What are the effect of using a piston designed for a 6.125 rod on a 6.098 rod? Example stock 346, just looking to change to forged pistons. My guess is boom, but....

 

Your piston compression height (top of piston to the centerline of wrist pin) will have been shorter with the longer rod combo....swapping it on a slightly shorter rod will bring the piston down the hole by the exact difference of the rods in question which in your case is .027

That will also lower your CR by the equivalent of 4.5 cc's unless you run a thinner gasket but I wouldnt recommend any thinner than a .030 gasket which still leaves you a little shy on compression and not an optimal quench distance. Assuming your piston was .007 out of the hole previously it will now be .020 in the hole so you would have a quench distance of .050 even with the thinner gasket.

Will it go boom??.....definately not. Will it run as good as it could with the right piston and quench distance?....definately not as well. If your putting something together on a TIGHT budget is it a really bad move....not really, but only if your funds are tight would I run it that way. A set of pistons really isn't gonna break the bank when you consider all the time and money you just spent doing the motor build and swap anyway.

And great name btw....not easily forgotten thats for sure!

Hope this helped,
Tony

 

Thanks Tony! I was looking at buying a set of used pistons for $250. They are -2cc Diamonds for a 6.125 rod, and a 3.622 stroke. I planned on purchasing these, installing a cam matched to my heads, home port my 243 castings, rod bolts, and hit it with a 250ish shot direct port, with a secondary fuel cell of 116. Right now I'm stock internal, so this is all hypothetical with the .030 gasket you recomended.

... I have a stock set of 243 castings sitting in my room. With the .020 in the hole that would result and the loss of 4.5ccs that would make my heads equivalent to having 68.95 chambers. The pistons are rated as so:
62cc = 10.5:1 cr
66cc = 10:1 cr
70cc = 9.5:1 cr

Without doing the calculations that would put me at around 9.6/9.625:1. Now let's say I got my heads milled:
68.95-62=6.95
.007 x 6.95 = .04865.
That would put me back to 10.5:1 and help with the quench. I see guys milling stock heads .040 quite frequently here, but this is a little more.

Here's a few questions though, how much can I mill the heads and maintain adequate P2V clearance with stock 2.00 and 1.55 valves. I say adequate because I plan on porting these heads myself and after acquiring the flow numbers choose a cam based upon the results (maybe mid/high 23x duration and again I know that's vague without the valve events and LSA).

Another question I have is would this help deal with quench distance? Quench is somewhat of a slight mystery to me. I understand what it is, but what affecting it does to the performance = a mystery. I know better quench is equal to better performance, but how is better quench acquired other than milling?

Ultimately, would the milling result in putting these back into the performance range of an appropriated designed piston, all aspects included. Or am I going to have to spring the extra money and buy a set new of pistons (6.098 rods) or for a set of 6.125 rods (used, have any for a 3.622 stroke)?

I don't think you understand how much of I appreciate this help, thank you. Thanks also for the compliment on the name, I use it for everything and it seems to work the way you say, because people seem to remember me.

Edit: I've thought of one last question. I was planning on asking Diamond, but maybe you might know also. These pistons are made from the 4032 material. It says they are good for "small nitrous applications". So what exactly is a "small nitrous application"? Would these take a 250/300 max shot reliably?

 

i'm no piston whiz but i'd say 150-200 max on the nitrous with that type of piston.

 

Bump, for answers and for checking out my new thread.

https://ls1tech.com/forums/generation-iii-internal-engine/620345-putting-forged-pistons-only.html

 

Beaflag,

Also make sure the piston pins are the same between them. The stock rods use a metric .945 pin and the 6.125 aftermarket rods usually use the normal SBC .927 pins.

 

You can also deck the block

 

I was thinking of that too, .005 per 1cc.

The good thing though is that it looks like I've found a correct set of pistons. They're the exact ones I was looking for that a sponsor is selling for a special price. Only $50 more than the 6.125 rod ones I was looking at so. I'm going to go with those most likely.

 

I'm going to bump this up to ask another question.
What would happen if you use a 6.100" connecting rod on a 6.125" piston? Basically the same question I guess but different circumstances. Someone said that the skirt of the pistons can come into contact with the crank bobs. Is this true with only a .025" difference? After reading Tony's post it doesn't seem so.
I am going for lower compression (FI build) so if the only side effect would be that the piston would be lower in the combustion chamber, well that would be terrific! Looks as though that would bring my compression ratio down a total of over 12ccs (-8.6cc dished pistons plus another ~-4ccs from the difference in connecting rod lengths). On a stock head and cam '98 LS1 with stock head gaskets, this should bring my compression ratio down from 10.5:1 to low 9s:1 correct?