Ajgkirkwood Posted April 5, 2019 Report Share Posted April 5, 2019 Hi everyone. My question involves (2) 251 motors. One motor is from a generator and I'm thinking of using this block to build a freshened up motor due to it having less wear on the walls. However the pistons look like rough. Now the second block that has more ridge wear, those pistons look in much better shape. The thing is, it's different. It has 3 rings (where I though dlathead had 4 rings) had a thicker looking wrist pin. Its 100 shorter in height. It seems to be about 0.01-0.015" taller then the other piston from the pin up. Diameter is about the same, maybe a few thousand bigger. Piston on the right came from the block I was thinking of using. Quote Link to comment Share on other sites More sharing options...
Ajgkirkwood Posted April 6, 2019 Author Report Share Posted April 6, 2019 One thing I should also ask is the piston diameters. The very top is 3.400 give or take a thousand, where the bottom is 3.43. Is this normal for flathead? Is it for expanding? Quote Link to comment Share on other sites More sharing options...
kencombs Posted April 6, 2019 Report Share Posted April 6, 2019 Look inside for casting or PN info. I suspect the three ring is an aftermarket part. That would account for the greater pin to crown height to raise compression. Generators and other industrial applications often have heavier parts and lower compression also, compare the rod lengths to be sure they are the same. Quote Link to comment Share on other sites More sharing options...
Ajgkirkwood Posted April 6, 2019 Author Report Share Posted April 6, 2019 Will do. Inside the 3 ring piston(conformstic) has this part number: p-672-a. Looking up on Google isn't bringing up anything though Quote Link to comment Share on other sites More sharing options...
kencombs Posted April 6, 2019 Report Share Posted April 6, 2019 As I suspected: Conformatic is an old piston line, since bought by Federal Mogul. Maybe a call to their tech line may provide more info. I see the 3 ring design as an improvement. Even if the call doesn't pan out, you can always source rings by bore and width. 1 Quote Link to comment Share on other sites More sharing options...
Veemoney Posted April 6, 2019 Report Share Posted April 6, 2019 3 hours ago, Ajgkirkwood said: One thing I should also ask is the piston diameters. The very top is 3.400 give or take a thousand, where the bottom is 3.43. Is this normal for flathead? Is it for expanding? From what I have always understood the piston rides on the rings at the top and that is why the piston is smaller at the top by the piston grooves. It does allow for expansion but the rings maintain the compression barrier. The lower portion of the piston barrel 90 degrees out from the piston pin towards the bottom is where you should measure diameter for bore size as some pistons are barrel cut and may give a smaller measurement at the bottom of the skirt when measuring inline with the pin. Hope this helps and makes sense. Quote Link to comment Share on other sites More sharing options...
Ajgkirkwood Posted April 7, 2019 Author Report Share Posted April 7, 2019 Yea it make sense, measure at wrist pin height, perpendicular to the pin. I know some pistons are oval to allow it to expand. With more digging I learned the tops are a smaller diameter for heat expansion. Also from what I read before, 4 rings has more friction then 3 so that's a plus for performance Quote Link to comment Share on other sites More sharing options...
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