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Bore or hone to .030 oversize


jlasater

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Can you hone a cylinder to .030 oversize, or do you have to use a true boring bar? This is on a 3.5" (approx) Briggs aluminum cylinder. The Intertec manual mentions just getting a hone from Briggs, but that seems like a heck of a lot of metal to take off in a controlled way. I'm also contemplating getting the oversize pistons moly coated, but no one seems to be able to tell me if it's worth the trouble for reliability.
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Most of the better machine shops will bore to with-in a few thousanths and then hone to fit. Can't say that I had ever heard of anyone going near .30 with a hone. A boring bar not only bores to size, it gets the bore centered once again. A worn cylinder don't wear on all sides evenly. And though there are many machines out there I have never seen, I can't see how you would center a bore with a hone..
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Also, a boring bar will clean up the taper in the cylinder, where a hone will simply follow the taper.
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Are you sure you can bore the alm cyl? Was told on an 8hp IC you can't even hone them as you remove the glaze of somesort thats used and the cyl will gall the first time it gets to operating temp. Not stating this as fact, just info.
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quote:
Originally posted by MPH
Are you sure you can bore the alm cyl? Was told on an 8hp IC you can't even hone them as you remove the glaze of somesort thats used and the cyl will gall the first time it gets to operating temp. Not stating this as fact, just info.
Yes, you can rebore aluminum cylinders. I have done it several times with excellent results. The problem you describe is usually caused by improper boring. Many machinists are not familiar with aluminum cylinders. They bore to fit the piston, just as they do with cast iron cylinders, but the difference between cast iron and aluminum in response to heat creates a different situation. Particularly with aluminum cylinders, Briggs does not recommend reboring to fit the piston. They recommend reboring to the proper increment over standard, then using the appropriate Briggs piston. If an aftermarket piston is used, the cylinder should be bored to fit the piston, but should be done by someone familiar with aluminum cylinders. Jeremy, as to moly coating pistons, I can't answer that question. Of the piston and cylinder, one has to be harder than the other in order to accept the bulk of wear. Both cast iron and aluminum cylinders use aluminum pistons, BUT the pistons for aluminum engines are tin plated. In a cast iron engine, you will find the majority of wear on the piston. On aluminum engines you will find the majority of wear on the cylinder. Pat
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Jeremy, One more tip. Be sure to fit the individual rings to the bore. Make sure that you have the end gap on the rings right. If you have it too close, you will gall the cylinder wall immediately upon trying to start the motor. You also want a very fine finish hone job done on the fresh rebore.
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Thanks Pat. Maybe I will have to fix that commercial grade 'Bobcat' snow blower yet. Its one heck of a built machine, just had a rod outta the gear reduction briggs.
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From what I'm reading in this Intertec manual, what Pat says seems to be correct. They don't list matching pistons to bores at all. Just honing to the overbore size and getting the correct piston. Seems like some trust is needed there in that the pistons will just fit. So...to recap, standard bore on this engine is 3.4365 ~.4375, so just tack on .030 to the bore through boring/honing, and get the .030 oversize pistons and call it good? The damage I'm trying to fix is this, only in one cylinder: [img]http://www.ttyr2.com/briggs_rebuild/pictures/image15.jpg[/img]
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quote:
Originally posted by jlasater
So...to recap, standard bore on this engine is 3.4365 ~.4375, so just tack on .030 to the bore through boring/honing, and get the .030 oversize pistons and call it good?
That's all I've ever done and it's worked well for me. I've done well over a dozen without a problem. I would also follow DaleC's suggestion above. If you're thinking of reboring only one of the cylinders, I'm not sure how that would work. Never tried it, but I would have some concern about balance. Pat
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The small engine shop that I deal with, bores the block till it cleans up, then takes a final cut to the nearest oversize, then gets the correct piston. I don't know about B/S, but Kohler makes their oversize pistons the same weight as the standard bore. This way they are not out of balance when re-bored.
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I planned on balancing the piston/rod combos before putting them back in the engine, just to make sure it runs as smooth as possible. Thanks for all the input!
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