Briggs & Stratton piston to bore clearance - Page 2
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  1. #21
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    Seconding hms50s method of opening up rings. Old Landys need this sometimes as well and that was my method of choice.

    If you have the piston clearances you need and it's just the rings a bit under-spec I wouldn't hesitate to re-gap the rings.

    Alan

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    Don't rub back and forth,you don't want to chip the ring face.File in one direction only.Have the ring face the leading edge and if it chips,it will be on the piston land side.

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    According to the Briggs manual, if the cylinder is bored exactly .020 over proper piston to cylinder clearance and ring end gap will be maintained. I have occassionally found rings that have a tight end gap. The method I was taught for adjusting end gap is to clamp the ring in the vise with soft jaws and lightly file the end of the ring being careful to keep the end square. File lightly and install the ring in the bore and recheck. Continue this until clearance is within specs. Are you using genuine Briggs parts or aftermarket?

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    What jcarter said is the way. I don't think I ever had to file the rings on real Briggs parts but that was 30 years ago. The main thing is to have enough clearance.

    hms50, the problem with honing out .010 or .020 or .030 is the cyl will not be a true circle. A cyl is egg shaped from wear and the hone will not make a true bore. As a matter of fact the egg shape migrates around the bore as the hone works.

    There is an interesting thing about spacing the ring gaps around the piston. Years ago Cummins was getting some oil consuption claims that said the ring gaps were all in a line and caused the oil consumption. They did tests where the rings were locked in line and the engine dynoed with no oil consumption. They used extra wide gaps with no problems. They lined the gaps up and ran the engines and tore them down and found the gaps in various positions. It was determined the the location of the ring gap at assembly did not matter because the rings turn in the bore during running. They continued to tell us to space the ring gaps at installation but not to be concerned if they are lined up or near lined up at a tear down.

  5. #25
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    jcarter,

    These are genuine briggs parts ordered right from them. There were no part numbers supplied with the rings as they came in the box with the piston. The box is marked 694168 piston assy .020. I went by the book & fit them tonight @ work. I will keep you posted how it turns out. Thanks again to all. Tom

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    ( I also checked the bore with a dial bore guage & on a cmm. It is concentric within .0001 )


    I think you mean "round" to .ooo1...it would not be concentric to anything

  7. #27
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    Booger, Yes, I see why your method would be safer. It also is important to have a very smooth file. The best I've found were sold for sharpening scissors.

    Carl, That is a good point about the failure of honing to produce a round bore. By all means bore first if you have the equipment. I tried some special rings once that made to have no gap. If my memory is working correctly, they were called "Total seal or Perfect seal" or somthing close to that. The company that makes them starts with a conventional compression ring and machines a rebate on the outside edge. A seperate ring that looks like a rail for an oil control ring snaps on this rebate with it's gap 180 to the main gap. This allows for expansion and at the same time should prevent some high pressure gas leakage.
    hms50

  8. #28
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    Just for the record:How to correctly bore a silicon impregnated block.

    First bore the block .003-.004 undersize,hone to correct size,then lap the aluminum away,leaving the silicon bore.It should have a full chrome ring set for it.

    You need to leave a couple thousands to hone out or you will NOT remove the boring marks.If laping is not done,the rings will lap the bore for you and you will be pumping aluminum laping compound through the engine.

    As far as piston clearance,these days pistons are ground/turned with the correct clearance in them.By all means go with what B&S says to run.As a rule of thumb,a cast piston is run between .0008-.0015 clearance and a forged piston .0015-.003 clearance(sometimes alot more).I've had pistons on fuel cars that run .010 clearance,the pistons sound like they are changing holes when you first start it up.When piston skirts are turned/ground,they are NOT round and straight.The skirt is tapered and egg shaped.The piston gets smaller as you go towards the head and the skirt only rides on about an 1" of the center on the skirt.The skirt is cam ground/turned.So if you measure the skirt close to the pin,it is smaller then measuring at the middle of the skirt.Even if it's a full skirt piston,it will measure the same way.

    Another way of "filing" the rings is to put a cut-off wheel in a 3/8 drill motor clamped in a vise.You can get wheels that are very thin so you don't risk distorting the ring,much less chance of chipping and you will end up with nice square end gaps.

