I have a 347 stroker motor and have recently blown it. I have it torn down pretty far and i noticed that every one of the pistons will rock back and forth in the cylinder wall. If i push on the bottom of the piston it will move every so slightly and the same will happen if i push on the top. It is like the pistons are too small for the holes. I have over a thousand miles on the setup like this with no marks on the cylinders and it ran fine. Is this normal? The cylinders have been honed out several times. Was this bad on the motor? I have since ordered .030 over pistons. Can i have a machine shop not bore it all the way out to fix my rocking piston problem or is this normal. Thanks for any advice!
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Iowa91LX
Founding Member
Take out your piston rings and put them in the cylinder and then use a feeler guage to measure the ring gap. Measure the ring gap in at least 2 places on the cylinder to get see if the bore is tapered. If your rings have excessive gap then you have a bore dia. problem. They should have been file fitted before assembling the motor, were they?
As far as getting it bored to .030 The cylinders will all be enlarged by .030 over standard bore. The machine shop cant bore it halfway with the same stone. It can however be bored to .010, .020, .030, etc.
Why has it been honed so many times?
As far as getting it bored to .030 The cylinders will all be enlarged by .030 over standard bore. The machine shop cant bore it halfway with the same stone. It can however be bored to .010, .020, .030, etc.
Why has it been honed so many times?
EMW150
Founding Member
It's normal for the pistons to have some rock when the engine's cold. Heat will expand them some. Like Iowa91LX said you could check you're origional ring gaps and see what it looks like. Put it all the way to the bottom of the bore and check it where the rings don't travel and then at the top which will show you any taper. I really doubt it's out of spec with only 1000 miles on it. Honing a few times shouldn't hurt it either because it takes a long time to remove material with a hone it really only etches the surface.
the pistons will rock slightly even in a freshly rebuilt shortblock - although it is usually difficult to discern w/ the naked eye. If your piston to cylinder clearance is too great then when the motor is cold, it will sound strange at idle, kind of like a diesel engine. That noise is the piston skirts slapping the cylinder wall. It usually goes away when the engine heats up. However, on a loud motor - it can be tough to hear. The downsides are that the piston skirts will get beat up over time and actually collapse slightly. Pistons have a taper to them, where the bottom of the skirt is generally a few thousands larger in diameter than the middle of the piston. Lots of slapping will get rid of this taper. When that happens, the piston really starts a rockin. As long as your pistons haven't collapsed, you shoudn't notice anything that would hurt performance. At the machine shop i used to work for - they had a lot of customers than ran NHRA stock eliminator. That class really limits mods on the motor, and you are supposed to run stock carb, intake, rods, crank etc, w/ no porting. Basically, the only change is duration of the cam. Even have to run stock lift. Long story short - every advantage is sought after in this class. They would intentionally make the pistons super-sloppy in the bore to reduce friction. As they went down the track, you could see a faint blue cloud of smoke trailing the car. Made more horsepower though!!
