[troyp]

Thoughts and opinions please. 408 stroker, 4340 crank and H-beam rods, forged Probe SRS pistons. Piston to bore clearance of .007". No plans for power adders just yet. Hotrod setup - street manners are not a big deal. Any opinions on how reliable this will be given the proper care and useage? This engine will see some good rpm (vic jr, agressive cam etc.). Trying to stick with my current block and make these pistons work.

Any experience? What are the limits? I don't think I could stomach a serious cold startup rattle and banger, maybe I could if the performance was there...

Thanks in advance.

Troy

 

[mstngjoe]

That's at the max recommended by Probe.

http://www.probeindustries.com/Instructions/fps_srs_pistons.htm

I've also got the SRS and my clearance is right .003.

 

[troyp]

I called Probe the other day and was told something like 2-4 thou for naturally aspirated. That was it - no discussion even though I told him their own website said differently. Some would say .003" for a forged piston is tight. I don't have enough experience to know, that's why I appreciate any input.

 

[mavman]

I always use .005" minimum on a 4" bore with Forged pistons. I've run as much as .015"...yeah it was a little noisy in the mornings but it ran like a scalded cat.

 

[troyp]

Thanks mavman.

Does anyone disagree that .007-.008" of clearance on a 4.030" bore with a good forged piston should be spin happy and will not rattle itself to death?

 

[Blue Fastback]

All forged piston are not the same. The silicone content of the forged pistons and manufacturing determine how much the piston will expand when hot and what the clearance should be. The old forged pistons did expanded a lot and you had to run them on the loose side. The new ones fit tighter in the bore for quieter operation and better ring seal. .005-.007 is not that bad but remember the sloppy fit will cause the piston to rock back and forth and the rings may not seal as well as a tighter fitting piston. Why dont you get a .040 over piston? That is what I did when my motor was already .030 over.

 

[troyp]

Thanks Blue Fastback - 2 reasons for not going 40 over. First - this kit is new in box being resold due to change in plans (i.e. good deal for me - if I can make it work) and the pistons are 30 over. Second - if I did order a kit with 40 over pistons, what clearance would I build to (same question). In the end, this is a quest for knowledge in the persuit of reliable hp.

All opinions welcome and appreciated.

 

[Blue Fastback]

It will depend on the piston manufacture. The pistons from Sealed Power are marked on the box what kind of minimum spec for piston to wall clearance. You really need to find a brand you want and contact the manufacture about the clearance. My 351C were marked for .0015 minimum clearance. These were new forged pistons. It is better to be a little on the loose side then to tight otherwise you will score up the cylinder wall. A shop that rebored my block honed it for .002 clearance. The old engine had .007 clearance with forged pistons and it was a little noisey. The Dura Shield pistons from Sealed power are stamped right on the box exactly what they want for clearance. The pistons are also matched for exact size so they say, so each cylinder is bored and honed the same demensions. Most manufactures want yo to measure each piston seperatly so you can hone and fit the piston to the cylinder. I went to Federal Mogul's training center for 3 days on performance engine work which is why I know about Seled Power and thier pistons.

 

[troyp]

Very good info guys - much appreciated. Looks like I will either find a block to match the piston deal, or find a piston deal to match my block. it always comes down to cash...