bore growth

[gdaddy01]

how much does the bore dia. enlarge from 70 degrees to 200 degrees F on a complete engine? do the pistons grow as much as the dia. enlarges?

[pbf777]

     Not really going to be able to answer that question accurately as there are many factors to be considered but:

     To start, since a generally accepted value for the coefficient of linear thermal expansion of 'iron' is something like .0000059" inch per °F and 'aluminum is .0000126" inch per °F they obviously don't grow at the same rate.

     As far as to how much does the blocks' bore grow with heat?  Well perhaps we can generalize that if we convert the diameter to a circumference distance, and then choose to identify it as a linear length, multiply this by expansion rate, times the thermal gain value, we can claim to have concluded something!   

     But, it just doesn't work that simply as there are many variables which will effect even this simplistic calculation.  For one there are different alloys involved which effect the thermal expansion rate, then when dealing with circular formations there is an inside diameter, an outside diameter, and everything in between (wall thickness) which means the calculation and solutions change, but also since this is of one structure there are stresses that that are created as the material attempts to expand; and then as the two ends to our fictitious column collide as it is actually a tube then this sets up compressive and deflection forces not being accounted for.  And of course then the other attachments to this cylinder tube in the block which present additional challenges, and also represent additional potential effects that cause the cylinder not to expand it's diameter equally about its' circumference.  And of course then the temperature throughout the casting is not consistent!   

     All of these issues presented in this very quick consideration also applies to the pistons!   Soooooo.........

     Scott.

         

[frnkeore]

First, if you use circumference (dia x pi), after you do you calculations, you have to divide by pi to get the dia for clearance. It makes no difference, just use dia.

using a 4" bore, if we use pbf's expansion rates, we get  .003068" for the cyl and .006543 for the piston. Off hand, I don't know the expansion rate of 2618 aluminum, used in pistons but, if it's the same rate that pbf offers for aluminum then you would use ~.0055 clearance and the piston would be 3.9945.

Cyl will be 4.003068 and the piston 4.00104, when both are heated 130°.

I believe the expansion rate, for 2618 is less than the .0000126 used.

[fe468stroker]

I would think that the expansion on all components would be different between the individual cylinders.  Some run hotter than others due to various factors.  The difference would be minimal and not cause for concern.  Even if you have a cast iron block with cast iron pistons, the expansion would be different because the castings didn't come from the same "batch".  Same for aluminum.

[pbf777]

     Yep,  just to many considerations, with multitudes of variables, and not enough accurate information to base our calculations upon, at least not for us here in the field anyway!   

     This is why in the era of the use of the cast iron pistons stuck (well, hopefully not "stuck"! ) in the cast iron blocks was practiced, if only it just reduced some of the confusion; and it made sense in the engineering environment and applications of the time of course. 

     As another example:  Do you think the aluminum piston growth is linear or that the circumference remains round as it is heated; if so, then you haven't looked very closely at examples that suffered from excessive overheat scenarios.   

     I think one would find that the acceptable required clearance sums, and the effects associated with heat expansion, has been for the most part, in the end, established in the tried and true process of trial and error, no matter what claims may be made by the engineers.   

     Scott.