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Discussion Starter · #1 ·
Reading notes in this forum by experienced 2-cycle Lawn-Boy rebuilders, I have been frustrated at not finding an exact procedure to test crankshaft clearance to decide whether or not to replace bearings and/or rod bearing inserts.

Let's assume that I attach a dial indicator pushing against the crank's side just beyond either the top or bottom seal retainer bore. Then I move the crank back and forth so as to record the maximum clearance range that it can move side to side. Normally I'd expect to find suggested rebuild clearance limit valuess beyond which replacing bearings is suggested.

We know that older points-triggered magneto type engine ignition timing depends on where the rotating (major movement) and oscillating (minor movement) crank-mounted ignition cam's location curve travels. Designers like to assume that its only movement is rotation with zero oscillation within bearing clearances. But that's not precisely true of even newly manufactured engines. They all have some crank oscillation within their bearing clearances. As bearing clearances increase with wear, those oscillation values just keep increasing. Some Lawn-Boy rebuilders have reported about engines which still ran pretty well at certain rpms despite having developed tremendous top bearing clearance. Others have talked about so much crank oscillation that their flywheel starts abrading away CD inductors or magneto laminations. Oscillation curves within bearing clearances usually follow almost same path with every rotation for any specific rpm, but they are both rpm and load dependent. So a sloppy top bearing may cause poor ignition timing at low rpm yet allow good running at 3200 rpm.

Sloppy lower bearings allow blades tips to wander up and down rather than follow a more tightly defined circle. Also excessive top and bottom bearing clearances keep hammering against crankshaft seals, progressively degrading their ability to seal.

So how do you experienced rebuilders make the decision as to whether to replace bearings? I'd like to use my dial indicator. I would greatly prefer to be guided by clearance readings rather than loose descriptive language which can fairly be interpreted differently by various readers. How sloppy is "too sloppy?"

Also, how about end-to-end crankshaft play? What's a suitable maximum value to tolerate before corrective action? And what specifically do you do to correct it?
John
 

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I wish I could help. This is becoming one of my frustrations as well. I've seen really sloppy engines run just fine, and others that appear plenty tight have surprised me with problems.

So when in doubt, replacing bearings is the way to go. But even brand new bearings have some play which leaves you wondering if everything is truly OK.

I have also seen wear on the crankshafts - that's fairly easy to identify. I think no wear is the only acceptable tolerance on cranks.
 

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I don't worry about crankshaft clearance as long a the crankshaft seal will hold. IF it goes to getting pretty wet, time to change. I worry about rod bearing when it throws the rod.

Never had that happen yet.

Walt Conner
 

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Lawnboy Green
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There were engine specifications in the files section of the yahoo group called lawnboysrus. I just went over there now to check on them for you and cannot get them to come up. I could be that yahoo is updating their system as its 2am here or it could be that theyre missing because of the transition of the whole yahoo groups concept. I have emailed the moderator of the forum to ask whats up and will let you know what I find. In the meantime if you know anyone that has a shop manual, under specifications for the style of engine the bearing specs are listed.

Best wishes,
Bob
 

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Lawnboy Green
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Lets see if this helps.
D400, D600 and F engine specifications. Which engine are you working on?

Thanks,
Bob
 

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Lets see if this helps.
D400, D600 and F engine specifications. Which engine are you working on?

Thanks,
Bob
Where can we find this for DFs?
 

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Discussion Starter · #7 · (Edited)
Thanks very much guys. Sharkey's reference pdf. file lists the following crankshaft clearance specifications for these three engine classes: F-Series, D-600 Series, and D-400 Series. Lawn-Boy's design limits, in inch units, for these engine series, listed in the same order are:

Crankshaft End Play: .006 to .016, .007 to .017, and .007 to .017.
Crankshaft Side Play Next to Seals: .0065 maximum, .0065 maximum, and .0065 maximum.

Those are perfectly clear rebuilding decision guidelines.
If these engines are run at 3200 rpm which is about 53 cycles per second, those seals must continuously flex by their actual side clearance values.

I've never seen shaft seals like what I hope the following description brings to mind. The outer ring would be rigid but covered with a thin layer of elastic material to enable tightly sealing into the outer bore for the seal's service life. The inner ring would also be rigidly reinforced covering two thin-but-durable wearing rings which would slide against the lubricated shaft's surface. Between these rigid outer and inner rings would be an elastic membrane covering a very elastic "spider web" of inwardly-spiraling springs which would reinforce the membrane while allowing generous lateral flexing. The outer seal section would seal but not flex. The inner seal section would only flex at the microscopic level, but maintain a sliding seal. Only the spiral-spring-supported flexi-spider membrane would flex, but it could flex a lot for a long time without damage. The flexi-spider membrane could be configured as concentric accordion folds. Its configuration would enable it to endure a lot of lateral flexing over long service lives, yet resist limited positive and negative gas pressure pulses. Piston-type loud speaker manufacturers maintain tight clearances between magnetic field induction coils and strong permanent magnets without allowing them to touch each other while maintaining long service lives. Those speaker spiders flex with the shaft bore's axis, whereas the spider design I just invented, if new or only described if already existing, would flex perpendicularly to their shaft bores axes. Expecting the seals that I've pulled from the tiny Lawn-Boy engine population sample I've seen to continuously sustain significant flexing perpendicular to their bore axes seems like those seals were poorly designed to endure that kind of flexing. Just my opinion.

