shown, but at least you can get to most of the nuts, bolts, and studs. Perhaps it was best the jet age came into being after all, saving some poor crew chief the difficulty of working on a Sabre.