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post #1 of 53 Old 06-08-2009, 03:53 PM Thread Starter
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Potential Lawn-Boy tweaks - hand-lap reed, double crank seals, crankcase stuffing

It has been 45 years since I was drag racing my Yamaha YDS2 250cc bike on nitro mix fuel turning high 13 second times in the quarter. So while dated, I've tweaked various 2-cycle and 4-cycle engines to improve their performance.

Now I'm rebuilding a Lawn-Boy F-series engine. I can't help thinking about tweaking again.

The first thing I noticed was that the reed valve did not seal tightly shut when I tried to pull a partial vacuum with my mouth on the intake bore leading into the upper crankcase. So I pulled the reed, checked for flatness and obstructions, but found no apparent defects. Still leaking. Next I flipped it side for side. No better.
Years ago we commonly applied various grit lapping compounds to "salvage" plate glass pieces we'd get from glass shops, then use that abrasive-covered surface to hand-lap cylinder heads and cylinder barrel tops to flat surfaces so they could fit together forming a gas-tight thermally-conductive sealed with only a little aluminum-particulate paste from the bottom an aluminum paint can. I thought about cutting and breaking a little glass piece to bridge across the reed valve opening surfaces, and that still might be worth trying. Instead, I just applied some lapping compound to the reed sealing side, then vigorously rubbed it in circles over the aluminum sealing surface, mounting surfaces and stand-off pillar between them. This would be a good task assignment for a child to perform while watching a TV show. Boring and slow going. Eventually it cleaned up and was displaying a uniform wear pattern. I also worked on that plane with the edge of a single-edge razor blade. On reassembly, pushing air into the cylinder with my mouth caused the reed to audibly buzz like a quiet "duck call." Great it was making music. Better yet, when I tried to pull a partial vacuum on the enclosed volume between the reed and the outside bore, it momentarily sustained a vacuum and did NOT allow perceptible flow. It's not sufficiently gas tight to make a good balloon seal, but there's no way it's going to let a fog flow backward into the carburetor. I see in the old OMC Lawn-Boy shop manual that they suggested reed clearance of no more than 0.015 inch. I'll bet crankcase pressure applied to reeds installed that way never seal nearly as well as this tweaked reed now seals. Naturally, any air/gas/oil back-blast past the reed into the carb reduces intake charging per induction cycle. So this little trick increases induction charging volume per stroke with all subsequent expected benefits.

Ambient air pressure near this planet's surface is typically about 14.7 psi. When a Lawn-Boy piston rises, gas pressure within its crankcase is pulled well below external ambient air pressure by piston-movement-volume displacement. Seals into that space are the piston, piston rings, piston-covering transfer ports, crankcase parting line seal, two crank end seals and the intensionally-leaky reed valve leading to the carb. We want all those seals to be leak free except the reed valve. Form discussion postings indicate that crank end seals commonly develop leaks. As I looked at the crankcase seal-capturing bore length, it appears to me that bore is probably long enough at each end to accept two rather than one seal. Humm. If replacing those seals, how about soaking them over night in some automatic-transmission-seal softener, then reuse each with a second new seal stacked above or below? Increased friction from two seals at each end would be trivial but increased sealing might be significant. Low cost but potentially useful benefit? Has anyone re-softened old Lawn-Boy crank seals with seal-softening solvents like those sold as automatic transmission fluid additives? Experience teaches that those softeners DO enable old hard seals to sometimes but not always resume performing their sealing task. Just a thought.

At least 50 years ago, 2-cycle engine crankcase "stuffing" was popular. The idea was to reduce crankcase volume so it would pump more inducted fuel/air/oil mix through transfer ports above the piston. Cork and other low-density solids were used to fill any voids not periodically occupied by moving parts. It turns out that securing cork and work-alike substitutes to fast-moving parts without having it self destruct from acceleration loads or disconnect and be chopped up by moving engine parts and spit out and burned within the exhaust system requires tweaking-designer ingenuity. Factory designers understand these tricks well. So they don't leave much empty space that can easily be stuffed. More stuffing and better sealing translates into increased fuel/air/oil mix precharging into the volume above the piston before it begins its partial-stroke compression.

