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Discussion Forum

Pistons

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Al

12-07-2000 05:13:57




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gidday,
anyone ever try turning down a piton,say .030.this would give me a .030 over piston,what about rings.just anouther idea from a guy with lots of junk and little money




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Rick

12-15-2000 21:04:56




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 Re: pistons in reply to Al, 12-07-2000 05:13:57  
I must have missed something here. I thought if you turned a piston .030 it would be .030 under.



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Neal

12-09-2000 06:52:43




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 Re: pistons in reply to Al, 12-07-2000 05:13:57  
Well I own a machine shop ( I'm a moldmaker ) and I bore small engines ( outboards, motorcycles, jet skis, brigs and straton, ect ). Brian is correct that the pistons are not always round and almost always tapered. I would not recomend turning down oversized pistons. One MAJOR reason is some pistons are made thiner to to keep the same rotational weight. By turning it down you may weaken it enough to fail. Another reason is unless you have a computer controled machine (CNC) it would be very dificult to match the original taper/oval which is important in the expansion of the piston. If it expands too much in one spot you have a seized engine. Thats my two cents. Good luck. N.

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Bill Stanley(NC)

12-26-2000 18:51:25




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 Re: Re: pistons in reply to Neal, 12-09-2000 06:52:43  
An engineering perspective.

Pistons are not round when cold but egg shaped and are not the same diameter from top to bottom. Pistons are generally cam ground to form the shape. There should be greater skirt drag when cold. When the engine is up to operating temperature the pistons generally grow inwardly to a round shape. The shape of the piston is up to the designer and the intended use of the engine. Round as in racing pistons will generally crack the skirt if revved to about 3,000 RPM when cold. Some pistons have a steel strut cast in to control the expansion from cold to hot.

Make the hole bigger not the piston smaller.

Thanks.

Bill Stanley(NC)

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Al

12-09-2000 07:11:50




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 Re: Re: pistons in reply to Neal, 12-09-2000 06:52:43  
i hear ya Neal,don t have cnc,know pistons are cam ground,but just playing with ideas to freshen up a sloppy engine with out major investment.i dont want to blow her up so im asking lots of questions
thanks
Al



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bill Lord

12-09-2000 14:20:57




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 Re: Re: Re: pistons in reply to Al, 12-09-2000 07:11:50  
Not all pistons are cam ground, or tapered, or barrel shaped. If you have enough stock on piston to turn o/d and deepen ring grooves correctly, go for it. Many of these shapes currently in vogue, are a result of emission compliance requirements. If your piston meets the above criteria on stock thicknesses, and you give the piston enough clearance, (.003/.004) it might just work as well as new. Nothing ventured nothing gained. I was mfg. engineer over piston dept for 5 yrs.

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Al

12-09-2000 15:06:47




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 Re: Re: Re: Re: pistons in reply to bill Lord , 12-09-2000 14:20:57  
Thanks Bill,we have the same atitude.Could you comment on min wall on the piston,and when you say .003 clearance do you mean piton dia should be .003 smaller than the finished bore,what is the criteria needed to determine ring depth?
thanks
Al



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ltf in nc

12-10-2000 09:51:11




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 Re: Re: Re: Re: Re: pistons in reply to Al, 12-09-2000 15:06:47  
I'm with you guys. Most of my adult life I have fixed "foul" ups. My thoughts are to turn the piston about .005 under the bore diameter and then knurl the skirt to a slip fit. This will assure the normal clearance plus a margin for error. If the piston wants to snug up as the engine warms the knurled area will permit some displacement. Whatcha think?



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Brian G.

12-07-2000 06:46:27




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 Re: pistons in reply to Al, 12-07-2000 05:13:57  
Years ago I had a piston pin let go in a '51 Ford V-8, I had a set of .030 pistons, so I had a guy turn one down on a lathe to fit the .010 bore of my engine. The first time I started it up, it ran a few minutes and siezed up in the cylinder. When it cooled down, it started up and did the same thing. My Wife's uncle (a mechanic since the Model A days) told me to put a quart of oil in the gas tank, run it til it stopped, let it cool and repeat til it kept running. He offered to do this for me while my then girlfriend (now wife of 40 years) went to a movie in his '56 Ford. The engine would then idle and drive fine until you heated the engine up going up a steep hill when it would literally bring the car to a halt. After a week or so of this inconvenience, everything worked fine again. He told me that the reason was that pistons are "cam" ground or turned slightly oblong to allow for expansion within the cylinder. So.....I guess if you didn't have access to a machine shop that could do the "cam" thing, you might get away with having it turned down a "little extra" to allow for expansion. This would probably be no problem in a low RPM old tractor engine. Rings the correct size for the bore will work. Incidentally, I assume when my guy turned down the piston, he cut the ring grooves correspondingly. Now, let's hear from the real machinists out there.

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Al

12-07-2000 08:07:06




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 Re: Re: pistons in reply to Brian G., 12-07-2000 06:46:27  
brian,
i am a"real machinist",i can make anything given the proper dimensions,machine,and material.i do not work on motors,this stuff is all new to me,but with a little advice i will get it right.thanks for your input.
Al



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