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Old 10-30-2018, 10:32 PM
amerkarim amerkarim is offline
Join Date: Nov 2017
Location: Houston Tx
Posts: 41
Question Fuel for break in

Hi All,

I am getting close to my first start and wanted some advice about break in, fuel and oil.

I have a stock 540 thunderbolt with 1 mag and 1 electroair. Am planning to run mogas full time so I have the electric boost pump, duplex fuel valve and a return line installed to both tanks. Put 10 quarts of straight shell mineral oil in, the engine will dump what she doesn't want in time I presume.

Regarding fuel, the engine has stamped on the ID plate - 91 to 96 RON. I presume that is the fuel rating you have to use. As I am planning to run only mogas and I don't like the idea of having any lead in the system if I can avoid it, can I just run mogas for the start and break-in? I can get good Shell V power 99 RON unleaded fuel locally, can I just use that from the first start to TBO or is there a reason for using Avgas for break in?

Will follow the instructions I have read in the posts before me and do a staged start and break in with cowl off, on, fly, then seat the rings well with high power until oil consumption stabilises.


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Old 10-30-2018, 11:27 PM
BobTurner BobTurner is offline
Join Date: Dec 2011
Location: Livermore, CA
Posts: 5,716
Default Are you sure?

RON is ‘research octane number’. For many years, the standard for aviation has been MON, ‘Motor Octane number’. In the US, by law, posted octane numbers for car gas are (R+M)/2, e.g., the average of tbe two. RON is typically 7 or 8 points higher than MON. I suggest calling Thunderbolt to confirm these numbers (I have never seen a range of octane numbers quoted, usually just the minimum needed).
While you’re on the phone, see if they have any objections to lead-free break in.
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Old 10-31-2018, 01:47 AM
tgmillso tgmillso is offline
Join Date: Nov 2012
Location: Launceston, Tasmania, Australia
Posts: 689

I just finished my initial break-in of an 8.5:1 IO360-M1B using a combination of 100LL and BP 98 RON. Used the 100LL on takeoff and landing, then switched over to the mogas once the power was back to cruise settings (approx 75%) and I was at the desired altitude (3500' AMSL). The 100LL will give you a superior detonation margin at the high takeoff power settings in the case your cylinders are running hotter than anticipated because either your fuel servo is set on the lean side of normal or your baffle sealing isn't perfect (experienced both) not to mention the normal high friction experienced during break-in. Once you're at altitude and the temps appear under control, switch over to the mogas to reduce lead fouling, as you'll be running full rich during the break-in period. It's amazing how quickly the lead builds up when running full rich the whole time. I'm 14hrs in and now running mainly BP 98 RON, except when I am taxiing in, as I've found the mogas more susceptible to vapor issues when parked and subsequent hot starts, so I just make sure I've got the lines filled with 100LL.
Not a recommendation a such, just my experience.
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Old 10-31-2018, 06:27 AM
Robert Anglin Robert Anglin is offline
Join Date: Mar 2008
Location: houston, texas
Posts: 900
Default Not in the book.

The old rule of thumb is to use 100 LL for the break-in period. that being the first 25-50 hours or so. Partly because of the higher CHT at break-in and also because it was to get some lead into the meatal of the valve seats, stems and guides, for protection of the metals used. Most of the newer engines have switched to metals that don't need the lead for them, but it is still a good idea to give them some anyway. We still break-in with 100 LL. I start switching to 50% 100 LL and 93 Oct pump gas with no "E" at about 50-80 hours and then if I am running less than an 8:1 compression ratio will run the pump gas. I still like to give the engine some lead every few months, in the hot summer for sure with hard fast climbs in hot weather. In the winter with pistons 9:1 or higher I don't like running less than 50% mix, but with 8:1 or lower I don't have a problem with pump fuel of 93 Oct. or higher, as long as you watch your EGT's and CHT's close and back down on the throttle if they start up the scale. Very few small aircraft engines have Knock sensors on them these days so it is best to track you engine cooling close when it under heavy stress and heat loads. And yes that's where the high octane fuels come into play. Just some miner thoughts, it's your call.
Yours, R.E.A. III # 80888
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Old 10-31-2018, 07:07 AM
amerkarim amerkarim is offline
Join Date: Nov 2017
Location: Houston Tx
Posts: 41

Hi All,

Thanks for the replies.