Another tweak I've been considering. Years ago I paid high prices to have certain combustion chamber surfaces ceramic coated to reduce combustion heat loss. The ideal gas laws (combined Boyle and Charles) describes how gas temperature reduction with unchanged volume causes lower gas pressure. Gas pressure from combustion heat pushes the piston down which is how we get crankshaft power out. We want high piston pressure to get more work out, so we don't want to uselessly reject heat into the head. We learned that some "barbeque" high temperature paints would stay affixed to the combustion chamber surface of cylinder heads. Run it for 100 hours, pull the head and that silly paint was still there. Amazing. Since then, moderately-priced super-insulating micro balls have been invented which are being added to paints applied to roofs, greatly reducing sun heating loads into those roofs, thereby lowering air conditioning loads. Those micro balls are sold by on-line vendors and even eBay sellers. It seems to me that we could clean a Lawn-Boy cylinder top, then paint it with super-high heat rated paint to which super-insulating micro balls have been added. Instant pauper's priced combustion surface insulation. We've all seen how you can boil water in a paper cup over a flame without burning the paper because liquid water's maximum temperature at atmospheric pressure is only 212 degrees F., well below the flash point of paper. I know it sounds crazy, but I think that insulation barrier would survive Lawn-Boy combustion chamber top surface heat exposure just as water-filled paper cups survive flames. These engines need cylinder walls to stay cool enough to sustain a molecular lubrication barrier between rings and cylinder walls. Engine power production is reduced by pulling combustion chamber heat DIRECTLY into cylinder heads. As a matter of fact, I suspect that this cylinder-top insulation strategy might enable better cooling cylinder walls because they are formed as a single piece with the head. If the head is not DIRECTLY heated from its combustion chamber surface, it can accept more heat from the cylinder which it can then reject with its top air fins. Similarly, but different are piston surfaces. Piston temperature during sustained engine operation balances heat absorbed from the top combustion chamber-exposed surface against heat rejected into evaporation-cooled inducted air/fuel/oil mix below the piston. Insulating the top piston surface would rebalance piston temperature to a cooler temperature range and slightly increase combustion chamber pressure against the piston top, slightly increasing output power. More of the same insulating paint.

We used to apply thin high-thermally conductive flat black paint to cylinder fins. Standard charts have existed for at least 75 years showing that flat black surfaces reject heat better than any other of the many surfaces those labs tested. Decades ago, Cycle World published an article stating that a racing team's tests showed that sustained peak horsepower could be as much as 5% higher by running thin flat-black paint as compared to bright shiny aluminum painted fins. Shinny aluminum fins may appear better but flat-black aluminum fins reject heat better. Shouldn't we be flat-black covering our Lawn-Boy fins?

Some fuel additives reportedly cause combustion chamber, piston and piston-ring bonded carbon deposits to slowly detach, one molecule at a time, eventually revealing previously hidden clean piston and ring surfaces. Has anyone EXPERIENCED this effect with any Lawn-Boy engine? I'm not looking for advertiser claims. Actual observations, if they exist, are what we should share. Lots of product advertisers claim their products can do this. Most are clearly wrong.

Those little Lawn-Boy foam air filters restrict fast flows. I see from their DuraMax engine series that engine developers increased air cleaner size about 50% compared to their F-Series air cleaner. I'd feel more comfortable with a much larger air filter. Consider this: At 3200 rpm, each full Lawn-Boy engine cycle is 60/3200 = 0.01875 second duration. The intake stroke must occur during less than 50% of that time. Intake cycle charging opportunities only occur during those intermittent periods which last less than 1/100th of a second. We want a tight seal AROUND air filters to prevent unfiltered air entering the induction tract. But we would benefit by larger air filter surface fitted at least a little further up-stream from the intake bore leading into the carb. I've seen engines forming a back-blast fog visible away from carburetors caused by intake valve sealing failures. I expect that keeps foam air filters wet which would help capture particulates from intake air streams. If someone swapped in a paper air filter, that wetting effect from back-blast through the carb would also wet that filter and greatly increase its restriction. I'd rather run better sealing valves.