    Honing with a GOOD hone(not a glaze breaker)will give you a perfectly round hole(within a tenth).The bore wears like a bent snow cone,big on top up to the ridge,small on the bottom,with more wear on the thrust side of the bore.The only disadvantage to honing a bore out is that it will relocate the bore to the thrust side of the block a couple thousands.No big deal for a street motor.I hone blocks all the time .020 over,much faster then 2 setups, boring then honing.You have to overstroke the bore on the bottom and dwell the head at the bottom in order the straighten the bore.A good hone will have 2 carbide guide shoes and 2 sets of stones,not 3 stones or felt wipers.

    Total Seal rings are good for a 2%-4% gain in a leakdown test,not much use on the street,but if every 100th of a second counts...

    [ 11-09-2006, 05:53 PM: Message edited by: Booger ]

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    scrapdaddy,

    Sorry if that sounded vague. After I bored the cyl. I swept the bore with an indicator & it was concentric to the spindle by .0001

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    I just wanted to say thanks again to everyone for all the help on the Briggs rebuild. I started the motor today & it ran great. I ran it for 15 minutes @ different rpm's, changed the oil then hooked it up to my refrigerator. I ran it under load for 1 hr & it did not seem to bogg down @ all. It looks like my first rebuild is a success. Thanks again, Tom

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    Glad it all came together.
    A nice gesture on your part, taking care of the folks.
    Sometimes the "warning no oil" stickers are not very conspicious.

    Also, a very interesting thread.

  12. #32
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    Thanks Kenh,

    I learned a lot from this project & I hope new aspiring machinist's here can learn the same lesson that I did. After being a machinist/quality inspector for 13 years, I thougt rebuild on a small engine would be an easy task. It was not by any means. Even with a manual in hand I still had a ton of questions & that is why I came here for advice. Even though I had the proper machines & tools on hand to do the job the right way, I still had to be patient. That is something you can not teach on a forum & if you think you know it all, you have not done every thing yet. There will always be something out there that will bite you. Thanks again, Tom

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    Going to piggy-back off this post, hope it's OK
    Several years ago (early 80's) I built a wood splitter. The power plant is an old vertical shaft 4hp briggs that when I got it was missing the original shroud, I had a shroud that fit but have no model number to order new gaskets, piston and rings.
    The engine is using oil badly and needing rebuilding that will probably require boring.

    Is there a way to order parts without a model number?

  14. #34
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    Quote Originally Posted by Carl Darnell View Post
    What jcarter said is the way. I don't think I ever had to file the rings on real Briggs parts but that was 30 years ago. The main thing is to have enough clearance.

    hms50, the problem with honing out .010 or .020 or .030 is the cyl will not be a true circle. A cyl is egg shaped from wear and the hone will not make a true bore. As a matter of fact the egg shape migrates around the bore as the hone works.

    There is an interesting thing about spacing the ring gaps around the piston. Years ago Cummins was getting some oil consuption claims that said the ring gaps were all in a line and caused the oil consumption. They did tests where the rings were locked in line and the engine dynoed with no oil consumption. They used extra wide gaps with no problems. They lined the gaps up and ran the engines and tore them down and found the gaps in various positions. It was determined the the location of the ring gap at assembly did not matter because the rings turn in the bore during running. They continued to tell us to space the ring gaps at installation but not to be concerned if they are lined up or near lined up at a tear down.
    What kind of a hone were you using that wouldn't make a round bore?

  15. #35
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    Default Donnot second guess piston manufactures

    Quote Originally Posted by gappmast View Post
    What kind of a hone were you using that wouldn't make a round bore?
    Second guessing piston manufactures is tricky, and that being said if you want to, call them. But lets say your putting a piston in to something it was not designed for, they will just tell you don't.
    These are some rules of thumb I've picked up over the years:
    - .0003 per inch of bore on many modern motors from factory on a cast piston
    - .0005 per inch is the tightest I will normally go
    - .007 if the piston is going to run very hot or sever abuse
    - probably .001 to .0015 per inch of bore for win or break situations where longevity isn't even on the table, and piston slap probably will be herd on cold starts.
    - Forged pistons a little looser still, they grow with heat normally more than cast or hyperutetic cast which grows the least.
    - always error on the side of loose, it may burn a tid bit more oil(maybe)but if it over heats for some reason like the piston being too tight or other at least it won't heat sease or gaul.
    - Generally the top ring I will fit per manufacture spec if I like it, but in normal stuff .004 per inch of bore the second per "man" or the same as the top or slightly looser. The thought today or 6 months ago is that having gasses under pressure in between the top and second ring(because the second is tighter and holds more gasses, causing the top ring to float), is bad. The pressure is to push down along the piston, onto the ring and then into the ring grove and thusly push out on the top ring ever so slightly with pressure. the reason the top ring can be the same gap as the second is that it runs hotter and thusly grows more and its generally made out of a chrome steel which grows more than cast material.(I saw that all those early briggs things were cast top and second.). The oil ring generally is big clearance wise.
    -Always clean bores with a clean white rag or paper towel and a wd type oil, continue wiping the bores with fresh towelettes until there is no remnant of any thing dark on them. You will be surprised on what you've left after cleaning with hot soapy water.
    Ok so now why I'm posting is because I'm rebuilding a Kawasaki 18 hp twin and the damn factory manual is specking .012 thou for clearance in a 2.892 bore, which is a big pile of hooie. It must be a miss print in a factory manual. Almost the same motor with a slightly larger bore a 20 hp, still air cooled. Is calling .0006 to .0022 that sounds like in the middle of the road is rite.
    Enjoy I hope it helps some one

