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I just rebuilt this Model 5083 Lawnboy with a D-400 series engine. I put new rings in it, among other things. This has a 2-ring piston. I started it and adjusted it and mowed some with it, and after I finished one small yard, I noticed when I switched it off, that it appeared to be dragging (and no brake on this early model). I tried moving the blade and it seemed the piston was "almost" frozen, or locked up, but not quite. The engine was quite hot. Now I used STP as assembly lube when I reassembled the piston into the cylinder. I took out the spark plug and squirted some light oil into the cylinder and worked the piston back and forth using my hand on the mower blade, and got it freed up. Now I was fueling the mower with my standard mix of 8 oz. Lawnboy oil, to 2 galllons non-ethanol gas, with a dollop of Marvel Mystery Oil. It was my understanding that the 2-ring piston Lawnboy used this ratio of oil which is 1:32. This engine has a roller bearing top bearing and a sleeve lower bearing. Anyway, I mixed up a fresh gallon of fuel with double the lawnboy oil 1:16, and continued mowing and it seems to be OK. I find this curious and can't explain it, unless this mower "actually" needs the 1:16 ratio. Any ideas why it almost froze up? In any case I'm continuing with the 1:16 oil ratio for now.

If my memory is still working Lawn Boy didn't switch to a 32/1 fuel mix until the mid seventies when the Series F engines came out. The series C and D engines used a 16/1 mix. There are a couple of Lawn Boy lovers on this forum who will be able to tell us if I'm right or wrong.

Whether a mower, outboard motor, motorcycle, etc. ALL the wise old sages say that when reringing or rebuilding a two stroke motor, run it with double oil to seat the rings and wear in the bearings . Run a few tanks of double mix, then you should be good.

Piston installed correctly? The word TOP cast into the piston skirt should face away from the deck.

Jeff

jp1961 said:

Piston installed correctly? The word TOP cast into the piston skirt should face away from the deck.

Jeff

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Yes, I didn't take the piston off the con-rod or crankshaft, so it could only go back the way it was intended. I just removed the cylinder, exposing the piston.

UPDATE: So far, running on 1:16 oil ratio, its running fine, and no smoking.

Piston bore clearance is a tricky thing. For instance going from cast to forged or a new mfg can cause problems. Bore clearance difference between measured bore ID and piston accross the skirts? Some require a feeler gauge and a "fish deliar" pull . Just ideas.

motoman said:

Piston bore clearance is a tricky thing. For instance going from cast to forged or a new mfg can cause problems. Bore clearance difference between measured bore ID and piston accross the skirts? Some require a feeler gauge and a "fish deliar" pull . Just ideas.

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Please note: I did not change the piston or cylinder. I merely separated them, and installed new rings on the existing piston, then put back together. I did not take the piston off the con-rod, nor did I remove the con-rod from the crankshaft. Minimal changes here.....just new rings. The cylinder was not scored or otherwise damages, likewise, the piston was fine. The mower was working before, it just had low compression, which the rings cured. I also did other external things, like new points, new seals, rebuilt carb, new blade, etc.

photo, Good show and hope it evens out for you.

Lawn Boy service manual shows a 32to1 mix for 1972 and up motors and 16to1 for 1971 and back motors. In 1970 they changed to a 2 ring piston with needle bearings at both ends of the rod. 1969 and older motors used a three ring piston with a bushing at the piston end of the rod. When they added needles to the piston end of the rod AND changed the oil quality the motor was then a 32to1 mix motor. The combination of these two changes in 1972 made using a 32to1 mix ok. Using the current LB quality oil in a 1970 or 1971 motor would be ok at a 32to1 ratio because they have the needles at both ends. Now back to the motor running hot and getting tight. When you installed new rings , what was the end gap of the new rings?

so it would be safe to say you did not hone the cylinder or cross hatch the piston skirt?

Now that it's been run for some time (an hour or two -- how many hours?) how much better does if feel if you turn it by hand? If it's improved a lot since that first day then I'd guess the new rings just needed to be run in. It's still not a good sign that it got hot and stiff like that at first -- maybe something was a little off during the re-assembly process.

Anyway -- that's a nice looking old bricktop. Thanks for the photo and good luck. :thumbsup:

Two-Stroke said:

Now that it's been run for some time (an hour or two -- how many hours?) how much better does if feel if you turn it by hand? If it's improved a lot since that first day then I'd guess the new rings just needed to be run in. It's still not a good sign that it got hot and stiff like that at first -- maybe something was a little off during the re-assembly process.

