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Discussion Starter #1
Sorry for the basic bitch question but the new site design makes searching for this info a massive pain in the ass as you can't search one dedicated forum for key words ( unless I have not found a search function other than the main on on the header).

What is the standard amount of rod growth at temp for 4340 steel rods? For some reason I have .006-.008 in my head but can't find the info to be sure. All of the articles i've read in searching has talked about a .035 piston to head clearance for steel rods, but none talk about how much growth the rod actually undergoes during thermal expansion as the engine come up to operating temp.

I am trying to decide how much to deck my block with the new pistons to get them to a clearance without sacrificing compression unnecessarily.

4340 rod, 2618 piston ( No crown above piston head ). Planning .027 gasket. Trying to decide if a zero deck is safe or if I need to have the piston .008 in the hole. I know these pistons will grow a bit so I want to be safe but also maximize what I already have. Thanks.
 

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Wouldn't the block grow some at running temp that would negate part of that .006-.008 if that is the case? Piston rock has to be considered also.
 

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Discussion Starter #3
Piston rock is taken care of with the bore finished diameter. The 2618 pistons will be noisier and move more when cold but will seal well when hot. As for the block growth, i would agree with the growth but how much compared to the rods I am not sure of. That's why I am bringing it up in here to the geniuses ;) I am a garage builder and racer and do not know as much as the pros do for sure. My issue is that I run a .027 head gasket and with the engine apart now I can either zero deck it or leave it with a clean up and some extra clearance. If I ran a thicker head gasket it wouldn't be an issue but a zero deck with my .027's puts me in the range that some say is not safe with steel rods. ...Just trying to see which is necessary.
 

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6.83 microinches per inch per degree F is 4340's coefficient of thermal expansion. (6.83E-6 inch/(inch degree F))
Multiply by the temp delta between room temp and operating temp, like maybe 250ish.
Then multiply be the C to C rod length for a ballpark estimate. So, about 8 thou for a 6" conrod.

The block will expand also - 5.8 E-6 inch/deg F
The block will be cooler than the rod - maybe 200 degrees F.
Block will grow about 0.007"

Difference about 0.001" growth due to temperature.
 

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Too close and you'll know right away, too much clearance and you might never know.

I'd be slightly more conservative than aggressive in this area.

I come up with .3 difference in compression (13.43 vs. 13.11) on a 4.125" bore. Not worth it in my opinion.

Do you NEED every last HP for head up or class racing, or is it a bracket/fun car?
 

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I use the same as above .040 piston to head however you get there. Beyond that, the compression difference will be so negligible and the HP difference its not worth chasing. For me, I zero deck and .039 head gaskets.
 

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Can't just go by rod material. You need to consider piston weight and stroke and rpm etc. I got away with 0.027 piston to head with 6.2 rod, 4" stroke, 400g pistons limited to 7000 rpm but my next build may see 8200-8600 rpm, I will NOT be running 0.027 anymore.
 

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That was awfully brave. There was an Engine Masters 383 SBC one year that had mid to high 30s piston to head, and the builders said they could read the piston part numbers in the carbon on the heads when they took it apart. I wanna say it was 0.036" on one bank and 0.038" on the other but I could be off a little.
 

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Yup, I was off.
31 on one side, 32 on the other. And it was milling marks from the head in the carbon on the pistons, not part numbers.
 

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No way I'm running .027" if I have to ask these questions. I don't mean that as an insult, more so that it takes time to slowly work a combination closer and closer until you know the limits. RPM wasn't mentioned in the OP but certainly is part of the equation.
 

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Discussion Starter #16
Good stuff. Thanks all. More of a " I just want to know for my own edification" subject as I had .006-.008 in my head but couldn't remember for sure. I mainly asked because I have my usual stash of .027 mls gaskets i use for my engines and my machinist asked me if i wanted this set up zero deck during the refresh. It is safer to have a conversation on here than to destroy a new engine :D
 

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How much more compression does .005" less deck height achieve? Then with that amount of loss what power level are we talking? Since you didn't list you specific combo we don't know if you talking 10-1 increasing to 10.2-1 or 15-1 increasing to 16-1. Either way the power increase is minimal. Why risk damage?
Doug
 

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Sorry for the basic bitch question but the new site design makes searching for this info a massive pain in the ass as you can't search one dedicated forum for key words ( unless I have not found a search function other than the main on on the header).

What is the standard amount of rod growth at temp for 4340 steel rods? For some reason I have .006-.008 in my head but can't find the info to be sure. All of the articles i've read in searching has talked about a .035 piston to head clearance for steel rods, but none talk about how much growth the rod actually undergoes during thermal expansion as the engine come up to operating temp.

I am trying to decide how much to deck my block with the new pistons to get them to a clearance without sacrificing compression unnecessarily.

4340 rod, 2618 piston ( No crown above piston head ). Planning .027 gasket. Trying to decide if a zero deck is safe or if I need to have the piston .008 in the hole. I know these pistons will grow a bit so I want to be safe but also maximize what I already have. Thanks.
008 in the hole with an .027 gasket will be fine. That’s what I run in my engine and I turn it almost 9k. You are best off with a thin gasket and the piston in the hole.
 
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