Briggs 3HP with piston scoring

[creia]

I recently took an engine into inventory that came w/o a recoil shroud. As such, I do not have the model, type and code numbers. By the overall look of the engine it appears to be a 1960's-1970's -era 3HP:
1. The piston (standard bore) and stroke measurements match a 3HP.
2. There is an "8" embossed into the top left corner of the cylinder head.
3. Points/condenser ignition.
4. "Pulsa-Jet" carb/tank.
Would this make it a Model 81202?

What surprised me was that the bore has a steel sleeve- which is a FIRST for me. In addition, the top of the piston has a "L" embossed into it, which, according to the Briggs repair manual indicates it is a tin-plated piston (like it supposed to have) in a sleeved engine. The bore is not scored, well within specs, and even has some (very faint) cross hatching still visible in some areas. In addition, the ring gaps indicate some wear, however, still within specs. Crankpin, crank journals, and matching bore holes in the block and sidecover are all within specs.

Here is my question: How much piston scoring is allowable, and when is it too much and time to replace the piston?
1. Approx 50% of the piston skirt circumference has fine scoring, and some of which you can catch a fingernail into.
2. There is scuffing over most of the piston skirt.
I am guessing that the engine had been previously ran with a dirty air filter, or no filter at all. The dirty air caused the piston to score rather than the bore as the former is the softer aluminum, while the latter is the harder steel.
Michael

[KE4AVB]

I think you are on the right track about how the scoring occurred, Now I don't do many engine's piston replacements but if you can snag a finger nail that tells the scoring is fairly deep. It is a judgement call but you probably gotten more experience in this area than I have.

Personally not much get rebuilt here anymore as customers are just not willing to pay for the engines to be restored. Matter of fact I haven't even had my hones out in the last 5 years. Now I will do my own engines just to keep in practice if nothing else.

[bgsengine]

Here is my question: How much piston scoring is allowable, and when is it too much and time to replace the piston?
1. Approx 50% of the piston skirt circumference has fine scoring, and some of which you can catch a fingernail into.
2. There is scuffing over most of the piston skirt.
I am guessing that the engine had been previously ran with a dirty air filter, or no filter at all. The dirty air caused the piston to score rather than the bore as the former is the softer aluminum, while the latter is the harder steel.
Michael

Pretty much nailed it - for #2, pretty much any piston coming out of a well used cast iron or steel sleeve cylinder will show obvious skirt scuffing - they are, after all, the thrust faces or the piston, no? You'd tend to see worse on the face that is being pushed against the wall as the piston is being pushed down by ignition of the air/fuel mix - On an L-Head briggs, that's typically the valve side (think about it, visualize it, and you'll probably see) Without a photo it is hard to judge how bad the scoring might be, but in some cases I might run a file very lightly over the scoring, especially at the bottom if any of any grooves, and see if I file away any ridges - if you can see obvious heavy ridges (narrow shiny "lines" along either side of a scratch) you may want to consider a new piston, although it is also possible to get away with filing/sanding away the ridges carefully (600 grit wet or dry paper and a LOT of patience once done) and a very hot soapy water wash after...

It'd remove tin plating on the piston where you sand and file, of course, but still a reasonable rescue if those ridges are not too deep and can be cleaned up, especially for a collector engine that isn't going to get high operating hours or heavy loads.

[creia]

I think you are on the right track about how the scoring occurred, Now I don't do many engine's piston replacements but if you can snag a finger nail that tells the scoring is fairly deep. It is a judgement call but you probably gotten more experience in this area than I have.

Personally not much get rebuilt here anymore as customers are just not willing to pay for the engines to be restored. Matter of fact I haven't even had my hones out in the last 5 years. Now I will do my own engines just to keep in practice if nothing else.

Thank you for the reply KE4AVB!
Michael

[creia]

Here is my question: How much piston scoring is allowable, and when is it too much and time to replace the piston?
1. Approx 50% of the piston skirt circumference has fine scoring, and some of which you can catch a fingernail into.
2. There is scuffing over most of the piston skirt.
I am guessing that the engine had been previously ran with a dirty air filter, or no filter at all. The dirty air caused the piston to score rather than the bore as the former is the softer aluminum, while the latter is the harder steel.
Michael

Pretty much nailed it - for #2, pretty much any piston coming out of a well used cast iron or steel sleeve cylinder will show obvious skirt scuffing - they are, after all, the thrust faces or the piston, no? You'd tend to see worse on the face that is being pushed against the wall as the piston is being pushed down by ignition of the air/fuel mix - On an L-Head briggs, that's typically the valve side (think about it, visualize it, and you'll probably see) Without a photo it is hard to judge how bad the scoring might be, but in some cases I might run a file very lightly over the scoring, especially at the bottom if any of any grooves, and see if I file away any ridges - if you can see obvious heavy ridges (narrow shiny "lines" along either side of a scratch) you may want to consider a new piston, although it is also possible to get away with filing/sanding away the ridges carefully (600 grit wet or dry paper and a LOT of patience once done) and a very hot soapy water wash after...

It'd remove tin plating on the piston where you sand and file, of course, but still a reasonable rescue if those ridges are not too deep and can be cleaned up, especially for a collector engine that isn't going to get high operating hours or heavy loads.