Your thoughts?
John

Last edited by LoveLearn; 06-08-2009 at 04:46 PM.
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post #2 of 53 Old 06-08-2009, 04:13 PM
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Re: Potential Lawn-Boy tweaks - hand-lap reed, double crank seals, crankcase stuffing

I do remember reed valve "spacers" and such from my old 2 stroke motor cross days. I do like your idea on the reeds though. Have you done this and tried running it afterwards? Any difference?

Double seals-if doable, would certainly cure the propensity for even new seals to still ooze.

I always used flat black paint in hot engine areas because I was told that it "dissipates" or releases the heat from the parts the best conductor so to speak. I was told WHITE paint reflects the most heat from entering surfaces.

Probably you're saying the same thing just differently?
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post #3 of 53 Old 06-08-2009, 04:16 PM
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Re: Potential Lawn-Boy tweaks - hand-lap reed, double crank seals, crankcase stuffing

Welcome to MTF John,
Check out Home and Active Topics for new posts.
I understood about 2-3 sentences but, We have people on here that will
be very interested in what you have to say.
Give it a little time, I'm sure you'll get some thoughts.
Again and

Mike
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post #4 of 53 Old 06-08-2009, 04:39 PM
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Re: Potential Lawn-Boy tweaks - hand-lap reed, double crank seals, crankcase stuffing

John,
The reed sealing procedure should make a difference. OMC used to recommend making sure the reed plates used in their outboards was flat. The shop repair manuals called for using a glass plate and some 1000 grit compound with oil to polish the reed plate.
Dave
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post #5 of 53 Old 06-08-2009, 04:44 PM
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Re: Potential Lawn-Boy tweaks - hand-lap reed, double crank seals, crankcase stuffing

Uuuumhhh... what length do you cut your grass?
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post #6 of 53 Old 06-08-2009, 05:21 PM Thread Starter
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Re: Potential Lawn-Boy tweaks - hand-lap reed, double crank seals, crankcase stuffing

GPT wrote:
"I do remember reed valve "spacers" and such from my old 2 stroke motor cross days. I do like your idea on the reeds though. Have you done this and tried running it afterwards? Any difference?"
I performed those modifications on tiny motorcycle engines many decades ago. It was fun making kids bikes outperform bikes owned by their friends. These kinds of tweaks caused bikes which formerly stayed side by side with others of the same model to pull away from their stock peer bikes. I have not reassembled this Lawn-Boy yet. I'm confident that it will improve the power curve but I'm not as sure about how it will affect fuel burn rates. One issue I didn't mention, since my comment was already rather long, is that when part of an inducted charge is back-blasted back through a leaky reed valve, then re-inducted through the carburetor, that part of the inducted charge has been double carbureted (if that's a word). Passing the same air through the carb twice make the charge richer, thereby wasting fuel. So leaky reed valves probably cause more fuel to be burned while producing LESS power due to lower precharge volume driven up the transfer ports. I know that explanation may be a little hard to follow. Perhaps another writer who reads this can explain it more simply.

"Double seals-if doable, would certainly cure the propensity for even new seals to still ooze."
I agree. I'm building one engine from two failed engines. I intend to try this double-seal configuration on both ends using re-softened seals.

"I always used flat black paint in hot engine areas because I was told that it "dissipates" or releases the heat from the parts the best conductor so to speak. I was told WHITE paint reflects the most heat from entering surfaces."
Excellent. Most of my air-cooled bike racing preparation was for quarter mile drag racing where flat black cylinders would convey no significant benefit. Runs for so few seconds could not benefit from sustained operational improvement. I did prep a bike for competition at Bonneville including the flat-black fin trick, but that tour never happened.

"Probably you're saying the same thing just differently?"
Our language selection is different but our understanding seems to align well.

LilysDad wrote, "Uuuumhhh... what length do you cut your grass?"
For a moment I'll assume that you are not being sarcastic.
I tend to cut grass to a taller cut length than most people. Scotts, the lawn product company has long stated that cutting grass too short alters the balance of weed strength to grass strength in a bad way. Tall cut grass is so strong that it "chokes out weeds." Too short cut grass is weak and can easily be invaded by wind-borne opportunistic weed seeds.

