The truth of the matter is that modern model engines are incredibly tollerant of bad break-in procedures and virtually all of them will run just fine, regardless of the technique used.

Generally I just follow the manufacturer's instructions (which do vary significantly from one brand/model to another).

Saito says to run their engines *very* rich for the first 10-20 minutes-- in fact most of the time they run so rich that the glow-connector has to be kept connected or they die and you can easily hold your hand on the (only slightly warm) cylinder head during this process. I've used the recommended break-in on both my Saitos and they run *brilliantly*, now having *excellent* compression, stacks of power and a wonderfully low idle.

On my ABC/N engines I simply run a tank of fuel through them on the ground with the needle set just a few clicks lean of 4-stroking and vary the throttle from full to idle at intervals of about 30 seconds -- then I go fly with the same needle settings. I have a TT46Pro here that was broken in that way and after more than 210 hours (and a set of bearings 10 hours ago) it's still running perfectly, hauling a 4.5lb profile 3D plane around with authority.

Even the engines that have been somewhat "abused" during breakin (ie: an accidental lean run on the first flight) seem to suffer no permanent ill effects.

Maybe it's because I choose reasonable quality engines (Saito & Thunder Tiger) for most of my planes that I have no problems. The cheaper engines (GMS, TigerShark) that I've tried have been dysmal regardless of the careful breakin I've given them. For example, I had *one* slightly lean run with a GMS32 and it lost its compression. My TT42GP has (over the period of many years) had some even leaner runs -- so lean that the engine has actually sagged and stopped -- yet it's still 100% as good as new and hauling a 5.5lb trainer around for the club.

In short, unless you're *really* stupid, I don't think it matters too much at all exactly how you break in a modern engine of reasonable quality -- just follow the manufacturer's instructions and then go flying without stressing out about it.

That's right and why the carbs are set richer. Full power is a richer than economy, and especially the "save the earth" lean that automobiles run at.

I didn't say they did. I said they have looser clearances. I should have said they have looser clearances than automobiles, at least as far as the piston, rings, and cylinders.


If that were true I would have been replacing cylinders every time I started the engine up, instead of every few annuals. Funny but for each cylinder I had to replace I recall replacing two compression rings and an oil ring. Granted some have an extra oil ring and many have the oil ring located at the bottom of the skirt. I suppose they put the crankcase vent on just for the heck of it. And I guess that air that leaked on the compression test wasn't blow by. I don't know how they seal the rings so well, I mean they have gaps in the rings just like automobiles and trucks, they have looser clearance because of the higher expansion and contraction of the cylinders. They also have high lead fuel, valves that tend to stick every few hundred hours, magnetoes that still use points, and a hundred other thing much inferior to a standard automobile engine.

I dissagree, they break them in at a full four cycle. The running four cycle mode is actually a little leaner, for the engines tuned for C/L 424 break, will four cycle at a leaner mixture. So why don't they glaze after they are broken in?

Yup, me and many of the acknowledged experts in the field have all these misconceptions about Cox engines. One Cox web page shows the tolerances of the upper cylinder in Cox engines was BETTER than 1/10,000 of an inch. Cox was the only company that didn't have to sell matching piston/cylinders. The tolerances were so close that any piston would fit any cylinder. Clarence Lee was one of the NOBODY misguided people that pointed out that Cox had tolerances to 25 millionths of an inch. The is 2.5/100,000ths of an inch and it is accepted in the industry that Cox had those tolerances. Now if Daring Dave wants to dispute that that's his business. If he wants to look foolish that's also his business. Me and all the misguided morons with rock mentalities stick by those numbers.

Even Bill the resident "expert" agrees that tolerances that close are feasable.

So, if you guys want to believe people with no credentials other than they post their opinions at some publis forum, be my guest.

Engine breakin does not concern just the piston/ring/liner fit (or piston/liner fit in non-ringed engines). It also concerns the bearings, rod-to-crankshaft fit, and valve train in four-stroke engines.

I've been operating model engines since the late 1950's, and have been involved in model engine service for well over 20 years. I have worked with the designers of model engines from different manufacturers, and have literally run thousands of engines.

Ringed engines are typically run very, very rich. I don't know all of the thermodynamics involved, but I do know from practical experience that a breakin that involved some seriously-rich initial running, gradually working towards peak RPM insures that the engine will have good handling characteristics. Even newer engines that claim you need only one or two runs and go fly benefit from a longer breakin time. Different engines need different amounts of time before they really "come in". Once they have, though, they usually run very well.

I'll run the engine super rich with a fuel that has a castor/synthetic oil mix for about 15 minutes or so before I even think of leaning the enigne richer than 'four-cycle' mode. Then I'll gradually work the engine towards peak RPM in a rich-lean-rich-lean sequence, allowing it to warm up and cool off as the mixture is changed. A .40-.90 size engine will usually take a total of about 45 minutes of running time to get to the point where it can be leaned to just rich of peak RPM and hold the setting, except for the natural leaning that happens as the tank drains. Then I'll work on getting the idle RPM and mixture found. If the engine won't hold the needle setting, it's not ready to set an idle and fly. I'll richen it and work on it some more. Larger engines can take more time.

