piston measurements


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By RayCarter18 - 12 Years Ago
Hi I was wondering if anyone knew the measurement when micing the pistons skirts. They are 292 what would the standard 292 measure out too? and at what point do they become unusable? appreciate your help. Thanks Raymond
By NoShortcuts - 12 Years Ago
RayCarter18 (4/2/2013)
Hi I was wondering if anyone knew the measurement when micing the pistons skirts. They are 292 what would the standard 292 measure out too? and at what point do they become unusabe.


Hi Ray,

I don't believe that there is a definitive answer to your question! Piston diameters vary slightly even in an aftermarket replacement set. Even when FoMoco built your engine, their pistons varied slightly in dimension. -For that reason, at least back-in-the day in engine manufacture, pistons were 'selectively fit' to the bore they were going into.

IF you were to buy a set of pistons today to have your engine overbored, the machine shop would ask you to bring the pistons with the block so that they could bore the cylinder and hone the cylinder to the correct size for each individual piston based upon the manufacturer's recommendation.

The above said, the particular piston manufacturer may have a recommended piston skirt clearance that is different than another manufacturer has. The reason the recommended clearance 'fit' may be different from one manufacturer to another has to do with the aluminum alloy used, the eccentric cam shape used for the skirt, or the heat dam design used around the lower ring and piston pin area. The production method for the piston can be a factor in the recommended clearance by the manufacturer, too. Forged pistons often have a greater expansion rate than cast pistons.

In rebuilding an engine, the skirt to cylinder wall clearance is determined, and the piston skirt can be expanded by a process called 'knurling' to adjust for the wear that has taken place.

Hope this helps.
By Hoosier Hurricane - 12 Years Ago
Ray:

The clearance is machined into the piston. In other words, for your 292 the nominal size of the bore is 3.750. The piston, with .001 clearance, would measure 3.749. It is measured just below, and 90 degrees to the wrist piin. The bottom of the skirt would probably measure 3.750 on a new piston, again at right angle to the wrist pin.
By PF Arcand - 12 Years Ago
By the 57 Ford manual, the recommended piston to bore wear limit is 0.0045 at the lower skirt. Original clearance is 0.0009-0.0015.
Edit; should have mentioned that the average piston spread size is listed as 3.7491" to 3.7515"
By RayCarter18 - 12 Years Ago
Thanks I appreciate it.
By Ted - 12 Years Ago

As Paul mentions, the accepted wear limit in the service manual for the Y pistons is 0.0045” measured at the bottom of the skirt. That’s where many high performance engines actually start for piston/wall clearance. There is quite a bit of leeway though on a stock engine as far as the point the pistons are deemed unusable and much of this is determined either by budget or intended use. A thorough examination and measurement of the pistons can determine if the pistons are worth reusing. Don’t forget to also measure the ring grooves for wear. Obvious scratches or scuffing on the piston skirt will have it being replaced much quicker than if it simply exhibits smooth wear but the wear in the ring grooves can go unnoticed if not being looked for specifically. For anything put together outside of the recommended assembly specifications, it’s a judgement call on the part of the engine assembler.