I've get a set of used SRP 4.040" pistons for a 385 I got off a member of the board a while back. What can I do to check them out to make sure that all 8 of them are ok to reuse?

 

Mic them up on the skirted sides about the same height as the center of the pin. Make sure they are all the same and meat the specs for clearance in your bore.

 

Is this something I should let a machine shop do, or just check myself with my digital caliper? I want to make sure that they are all good to go before using them. Even if I have to buy 1 or 2 new ones I am still coming out ahead on the deal.

Second question, cleaning them, should I just let them soak in SeaFoam to break up all of the carbon, then try to polish the tops with a felt Dremel bit? I know to stay away from the sides and corners of the piston.

Also, they also still have the old rings on them, I assume letting a machine shop remove those would be best to prevent myself from scratching up one of the pistons since I have never done that before?

 

If they look good - no scoring or wear on the skirts, most likely they are ok to use again. The important things that would be measured are skirt taper to make sure they're not collapsed, and ring lateral clearance - that is a measurement of how worn the ring lands are. A feeler gauge set will get you that number - any more than a .003" feeler in there and the pistons are junk, over .002" I consider worn. As for the skirts, you will measure the nominal number at the position designated by the manufacturer(typically .125-.350" down from the bottom of the skirt) to get the skirt clearance - for example a piston that measures 4.0265" and specifies .0035" skirt clearance would get a bore honed to 4.030". The skirt taper is measured by taking that nominal number and position and measuring down the piston(while upside down) towards the oil control ring land to make sure the skirts are not collapsed. There should be generally .0025" of change within just over half an inch.

It takes significantly less time to actually check these things with the proper tools than to explain it.

 

Per the Haynes manual the Piston to Bore clearence range is .0010 to .0027 inch. And the area of measurement is .45" from the bottom of the skirt.

On a side note... I was playing with my pistons... (Yeah, I know, perfect setup for a signature quote.) Anyway, I was putting them in my block without rings or crank and noticed they wobbled. There is about .030" gap parallel to the pin. There is no gap perpendicular to the pin. (Ok, I know there is some gap)

Pulling them out and measuring I find the following:

Parallel .100 above the pin...3.995x
Perpendicular .45 above the skirt ... 4.026x

Is this the normal design? Will they "ride tight" once they have rings and are attached to the crank?

 

That is normal, pistons are not very round - they are a very complex shape.

 

^^^^agreed.....they will be fine once the rings are installed and fitted in a properly honed bore. Each piston manufacturer has different specs for different apps. IIRC the piston to bore clearance for my brothers 2.4L that we turbo'd was at .035....and that was specified by Wiesco for the application. And that was with forged pistons. And the guys that put his shortblock together did a wonderful job.....Eash engine is different and you should always listen to what the part manufacturer recommends...after all they designed and produced the parts