Briggs & Stratton piston to bore clearance
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  1. #1
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    Hello Everyone,

    I am getting ready to bore a 6hp ohv Briggs engine .020 over. The model# is 120412. The problem I am having is determining the piston to bore clearance. The manual says to take .020 for a .020 over piston but gives no numbers on clearance. The standard bore is 2.6875 & .020 over is 2.7075. The new piston is marked .020 on top & measures 2.705 @ the skirt& this seems a little tight to me. I have called Briggs & they were no help. If anyone has thoughts on this I would greatly appreciate it. This has an aluminum bore also. Thanks, Tom

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    i would set it up at .003 to loose at .005
    try for .0035 factory likes to set up the new b/s tight better make it dang straight if you go .0025 . they run fine at .003 to .005 jim
    make shure you use a good ridged hone and high sulfer oil . have it bored ,not honed out .020 ..jim

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    Thanks a lot Jim I really appreciate it. I was going to "guess" @ .004. I have had a hell of a time finding info on this subject & should have come here first lol. Thanks again, Tom

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    I was a Briggs dealer. Bore it out to exactly .020 and hone it to get the crosshatch pattern and it will have exactly what Briggs wants for clearance. It is as simple as that. Do not try to guess what it should be and do not try to use your or anyone elses percption of what it should be. When I did this under warranty or as a repair for a customer I did not deviate from Briggs instructions. I may be able to find the clearance for you but I DO NOT recommend using any other way.

    That is .020 over the stock bore size. The clearance is built into the piston.

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    Carl is correct. I am a Briggs dealer also. Cylinder dimensions for that model are 2.6875-2.6885". Bore it to 2.7075-2.7085". I had a local machine shop bore a block for me a few years ago. The owner swore by a thou per inch of bore diameter for piston to cylinder clearance. I took his word for it and it cost me a short block when the engine failed. I still send out blocks for boring and the shops I use will ask for the piston. I won't send the piston with the block. I tell the machine shop the dimension I want the block bored to.

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    I thought that anything less than half an inch was good to go. I have an old Briggs powered lawnmower 30 years old that still runs. The rod knock hasn't gotten much louder in the last ten years and the piston slap is almost unnoticeable with the rusted out muffler.

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    Thanks for all the info guys, I really appreciate it. I have been a machinist for 13 years & have never come across a situation like this. It really does pay to "ask" even if you "think" you know what you are doing. Thanks again, Tom

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    jcarter, I agree when you bore small engines you bore them exactly .010,020 or 030 over the original std. bore. I managed a machineshop for about 20 years. The reason I always asked for the piston they were going to use is to make sure it was the size it was suposed to be. You would not believe how many pistons I found that were either not the size stamped on them or were damaged in shipment and would have failed after a short time. These were not only small engine pistons some were automotive pistons as well. The average machanic doesn't always check each piston with a mic. before installing them. Who do you think is the first to be blamed if something goes wrong with a piston in a new bore job? Just my .02 worth.
    Gary

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    Yep always ask . A buddy of mine did some work for someone and the customer had 2 pistons that where stamped wrong and they did slap in the bores.

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    As JS said, always check the piston size(s) first. In 20 years, I bored and honed thousands of cyls. of all types and would not accept a job w/o the pistons. You'd be surprised at the difference in measurements of the 8 pistons in the same set, at times. I also came across more then 1 piston/set that was marked incorrectly. Rings also. Also nicks and skirt damage from dropping in handling or shipping. It's cheap insurance.
    Aloha, Doug

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    Another thought.Some engines, and maybe not BS, used some hard particles (I don't remember what anymore, maybe silicone carbide or?) in the casting material to help prevent galling of the Al piston in the AL bore. The process after boring and honing to size was to use just the Felt Wipers on the hone with some kind of a compound to actually wear the Al away a micro. amount and let the silicone(?)have the actual skirt contact. Also, as I said, bore and Hone to size. If you bore to size, then hone, the bore will be oversize as about .002 needs to be honed to provide the proper crosshatch pattern and also the proper grit of hone stones must be used to match the type of rings. IIRC, it's 220 gr for chrome and 400 gr for moly.
    Aloha, Doug

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    Reynolds Metals made aluminum alloys used by McLaren back in the 70's that had silicon carbide in them. After boring, the bores were treated with an acid etch that ate away a minute amount of aluminum leaving the silicon carbide as the wear surface, thereby making a robust bore that didn't require a sleeve.

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    It never took me more than a few passes in the cyl to hone the cross hatch. I doubt that it removed more than .0005 to .001 material. Actually I just used a cyl mop, the knoby clean up tool. All you are doing is knocking off the high spots, not trying to hone to size. The small air cooled engines are a little different from the automotive engines. I know I measured the clearance of new small engines for reference but after 30 years I can't find or remember the specs. Oh well.

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    Thanks again for all the feedback. The story behind this engine is kid of sad. My elderly neighbors bought a generator in case of a power outage as his wife is on a dialysis machine. They scraped up every penny to buy this & he didn't realize it wasn't shipped with oil & started it. The piston welded itself to the bore & they don't have money to fix it. I bought the parts out of my pocket so I can repair it for them & want to get it right the first time. You guys here are the best.

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    That's a nice thing to do. I hope it works well for you and them. Let the engine run in without a load for 15 min. to a half hour and it should be ok. If all else fails then short block it.

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    Re: the aluminum bores with silicon. That is the dicast GM Vega engine. A disposable engine in a disposable car. To bad it needed to be disposed of in under 10,000 miles. Sunnen used to have an extensive writeup about how to hone the aluminum blocks so the piston would ride on the silicon, and not the aluminum. Cast iron sleeves were much better choices.

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    RE: RE: the aluminum bores with silicon...

    Porsche too! -not exactly "disposable".

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    gbent, I agree the castiron sleeve was a better fix then just boring the Vega block. I sleeved a bunch of those blocks.
    Gary

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    jcarter & carl,

    I bored & honed the cylinder too 2.7081. When I put the rings in the bore to check end gap, the top & middle rings are all exactly .010 tight from what the manual calls out. I used feeler gauges & pins just to double check. I also checked the bore with a dial bore guage & on a cmm. It is concentric within .0001 with zero taper. Is this normal, & what would be the best way to get the gaps right. Tom

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    I have opened up B&S aluminum cylinders with a Sunnen hone rather than a boring bar. After the cross-hatch is complete, it is nessesary to wash out the bore with very hot water and a strong detergent. Big shops somtimes use a machine like a giant dishwasher and a rotating brush in the bore. As soon as you are sure you have the bore cleaned out completely, wash it 5 more times! When checking ring end-gap, if you need to open the gap, we used to clamp a fine file in a bench vise so both cutting surfaces are verticle. The ring is gently placed so the ends of the ring bear on the cutting surfaces of the file. The ring is then gently pressed aganst the file and dragged back & forth. This alows you to remove a controlled amount of ring material and with regular checking come in to the exact clearence wanted. This is an old, slow method but has given good results.
    Good luck and I salute you for being a good Samaritan.

    hms50

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