Anyway -- that's a nice looking old bricktop. Thanks for the photo and good luck. :thumbsup:

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After the first "issue" with the engine getting tight....I switched to a 1:16 ratio of Lawnboy Oil to non-ethanol gas. I have mowed quite a few of my yards now, and the mower feels and acts normal. No binding of the piston. The mower has plenty of power, does not bog down in thick grass. Starts up with 3 or 4 pushes of the primer (I did not rebuild the primer), and runs steady. Since I was using mowers of this style when this mower was new, I can say this feels normal in every way now, based on my past experience. I am reluctant to go back to 1:32 ratio oil use, though. I am not sure exactly when the Model 5083 was made, but I feel it is right in the transition period on the oil ratio change. It has a 2-ring piston, has a sleeve bearing on the bottom of the crankshaft, and a roller bearing on the top. Using a 1:16 ratio oil mix, the mower runs fine, and does not smoke. I did no honing or any other cylinder preparation other than cleaning. I did not replace the piston, just the rings.

What was the end gap of the new rings?

LB8210 said:

What was the end gap of the new rings?

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Sorry, I did not measure that, and it's assembled and running well now, and I cut yards with it. So I'm not taking it apart.

You are running 16to1 mix in a 32to1 motor. The extra oil is masking the problem. Any time the rings are replaced, the end gap has to be checked and rings fitted as needed. LB service manual calls for .005-.015 end gap. When running a motor without enough ring end gap, as the motor warms up to normal operating temperature the rings expand and the ends of the rings butt into each other, then this forces the rings out tight binding against the cylinder wall. This causes increased fiction and results in the motor running hot and over heating making the piston expand and getting tight in the bore. The extra oil will also cause binding and packing of the ring grooves sticking the rings and ports with carbon. The tight rings and carbon will cause cylinder wall scoring. This motor really needs to be taken apart and the end gap of the rings fitted to the proper clearances before it is too late and major damage happens. Also anytime the rings are replaced the cylinder needs to be honed to break the glaze so the new rings will seat properly to have good compression. Not honing the cylinder will keep the rings from never seating.

LB8210 said:

You are running 16to1 mix in a 32to1 motor. The extra oil is masking the problem. Any time the rings are replaced, the end gap has to be checked and rings fitted as needed. LB service manual calls for .005-.015 end gap. When running a motor without enough ring end gap, as the motor warms up to normal operating temperature the rings expand and the ends of the rings butt into each other, then this forces the rings out tight binding against the cylinder wall. This causes increased fiction and results in the motor running hot and over heating making the piston expand and getting tight in the bore. The extra oil will also cause binding and packing of the ring grooves sticking the rings and ports with carbon. The tight rings and carbon will cause cylinder wall scoring. This motor really needs to be taken apart and the end gap of the rings fitted to the proper clearances before it is too late and major damage happens. Also anytime the rings are replaced the cylinder needs to be honed to break the glaze so the new rings will seat properly to have good compression. Not honing the cylinder will keep the rings from never seating.

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Undoubtedly, what you say is true from a perspective of ultimate restoration. Of course, you know that only one set of rings is available for the D series engines. Are you SURE my model is designed for the 32to1 mix? I believe it was made right around the time of transition to 32to1 from 16to1. In any case, I am happy just the way it is. I am not going for a 100 point restoration, as I am using the mower.

Any D model mower with a 2 ring piston has needles at both ends of the rod and is a 32to1 motor. Any motor ( mower, car, truck, etc. ) that you replace rings on as part of the job you have to check ring end gap. You adjust end gap by filing the ends of the ring to fit the bore of the cylinder of the engine, with the proper amount of end gap. This is not just for a 100 point restoration , but a part of the job of any motor rebuild, a required step in the rebuilding process. The rings are not sized to fit any motor off of the shelf right out of the box, all must be fit to the cylinder bore that will vary between motors. Also before sizing the rings the cylinder has to be honed to break the glaze so the new rings will seat and because honing will enlarge the bore which in turn will increase the ring end gap. If the motor is running hot and is tight, you do need to now before damaging this motor.

LB Well said and after the end gap filing we'd hand hone a slight radius on the sharp ends of the rings.