Four-stroke engines are run as rich as possible at full throttle without quitting...even if we have to leave the glow plug battery connected. This insures that plenty of lubrication is getting to the camshaft gears, lifters, and rocker arms. After 15-20 minutes of this, the mixture is then gradually leaned. The multi-cylinder engines get run this way for at least 20 minutes before we start to lean them.

With the procedures we use, and are written in almost all of the engine instructions, we have never had problems with an engine not reaching its proper performance. If it wouldn't perform, it was something else, and no amount of running would cure it.

The only time we've seen any true "glaze" (burned-in residues of any kind), have been in engines that have been seriously mistreated and overheated. We've never seen an engine damaged by the procedures we use. We have engines that have hundreds of runs on them with no damage, just normal wear. We have seen engines that have had sufficient use that the cylinder liner was polished very smooth. Those cylinders were usually worn to the point that they needed to be replaced. Scuffing them usually doesn't help that much.

What can I say. My experience has shown that an extended breakin for a ringed engine does no harm whatsoever...and is usually a better idea than trying to get the engine in the air as quickly as possible. I prefer bench running for breakin over flying rich because the engine can be under close observation and much better controlled.

My two cents. Take them how you wish.

Hey Baxter,

I agree with everything you just wrote. I differ a bit from you. I will use a smaller than normal prop for break in. Say on a 60 I'll use an 11x7 cut to 10 inches. It will run at a reasonably high rpm but being very rich won't overheat. I usually do a series of 5 minute runs getting leaner each time until I use the 11x7 and the engine will hold a two cyclew without sagging. Thing I fly it still a bit rich until it haa about a half hour on it. That's how I break in most ringed glow engines.

do not over prop - do not over heat - do not overheat and do not overheat also vary engine speed and do not overheat - that is a good break in sequence - use plenty of good oil 20% min on ANY glow engine
also do not allow to over heat .
we ran .049 engines in --1949 or 1950 I forget - but it was the same then . do not overheat .
the worst tolerances in an engine, which also needed refitting very often was- FOX - some of the engines we had to refit twice to get proper running clearances. Early Enyas were a close second fit far too tight in many instances. The Chinese early engines were simply almost unusable- horrible fits.
Cox did the best job of temperature controlled machining - and could hold better tolerances than anyone .
Any of the current engines I have run - typically just needed a few trial run ups to see how they handled heat - NO extended running -I can tell in ten seconds if the fit is OK to fly- Then we fly varying th rpm and if I did not mention it - making sure to not overheat it.

Hey Dick, is it true we shouldn't overheat an engine while breaking it in? Over propping is the surest way to cause overheating. That's why I use a smaller prop and run very rich during break in.

Yup Cox had the tightest tolerances of any engine manufacturer.

When Clarence Lee makes one of his custom K&B61's part of the procedure is to measure the pistons and cylinders and get the biggest piston in the smallest cylinder with out over doing it. Ie, the piston still has to be a shade smaller than the cylinder. Ganging of tolerances is what makes a poorly fit engine. Cox had the best fitting piton/cylinder fit overall.

Some of the old lapped Enyas had very tight piston/cylinder fits but when properly broken it had unbelievable compression. They could sit a TDC for 5 minutes and still pop out when turned. Johnson's were like that too.

You are right. Some of the old Fox engines were horrible and the Chinese stuff was one step above junk.

Now thanks to production equipment that measures as it goes etc., poor fits are a rarity.
The best stuff - for years -- the OS line - I did .61abc (actually not abc) pattern engines -for me and others - I could purchase pistons and liners - separately and drop a piston into a liner - --they all fit and stopped at almost exactly the same place in the bore . and the same finger pressure to move to "solid" fit.
the current line of OS four strokes are still a favorite .
powerwise the Saitos beat em for power to weight but the OS 70/91 four cycle engines are bulletproof
I ran em on 40% nitro and 20% oil for hours - no undue wear and simply fantastic power for the size. None of the gasoline engines have hit this mark of overall precision----yet-- but they are rapidly getting there ---

Haven't you heard?? According to the people here, OS is turning out junk now.

Not junk, Bruce, just not worth the money. And yes, that's an opinion; mine.

Bill.

Bruce,


Most current sport .61 engines are under-propped with an 11x7 prop.

The late 70s/early 80s piped F3A engines, like those used by Dick Hanson on his Tiporare and by other pattern competitors in those days, which would spin an 11x7-11x7.75 prop, around 14,300-15,300 RPM, in ground static condition, would unload for more RPM in flight, reaching their real HP peak RPM of 16-17,000.

Most current sport .61 engines, as they come, with their original silencers, peak around 13,000 RPM, despite anything their manufacturers claim they can do...


So the right size prop is a 12x6, or a 12x7 (or one of equal load) that will have the engine spinning around 11,500 RPM on the ground.

The 11x7 size will not cause the engine to overheat, but if extra cooling is desired, a higher pitch prop of equal load will be much better than chopping the 11x7. A 10x9 is the best choice, if over-heating is feared with an 11x7.

Your opinion yes, but since everyone here seems to think you are one level below God, they take your OPINION as gospel and then spread the gospel.