I have found this article that seems to make a lot of sense.

I guess it is Avgas for me for the break in, then every 50 hours or so.



"Why is lead needed during engine break-in?

In a past column I gave some background on what happens during break-in of a new or overhauled aircraft engine. Since then I have received numerous questions about why leaded fuel is needed during break-in.

Unfortunately, I do not know anyone who has a perfect answer for this. My opinion is based on lab tests and reports from field overhauls and manufacturer’s reports.

An example of a lab test that we ran back in the late 1960s was one run in a 430 CID Buick engine. The engine was installed on a test stand and run for over 20,000 simulated miles at relatively high RPM and load. We monitored the valve stem height and, at the end of the test, there was no significant exhaust valve recession.

The test was repeated with a new tank and fuel lines, plus new heads that had never been run on a leaded fuel. (We found out that the original new engine had been run at the factory on a leaded fuel.)

Now after about 15,000 simulated miles, most of the exhaust valves had receded enough to take up the lash allowed by the hydraulic valve lifters.

As far as field data, about 20 years ago, a west coast oil company started to market an 80/87 unleaded avgas. This was perfectly legal since the ASTM D-910 spec for avgas specifies that the fuel only meet a maximum of 0.5 grams per gallon lead level. There is no minimum level.

A few months after the introduction of this fuel, I started to receive numerous valve recession complaints, but only from the west coast. Further investigation revealed that every failure occurred on aircraft that had operated on the unleaded fuel.

Subsequent to this I have heard numerous reports of valve recession on aircraft that have operated exclusively on unleaded mogas. Now it does not happen to every aircraft every time, but it does happen a significant part of the time.

So what is the mechanism here and what does the lead do to prevent exhaust valve recession?

This is one of those things that if you talk to 10 experts, you will probably get 20 to 25 different answers.

The most common ones are: Lead acts as a solid type lubricant to protect and cushion the valve and seat; another is that the lead alloys with the seat material; and still another is that it improves the heat transfer from the valve and heat tempers the seat.

In the Buick engine test, the low level of lead that was needed to protect the valves was due to the liquid cooling of these engines and the subsequent lower seat temperature compared to an air cooled aircraft engine.

When an engine is running, especially under high load, the exhaust valve is exposed to direct flame temperatures when it first opens. To keep the exhaust valve from getting too hot, engine manufacturers design a direct heat path up the valve stem to the guide, valve tip, and the oil flow.

But the most critical point is the valve tulip edge, and that is designed so that most of the heat is transferred to the valve seat during the time the valve is closed. There is not much time for the heat to be transferred, and the transfer depends on the difference in temperatures between the valve edge and the seat.

Since a liquid-cooled engine exhaust valve seat runs several hundred degrees cooler than that seen in an air-cooled engine, it is easy to see why aircraft engines are much more critical.

So what is the answer? I feel it is a combination of things.

First, in a new engine, the valves and seats are both ground. The surface of the valve and seat are not perfectly smooth. The micro ridges and valleys from grinding allow some leakage after the first startup, and provide a poor heat transfer path from the valve to the seat.

With a leaded fuel, the byproducts of combustion tend to coat the valve and seat, which yields improved sealing and a better heat transfer path. Over time, the seat does harden somewhat and the engine goes on to live a long productive life.

With an aircraft engine, it becomes much more critical, especially during break-in. After that a small amount of lead is required during the life of the engine to protect the valve and seat.

It is not a precise amount, but during break-in I would recommend running at least 25% 100LL for the first 50 hours. After that, a small amount every so often should keep all running well.
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Old 10-31-2018, 07:17 AM
Larry DeCamp's Avatar
Larry DeCamp Larry DeCamp is offline
Join Date: Apr 2010
Location: Clinton, Indiana
Posts: 806
Default Leaded wives tail ?

Does anyone have current facts about the metallurgy requirements for lead in Lyc/Superior etc. engines ? Octane for preignition is a separate issue,please.
It feels like this unsupported wives tail goes on and on like MMO and similar debates.
Larry DeCamp
RV-3B flying w/ carb / Pmags / Catto 2b / Steam
RV-4 fastback w/ Superior XP360/AFP/G3X/CPI/Catto3b
Clinton, IN
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