I have a set of #11503 Diamond Pistons (3.905 bore, stock stroke, -8.6cc dish.), building for 15-16psi of boost, but not sure on piston/cylinder wall clearance with these particular pistons. Machine shop set it at .0025-.003", but I'm not sure if that will work or not. I ordered a couple replacement pistons for it because 2 I had were dinged, I went to stick them in, and they're too big. The numbers on the bottom are the same, but when you mic them, the new ones measure 3.903, while the ones that were used to hone the block measure 3.900, which leads me to believe the bore SHOULD be 3.905 which would give .005" clearance, while the 3.903 (new) pistons should finish at 3.908 (.010 oversize), also giving .005" clearance. Please help, to sum it up, i have 6 pistons from the original set that fit, while the 2 replacements, same part # are measuring .003" bigger, and bind up. Any ideas?

 

Weird, did you try calling diamond?? I havn't measured my set of 3.905's yet. When I get home tonight I will measure them and see what they are. I seem to think the clearence was around .003.

 

I'm working with Victory racing engines, where I got the 2 pistons from. He's trying his best to help me. I'm gonna mic the cylinders today and see what I come up with. I just don't want to stick a piston because I didn't give it enough clearance. Thanks bud, appreciate it! =)

 

Most guys shoot for around .004" on a n/a aluminum engine. For your application I could see going .005" but I would rather gap the rings a little wider and run the piston tighter.

Nate

 

So me running .0025-.003 just isn't gonna cut it for clearance then?

 

I just measured 4 of mine. They all came in at 3.902...

 

That's strange, the ones I got from SR71 all measure 3.899-3.900. 2 New ones are 3.903.

 

seems .003 is fine for NA street engines and .005-.007 for street blower cars or thats what Probe recommends for 2618 alloy. JE pistons recommends .003-.004 for street and add +.0015-.0025 for forced induction.. Your machine shop should have called Diamond for specs for a blown motor... .003 is probably still fine if you are running meth....005-.007 is probably safer. I'm going to call Diamond before I go to the machine shop. You would think that since these are designed for boost, Diamond would have machined in the tolorance for the 3.905 bore...Maybe Diamond measures a specific spot on the pistons....

 

I measured about 1/4" from the bottom of the skirt...

 

Yeah, shoot... Guess I'll be taking the bottom end back out of it and driving another hour north to have it re-machined. I'll have them put something like .0055" in it, and call it done. Thanks again!

Tony

 

Now this is interesting, Wiseco 2618 pistons they say their LS1 piston is designed for as little as .004

My Diamons spec sheet that came with the pistons says...

Quote:

Install these pistons with .0035 clearence measured 90 degrees to the pin hole @ 1.250" down from the oil ring

 

I got 3.9015" at that location...

 

Have some pics at the link below... didn't want to bog the page down. The block mics out at 3.902, as seen in the picture. New pistons mic out 3.903, and old ones mic 3.899. Same part #, but different design. More ideas?

http://pontiac.wirelesstcp.net/tonypics/parts/pistons/

 

Hang on a sec...

 

Weird, mine are even different than both of yours...

 

Yeah, the new ones don't even have 1.250" worth of skirt to measure! I'm going CRAZY!

 

Need to call Diamond and do some bitching....

 

not only is the fit a problem, from the looks of the difference in the pics, the weight is going to be a few grams off too.

how about a pic of the dinged places on the 2 pistons??? I went over those things with a fine tooth comb several times before I ever posted them for sale here and on corvette forum. Chris can vouch for the fact that if you put something up for sale on corvette forum and don't note every single scratch in your post, you will be getting the parts back and paying for shipping both ways. if things aren't in EXCELLENT condition, they go in the trash can.

 

Yeah, I'm going to check the pistons for weight, just have to find a scale to get them on. I'll get some pics of the dinged pistons, like I said bud, they totally weren't your fault. I didn't see the dings anywhere on the pistons when I got them, and I completely stand behind you as a seller.

 

Did you get a chance to call Diamond and ask about the 3 different pistons all with the same part numbers...