  16. #36
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    Second guessing piston manufactures is tricky, and that being said if you want to, call them. But lets say your putting a piston in to something it was not designed for, they will just tell you don't.
    These are some rules of thumb I've picked up over the years:
    - .0003 per inch of bore on many modern motors from factory on a cast piston
    - .0005 per inch is the tightest I will normally go
    - .007 if the piston is going to run very hot or sever abuse
    - probably .001 to .0015 per inch of bore for win or break situations where longevity isn't even on the table, and piston slap probably will be herd on cold starts.
    - Forged pistons a little looser still, they grow with heat normally more than cast or hyperutetic cast which grows the least.
    - always error on the side of loose, it may burn a tid bit more oil(maybe)but if it over heats for some reason like the piston being too tight or other at least it won't heat sease or gaul.
    - Generally the top ring I will fit per manufacture spec if I like it, but in normal stuff .004 per inch of bore the second per "man" or the same as the top or slightly looser. The thought today or 6 months ago is that having gasses under pressure in between the top and second ring(because the second is tighter and holds more gasses, causing the top ring to float), is bad. The pressure is to push down along the piston, onto the ring and then into the ring grove and thusly push out on the top ring ever so slightly with pressure. the reason the top ring can be the same gap as the second is that it runs hotter and thusly grows more and its generally made out of a chrome steel which grows more than cast material.(I saw that all those early briggs things were cast top and second.). The oil ring generally is big clearance wise.
    -Always clean bores with a clean white rag or paper towel and a wd type oil, continue wiping the bores with fresh towelettes until there is no remnant of any thing dark on them. You will be surprised on what you've left after cleaning with hot soapy water.
    Ok so now why I'm posting is because I'm rebuilding a Kawasaki 18 hp twin and the damn factory manual is specking .012 thou for clearance in a 2.892 bore, which is a big pile of hooie. It must be a miss print in a factory manual. Almost the same motor with a slightly larger bore a 20 hp, still air cooled. Is calling .0006 to .0022 that sounds like in the middle of the road is rite.
    Enjoy I hope it helps some one

  17. #37
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    Maybe,......Caterpillar 3208 engine specs .020" piston clearance new pistons......you better believe it,fit at .015" and seized on the first run .Lesson learned.

  18. #38
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    Question, how do you measure a piston. I know with a micrometer but where do you measure it.

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  20. #39
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    Those who are saying that a worn bore cannot be honed perfectly round are incorrect,I have honed worn B&S cylinders, on a Sunnen LBB1660, straight to .020" oversize, with no boring, and they came out round, parallel and on size with a minimum of fuss.

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  22. #40
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    Quote Originally Posted by LexD View Post
    Those who are saying that a worn bore cannot be honed perfectly round are incorrect,I have honed worn B&S cylinders, on a Sunnen LBB1660, straight to .020" oversize, with no boring, and they came out round, parallel and on size with a minimum of fuss.
    There are many levels of making nice piston holes.
    Each has a working. Mega bucks to do what the big makers do but is it needed?
    No small shop can ever hope to compete with Honda, Toyota, GM, Ford in this but there is good enough and functional which is what counts and works.
    Step A is above member gapmastt. There the rabbit that I am sure Tony may have something of help.
    Why are pistons in the cold state not round but instead cam made? How does this work two stroke vs four.
    Why the huge effort in manufacturing to make such weird? Certainly basic round would be easier.
    Bob


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