But maybe you are suggesting that most LawnBoys deliver so much torque to their blades that their rpm isn't pulled down significantly by mowing "normal" height grass. In my case, my LawnBoy will only be used to trim. My 72"-cut diesel mower cuts all spaces where it can go. But I have some areas where it is too wide to fit. That's where I need a trim mower. I freely confess that I'm often driven by the urge to have fun tweaking to improve factory designs more than what others would perceive to be a "need."
John

Last edited by LoveLearn; 06-08-2009 at 05:29 PM.
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post #7 of 53 Old 06-08-2009, 05:25 PM
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Re: Potential Lawn-Boy tweaks - hand-lap reed, double crank seals, crankcase stuffing

Greetings, John. Glad you found us. I think that you'll be a valuable resource to all of us here with your knowledge of engine operations and characteristics. By all means, drop in often and share your knowledge! You will get all sorts of assistance with any Lawnboy questions that may arrise.

I assure you that Lilysdad was not being sarcastic. He was saying, in essence, "You lost me, but welcome to MTF!"

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post #8 of 53 Old 06-08-2009, 05:28 PM
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Re: Potential Lawn-Boy tweaks - hand-lap reed, double crank seals, crankcase stuffing

Man, thats a lot of info! I wish i could understand it!:

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post #9 of 53 Old 06-08-2009, 05:35 PM
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Re: Potential Lawn-Boy tweaks - hand-lap reed, double crank seals, crankcase stuffing

Yeah, you're over my head. But, it's great to learn new things. I'm looking forward to hearing how your engine turns out. I've wondered before if a 'F' series engine could be made to run like a Duraforce.
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post #10 of 53 Old 06-08-2009, 05:56 PM
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Re: Potential Lawn-Boy tweaks - hand-lap reed, double crank seals, crankcase stuffing

You could also do some work to the transfer ports, perhaps grinding a little "booster" port on the top edge so the piston would uncover this part first, and get a high velocity stream going before the rest of the port was uncovered. Model airplane engines have done this for decades with considerable increases in top end power.

I still don't know about the double seal idea. If it fits, try it. It certainly can't hurt much, but remember the secondary seal isn't going to get much, if any lubrication if the primary seal is functioning correctly. You'll end up just burning it up and still having just one seal functioning after a while.

Regular old brake fluid works well to soften and rejuvenate old oil seals. Basically, these ATF "conditioners" are not much more than that.

The other thing to remember about these engines is they're turning a relatively low RPM for a 2-cycle engine. While your heat transfer ideas and cylinder coating are great for high-load high-RPM engines, on an engine such as this, I see it being kind of a moot point. Torque is more important than gross horsepower, and that's the one area a typical 2-cycle engine lacks. Lawn-Boys are not bad in the torque department (the M is a little lackluster below its "sweet spot" in the powerband, but that's a piston-ported engine...Goes without saying.) the way they are, and I've never really thought much about doing things other than what you mentioned with making sure the reed sealed well. That'll definitely help low-RPM power a ton. I never really gave it much thought till I read this, but that's going to be one extra step I take on every one that I rebuild from now on.

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Re: Potential Lawn-Boy tweaks - hand-lap reed, double crank seals, crankcase stuffing

Quote:
Originally Posted by lovelearn
I freely confess that I'm often driven by the urge to have fun tweaking to improve factory designs more than what others would perceive to be a "need." John
Uhhh....You have touched on the reason you will fit right in here.
...and there are people here that will understand what you are saying.

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post #12 of 53 Old 06-08-2009, 06:49 PM
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Re: Potential Lawn-Boy tweaks - hand-lap reed, double crank seals, crankcase stuffing

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Originally Posted by TheDodgeGuy View Post
The other thing to remember about these engines is they're turning a relatively low RPM for a 2-cycle engine. While your heat transfer ideas and cylinder coating are great for high-load high-RPM engines, on an engine such as this, I see it being kind of a moot point. Torque is more important than gross horsepower, and that's the one area a typical 2-cycle engine lacks. Lawn-Boys are not bad in the torque department (the M is a little lackluster below its "sweet spot" in the powerband, but that's a piston-ported engine...Goes without saying.) the way they are, and I've never really thought much about doing things other than what you mentioned with making sure the reed sealed well. That'll definitely help low-RPM power a ton. I never really gave it much thought till I read this, but that's going to be one extra step I take on every one that I rebuild from now on.
Its true that added cooling isn't really needed in a stock setup. But I'm personally wanting to slip a 5hp F under the old 3.5hp shroud of my 6259, I've often wondered whether or not this would case cooling problems. These little tips help. I wonder if powder coating the surface would work.