I'm guessing that their gas-pressure loadings during each crankshaft rotation are something like from -10 psi to +30 psi as the 2-cycle engine case section is just acting like the first section of the engine's 2-stage pump. Full compression within these engines is the combined effect of both this first lower-pressure pumping stage and the piston's higher-pressure pumping stage. Then a spark triggers the exothermic-combustion chemical reaction. That combustion causes the confined-fixed-gas mass to be heated. Except for heat lost into the cylinder head and piston top, that heat simultaneously causes gas pressure to increase, pushing against all combustion chamber surfaces. The only surface of those which can move is the piston. Internal combustion engines generate their crankshaft-turning power by their DECOMPRESSION cycle which ideally should be AFTER rather than DURING combustion. When you see fire blowing out open exhausts, you should know that engine failed to efficiently convert its fuel's chemical energy into crankshaft turning energy. Hot gases are NOT luminous. Just heating exhaust gas components well above exhaust temperatures does NOT cause them to emit light. Only combustion well past the cycle period when the engine can convert pressure it causes into useful work can account for exhaust luminosity. Engine designs which fail to complete combustion near top dead center can't apply that pressure through the piston's full power-stroke travel. Obviously later combustion, like combusion part way down the power stroke or worse, after the power stroke has completed, can't push the piston during the entire power stroke. Current engine designs don't enable full combustion completion before the piston significantly moves down. Their fuel charges are still being converted into piston-pressing gas pressure long after "the ship has left the dock" so to speak. It's like the ship keeps being loaded after it has started moving away from the loading dock rather than starting with a full load. Yet the ship only collects power from load miles. If the ship isn't fully loaded until it is half way to the unloading end, that is a power wasting design path. It fails to convert as much combustion energy into crankshaft-turning energy as a better design would make possible. But that's how conventional internal combustion engines work.

Back to these pressure-seals. The word plastic was chosen to describe certain materials because they are able to deform without destruction. But most plastics loose their elasticity through time as they are exposed to heat. Any new plastic that a smell-sensitive dog can smell is emitting volatile chemicals which help make it able to flex. As it gradually becomes less smelly, it is simultaneously becoming less flexible. As "new car smell" from plastics eventually disperses, plastic dashboard covers, steering wheels, arm rests, etc. simultaneously become increasingly vulnerable to cracking. Are crankshaft seals that have been in Lawn-Boy service for 20 years as soft and flexible as they were when they were new? If not, why not. Can old seals be re-softened to some degree so they can flex more again? Some ATF transmission fluid additive producers claim their products can extend useful service lives of seals which have become less flexible than they were when new. Perhaps wetting them with brake fluid for some period would make them better or worse. These seem like reasonable inquiry lines some of our discussion group participants can try with little cost, then share our observations. If you're already planning to rebuild an engine that has developed barely tolerable seal leakage, how about pulling the top cover and squirting some seal softening candidate fluid on that seal, let it sit for a day or more, then seeing if it leaks less during later runs. Flipping Lawn-Boys over and squirting some candidate fluid up against the bottom seal wouldn't take long. This may or may not work. Unless we try it, how can we know? Testing cost is low. Potential pay-off to thousands of Lawn-Boy owners may eventually become significant by this contribution to 2-cycle owner knowledge bases. The same thing is probably happening to 2-cycle chainsaw engine seals. This is like a real world slot machine at this point in the inquiry.

I had a leaky automatic trans in a car which gradually stopped leaking after adding a bottle of ATF stop-leak additive to its fluid. It gradually leaked less and less, then stopped leaving drips everywhere ever it was parked for about 2 years. That decades ago experience is not a Lawn-Boy test. But that ATF stop-leak did something to swell or soften some leaky seal or seals.

I know that my approach is unlike others who participate in these discussion boards. I hope it triggers some of you to consider long-familiar issues in new ways. Again, I apologize if anything or any way that I've expressed myself here has offended anyone as it apparently did earlier. And thanks again for your help. Let our diversity be a strength.
John
 

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Just blew up and tore down a D-480 today. That passage in the reed plate is to oil the lower end of the engine. Bad thing is the lower bushing is shot and looks like that's a non-replacable part. Upper crank is supported by a bearing.
 

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Just blew up and tore down a D-480 today. That passage in the reed plate is to oil the lower end of the engine. Bad thing is the lower bushing is shot and looks like that's a non-replacable part. Upper crank is supported by a bearing.
There should be a thread somewhere and I expect some of the people here know the details, on boring out that bottom bushing and replacing it with a needle bearing from an "F" series. I MAY even have saved a copy somewhere.

But as I said before, if the seal isn't leaking so the crankshaft is very wet, forget it.

Walt Conner
 
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