Thank you Brian. After cleaning the piston and rings (some carbon deposits) I lightly sanded the skirt just to knock down any edges. There were no heavy or even moderate ridges, and I would describe the scoring as relatively minor (i know, that is subjective) and it is actually looking not too bad at all. Between a new piston/ring set and honing it would have cost me about $ 150.00 which the budget really did not allow on THIS engine. I'm going to reinstall the piston/ring set and give it a try. I'll report back after test running...
Michael

[KE4AVB]

Costs is why many otherwise good engines here die too. Just can't put more in than I can get back. And this don't count my labor.

And apparently these replacement battery powered walk behind mowers that Lowes is selling is not supported after the sale. Or at least I can't get Kobalt to reply to my emails.

[creia]

"Houston, we have a problem"....
UPDATE:
The bore ended up having only .0005-.001 wear. With the cylinder wall cross hatching still (barely) visible I decided to just put the same rings back in and reassemble everything.
I started it up today and it now SMOKES! (just when I open or close the throttle- not when its running). It did not smoke one bit before I took it apart. I was very careful to put the original rings back in their proper locations, and I did stagger the ring gaps properly as well. Does anybody have any ideas what is going on?

A couple of thoughts I have:
1. Both of the compression rings (top and middle rings) ARE oriented the proper direction, however, I do not know if the bottom (oil control) ring is oriented the same side up as it was when I took it out. While it is simetrical, and CAN be installed either way, I might have installed it in a different orientation than when I took it out- That is to say, it is now "upside down" compared to when I took it out. Maybe the oil ring had seated in a different direction and now I have it opposite side up and oil is getting past? Is this plausible and could be causing the smoking?
2. Ring gaps- I have never paid attention to WHERE the ring gaps are around a piston when I remove it. I just make sure when I install a piston that the ring gaps are separated by about 120 degrees, and that has never been a problem. Should I make sure that the ring gaps are in the same position when I reinstall a piston? It has always been my understanding the piston rings move around the piston as the engine is running so it would be a moot point.
Thank you,
Michael

[bgsengine]

"Houston, we have a problem"....
UPDATE:
The bore ended up having only .0005-.001 wear. With the cylinder wall cross hatching still (barely) visible I decided to just put the same rings back in and reassemble everything.
I started it up today and it now SMOKES! (just when I open or close the throttle- not when its running). It did not smoke one bit before I took it apart. I was very careful to put the original rings back in their proper locations, and I did stagger the ring gaps properly as well. Does anybody have any ideas what is going on?

A couple of thoughts I have:
1. Both of the compression rings (top and middle rings) ARE oriented the proper direction, however, I do not know if the bottom (oil control) ring is oriented the same side up as it was when I took it out. While it is simetrical, and CAN be installed either way, I might have installed it in a different orientation than when I took it out- That is to say, it is now "upside down" compared to when I took it out. Maybe the oil ring had seated in a different direction and now I have it opposite side up and oil is getting past? Is this plausible and could be causing the smoking?
2. Ring gaps- I have never paid attention to WHERE the ring gaps are around a piston when I remove it. I just make sure when I install a piston that the ring gaps are separated by about 120 degrees, and that has never been a problem. Should I make sure that the ring gaps are in the same position when I reinstall a piston? It has always been my understanding the piston rings move around the piston as the engine is running so it would be a moot point.
Thank you,
Michael

If it is smoking on snap throttle as you describe - as you may well know that's crankcase vacuum changes which tends to "show" blow-by - generally after putting rings back in after taking them out, they'll need some run time to re-seat - Yes they do rotate a bit in the bore so end gap staggering is not exactly an oil control technique (in theory it can help a bit) but more to avoid having gaps line up and resulting uneven wear and seating.
It's why a cross-hatch is wanted in a bore - more to aid in ring rotation and seating - It'd be impossible to put rings back in precisely the same spot so they'll need time to re-seat - and in a used bore and used rings, this may take quite a bit longer than you might see with new rings, since the old rings will have imperfect cross section (Some areas may be thinner than others) plus worn rings may have sharp corners that can break off and dig in to a bore

In a nutshell, sounds to me like you need to go through the usual "new rings" break-in process (varying RPMS and loads, etc over a few hours of run time, run to full operating temp, cool down, run again, etc) and then change oil (and look carefully for any metallic particles in it) - Rings will need time to re-seat in any case, and once they do, your smoking MIGHT go away (But watch for it to get any worse, which may indicate your old rings might have caused some scoring or galling...)

[creia]

Thank you Brian. I'll continue to run it and "break in" the ring reinstall.
Sooooo......
Do you think that installing the (bottom) oil control ring "upside down" as compared to how I took it out contributed to this? (I am thinking previous wear surfaces would now be different/reversed, and not seating as before.) I am not SURE that I did this- it would be a 50-50 chance...
Michael

[bgsengine]

Thank you Brian. I'll continue to run it and "break in" the ring reinstall.
Sooooo......
Do you think that installing the (bottom) oil control ring "upside down" as compared to how I took it out contributed to this? (I am thinking previous wear surfaces would now be different/reversed, and not seating as before.) I am not SURE that I did this- it would be a 50-50 chance...
Michael

Oh probably a wee bit yes - even if you got them back smack-dab exactly in the same locations, positioning and even ring gap staggering, they'd still need to re-seat anyways, unless you lucked in to nailing their positioning within a half thousandth of an inch of where they came out from..