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post #13 of 53 Old 06-08-2009, 09:35 PM Thread Starter
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Re: Potential Lawn-Boy tweaks - hand-lap reed, double crank seals, crankcase stuffing

TheDodgeGuy wrote: "You could also do some work to the transfer ports, perhaps grinding a little "booster" port on the top edge so the piston would uncover this part first, and get a high velocity stream going before the rest of the port was uncovered. Model airplane engines have done this for decades with considerable increases in top end power."
The top ring on F-Series pistons is so close to the piston's top surface that I don't think we can prudently remove enough metal to make a noticeable difference. These top rings are exceptionally close to their piston top. I'm also reluctant to alter the factory's port timing because port timing so strongly influences the torque curve. The periods when I most want more power from this mower is when I'm trying to drive the blade through too much load, causing an overload which lowers rpm. When that happens, I want more medium-rpm and low-rpm torque. I don't want to rebalance the factory torque curve. I'm sure the factory engineers spent a lot of time trying to artfully balance that trade-off.

But I'd love to improve transfer port flow characteristics which would help at all rpms. Again, I didn't mention this earlier because I didn't want to cause information overload with one long post. We have an easy opportunity to reshape those ports with low risk and high probability of improvement. Go-cart racers running Lawn-Boy engines would be well advised to consider the following strategy. The factory ports were formed by two intersecting machining operations. Their side machining cut passed through the cylinder side, through the vertical port, then through the steel cylinder liner. If these were racing engines, that "Y-shaped"-port intersection would be filled with something like JB Weld, then smoothed to form a nice smooth curved port shape. Instead, Lawn-Boy just capped off that side hole into the cylinder side with a metal plug. The intersection is rough, crude and probably creates turbulent swirls like a toilet bowl. I'd be willing to bet that filling that capped side hole to form a smooth tapered port would fatten the entire torque curve. So instead of REMOVING more port material, the first thing I'd do is ADD port wall material to fill that side breach which must cause lots of turbulence that reduces those charging pulse flows which are less than 0.01 second long. We want a nice smooth shot, not a toilet-like swirl just before the port dumps into the cylinder.

"I still don't know about the double seal idea. If it fits, try it. It certainly can't hurt much, but remember the secondary seal isn't going to get much, if any lubrication if the primary seal is functioning correctly. You'll end up just burning it up and still having just one seal functioning after a while."
Your point is well taken. Perhaps we can pack some wheel-bearing grease between them. Eventually that grease would leak out, but
if that's occurring, I'd think the inner seal would be seeping enough lube to keep the outer seal lubed enough to prevent burning.
This is pretty speculative since we haven't tried it yet. Maybe a year from now we will have field experience feedback. Thanks for pointing out that lubrication issue.

According to discussion group posts, many Lawn-Boy mowers have suffered from crankcase sealing pressure losses caused by crankshaft seals migrating outward along the crankshaft until they are no longer captured within the outer case bore hole. When that happens, the seal leaks at its outside rather than at its inside. I know that I'm new here, but I have a solution for that, and my solution would also hold double seals in their bore holes as well as stock single seals.

