piston ID

[VTStang66]

A few years ago when I built my 302 I bought speed pro forged pistons from Jeg's. At the time, they only had 1 set in their catalog for a 302, now they have 3. I'm about to redo the top end of my engine, and I need to know some things about the pistons so I know how much to deck the heads, how much valve lift I can have, etc. I was going to call speed pro and ask them this stuff but without a part number I can't do that.

What is the easiest way to figure all this out? Is there a part number on the pistons? I wasn't going to take the short block apart but I can if I need to. Or would it be easier to bust out the graduated cylinders and modeling clay and just measure it all? I haven't done that before and I don't own a graduated cylinder but I could probably get access to one. What would you do?

*edit*
I already tried calling Jeg's, their records don't go back that far.

*edit again*
Looking at my catalog, I think I might have it narrowed down to PN L248230F, which gives me 9.09:1 compression with 63cc chamber. What do I need to measure to verify this? And what size chamber do I need for 10:1 compression?

 

[SoCalCruising]

The only way to know what your chamber size is NOW (given that there may have been machining of your heads done in the past) is to measure the volume. You can just about look at the pistons and tell how large (CCs) the valve reliefs are. Large reliefs suitable for a 2.02" intake and 1.6" exhaust will be +/-6cc. Smaller reliefs will be about 4cc. That should be close enough, after you measure your chambers, to make a reasonable estimate of your static compression. Any competent machine shop can CC a couple of chambers for you for little cost.

 

[VTStang66]

Well I bought the heads (edelbrock performers) new and never had them machined, so I know they're 60cc chambers. The pistons are aftermarket and they're forged so I'm pretty sure the reliefs are up to the task of the 2.02" valves.

I guess I should just call a couple cam companies and see what they say before I worry too much about this stuff.

 

[VTStang66]

You can just about look at the pistons and tell how large (CCs) the valve reliefs are. Large reliefs suitable for a 2.02" intake and 1.6" exhaust will be +/-6cc. Smaller reliefs will be about 4cc.

So what do these look like? I called speed pro and the guy told me they're 2.7, but I don't think he knew what he was talking about.

 

[SoCalCruising]

You don't need to agonize over this too much. I'd use 4cc in the calculation and call it good. Actually, your piston to deck height is more important for the calculation, and you prolly haven't measured it, right?

 

[brianj5600]

http://store.summitracing.com/partdetail.asp?part=TRW-L2482F30&autoview=sku

 

[D.Hearne]

You need both piston top volume and the deck clearance volume to accurately figure the static comp ratio. 4 cc's doesn't sound like much, but in a small displacement engine, it's a lot. The gasket volume is 9.5 ccs with .040 gaskets. Piston top volume is 5 ccs with the reliefs and area between the piston top, cylinder wall down to the top ring. To get a 10 to 1 ratio, the deck clearance needs to be no less than .010 and the piston height needs to be as close to 1.610 as you can get, and then the chamber volume needs to be about 58 ccs. I don't have a calculator handy to figure this out at the moment. You add the swept volume, chamber volume, piston top volume (I use 5 ccs for this) plus the gasket and deck clearance volume, then divide the total, less the swept volume, into the total volume.

 

[65ShelbyClone]

I'm pretty sure the reliefs are up to the task of the 2.02" valves.

I have the standard bore version of those pistons in my 302/5.0. I hate to be the bearer of bad news, but there isn't a snowball's chance that those reliefs are big enough for 2.02 intakes. I have the normal Performers(1.90/1.60) and with my .512/.529 Crane and I had to open up both intake and exhaust eyebrows.

 

[VTStang66]

I have the standard bore version of those pistons in my 302/5.0. I hate to be the bearer of bad news, but there isn't a snowball's chance that those reliefs are big enough for 2.02 intakes. I have the normal Performers(1.90/1.60) and with my .512/.529 Crane and I had to open up both intake and exhaust eyebrows.

Is there enough material there to open them up enough for the 2.02" valves?

 

[SoCalCruising]

Yes. Isky sells the tool you need.