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Briggs 21 HP Single Cylinder. Bad rings?

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doug2500
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2025/05/15 17:12:22 (permalink)
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Briggs 21 HP Single Cylinder. Bad rings?

Just got this Craftsman mower for free. Nothing is free.....
Engine: Model 331877, Type 1371 B1, Code 100405ZD
Started right up after I cleaned the totally fouled plug.
Burns oil blue smoke.
Has 60 psi compression.
But runs pretty well.
Just wanted to get a second opinion that there's no way this is a valve seal oil leak.
The cylinder looks great except for that stain (see photo) where I think some dirty oil sat for a couple years. It's a horizontal cylinder so that is where any fluid would pool. The stain feels as smooth as the rest of the cylinder.
Spark plug and intake valve all oil crusted. (see photo).
Would an intake valve have this much crusted on on the top side of the valve if it was an oil leak through the rings?
I don't see oil leaking down the valve stem.
Also it blows a lot out of the dip stick hole.
Exhaust valve stem is flat black and dry. (That photo won't load)
 
Ok, it's obviously bad rings, but I'm still open to somebody else saying Duh!
 
If I replace the rings, do I need to clean up that stain even if you can't feel it there?
 
Thanks
:)

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    SRTsFZ6
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    Re: Briggs 21 HP Single Cylinder. Bad rings? 2025/05/15 18:35:51 (permalink)
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    You do have an oil seal on only the intake valve..
     
    With the valve in the head, just off the seat, is there any back and forth play?
    If so, (probably not), that would help with oil getting past the seal and burning oil.
     
    You could do a "Leak down test" to ascertain the health of the engine however
    I think you correct with the diag of worn piston rings.  After cleaning the head, 
    flip it over, spark plug installed and fill with say seafoam-Let it sit.  If the valves are leaking
    the fluid will pass the valve(s) and into the runners.
     
    Re the staining.  As long as you don't have any scrapes (usually vertical), I
    would just hone the cylinder before the new new rings are installed.  
     
    I have a similar engine from 2005 (18HP Briggs) which needed a head and piston (dropped a valve)
    A new head, honed block and new piston/rings, runs like a top (no damage to the block). 
     
    Honed and new piston installed:

     
     
     

     

    Scott
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    doug2500
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    Re: Briggs 21 HP Single Cylinder. Bad rings? 2025/05/15 20:41:55 (permalink)
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    Thanks for the info.
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    doug2500
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    Re: Briggs 21 HP Single Cylinder. Bad rings? 2025/05/16 13:32:08 (permalink)
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    Got the engine all apart. Can you tell me where the little pin in the photo goes that fell out when I turned it upside down?
     
    And there's a lot of cross hatching still. Do I need to hone anyway?
     
    And how do I tell that the rings are actually bad? I don't know what I'm looking for.
     
    I have photos, but nothings loading right now.
     
    thanks
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    SRTsFZ6
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    Re: Briggs 21 HP Single Cylinder. Bad rings? 2025/05/16 13:39:23 (permalink)
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    Are you referring to the two solid pins, showing in the top of my block?  Their just alignment pins.
     
    Re honing, yes, I would give it a light hone for the new rings to seat.  
     
    As for ring wear, put both the compression rings in the cylinder (about 1" down from the top)
    and measure the end gap...
     
     
    From another thread:
    Service doesn't have minimum end gap but .010" should be a target as .030" is the maximum allowed.
     
     
    .
     
    post edited by SRTsFZ6 - 2025/05/16 13:43:13

    Scott
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    doug2500
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    Re: Briggs 21 HP Single Cylinder. Bad rings? 2025/05/16 13:42:57 (permalink)
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    It's not an alignment pin, it is smaller and was loose. I didn't see it when I flipped it to drain the last oil.
     
    What stone number do you use for that level of honing?
    Quote  #6
    SRTsFZ6
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    Re: Briggs 21 HP Single Cylinder. Bad rings? 2025/05/16 14:01:58 (permalink)
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    This is the hone I have and used (for literally decades).  For 2" bores to 7" bores.
    There is no "grade" of the stone listed.  You may be able to borrow one from your local 
    auto parts store...
    https://www.summitracing.com/int/parts/shc-94633/overview 
     
     
    Can you open a free acct at Post image: https://postimages.org/login  
    Upload your pic, then copy and paste the link for FORUMS.
    A pic of your "pin" may help find where it goes...
     
    That and some more detail, how the engine was positioned when this pin came out.
     
    Admin may take a couple of days to allow it to post (for new folks).
     
     
    .
    post edited by SRTsFZ6 - 2025/05/16 14:03:19

    Scott
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    doug2500
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    Re: Briggs 21 HP Single Cylinder. Bad rings? 2025/05/16 16:31:57 (permalink)
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    I'll give that a try if the photos continue to not upload.
     
    The main body of the engine was how it would sit on the mower, with the bottom cover off. It's about 1/2" long and maybe 3/16" diameter.
    Quote  #8
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