Observe that both upper and lower case sections have two support "webs" formed to provide side support for their extended side sections which contain the side seals. If we drill a tiny hole through the inside corner of each of those web castings on each side (total of 8 tiny holes), we can run aircraft safety wire through the top web base, out to the end of the seal carrier, then diagonally over the outer crankshaft seal, then down where it would be wired to the bottom web base hole. So we would run two safety wires over the crankshaft seal from top to bottom on each end of the crankshaft. You can probably cut the holes and safety wire them in place in less time than it took me to write this. There's no way in hell those crankshaft seals can migrate out of their case bores if they are safety-wired in place this way. This would NOT significantly weaken the support webs and would eliminate a too-often cited seal failure mode. Many Lawn-Boys have been set out with trash at curbs because one of their crankshaft seals migrated out of its bore. Besides, this way you can definitely run double seals at each crankshaft end without concern that either can migrate out of its case bore. How do you guys like this proposal? Ten minutes of extra work and you've just eliminated one of these engine's most commonly cited failure modes.

"Regular old brake fluid works well to soften and rejuvenate old oil seals. Basically, these ATF "conditioners" are not much more than that."
Interesting idea. Perhaps we can look for discussion by people who many have tried adding brake fluid to automatic transmissions to see
if seal performance has been extended. I know that brake fluid can soften seals. I'm just not confident that it would leave these seals in as durable a condition as ATF seal leak products would. I hope you're right.

"While your heat transfer ideas and cylinder coating are great for high-load high-RPM engines, on an engine such as this, I see it being kind of a moot point. Torque is more important than gross horsepower, and that's the one area a typical 2-cycle engine lacks."
If we DON'T cool combustion gases as much while they can push against the piston top, they will push harder because that higher gas temperature within an equal volume also causes higher gas pressure against the piston top.
Going back to the ideal gas laws:
PV = nRT
where
P = absolute pressure
V = volume
n = number of moles
R = universal gas constant
T = absolute temperature
Thermally insulating the piston top and cylinder head bottom will increase combustion gas temperature, increase gas pressure, increase piston pushing force and increase crankshaft power output at ALL rpms, not just at high rpms. Granted, increased combustion temperatures will slightly increase nitrous oxide formation. The torque increase will occur across ALL rpms, not just at high rpms.

These tweaks will give slightly more crankshaft work output for slightly less fuel input. I doubt that we can tweak an F-series engine to become as powerful as a DuraForce which has a larger air cleaner, an extra transfer port and more displacement. But I'm confident that we can narrow their performance difference. In a way, I feel that the F-series engine is more elegant. How can you improve on casting the head and cylinder together as a single piece? That gives as nearly a perfect thermal lock between those sections as is possible without liquid coolant. And with no head-to-cylinder parting line, the F-series engines can never develop head gasket leaks.

LoveMySan wrote, "I wonder if powder coating the surface would work."
I think that powder coating would have a lousy cost/benefit ratio. Here's my reasoning. To "reject heat" (as thermal engineering classes describe it), we want the surface to have as much hot surface area as possible, and to be an ideal emitter, which is flat black, NOT SHINY black. Power coatings melt as they are baked on and become shiny. That reflective quality is only possible because it smooth. Flat blacks are rougher at a microscopic level so they present more exterior radiating surface area per square inch of fin material. Some "wrinkle finish" coatings can have about 2 square surface inches for every square inch they cover. We want lots of radiation not smooth low-radiation-rate surfaces. Also, keep the covering thin rather than thick so the coating does not become a significant insulating barrier. Flat-black high temperature barbecue spray paint works well and can be purchased cheaply.

Just some of my opinions.
John

Last edited by LoveLearn; 06-08-2009 at 10:20 PM.
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post #14 of 53 Old 06-08-2009, 10:16 PM
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Re: Potential Lawn-Boy tweaks - hand-lap reed, double crank seals, crankcase stuffing

I am wondering why you would need to tweak a LB engine? They are plently effecient for cutting the grass just as they are. Did you realize that the DF engine uses a tuned exhaust and a rear boost port?

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post #15 of 53 Old 06-08-2009, 11:00 PM
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Re: Potential Lawn-Boy tweaks - hand-lap reed, double crank seals, crankcase stuffing

Quote:
Originally Posted by mtgrs737 View Post
I am wondering why you would need to tweak a LB engine? They are plently effecient for cutting the grass just as they are. Did you realize that the DF engine uses a tuned exhaust and a rear boost port?
I wonder why people collect lawnboys. Your missing the point. His hobby is modifying stuff. I do the same thing. Its about having a good time.

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