Okay folks I have a decision to make. I've been gathering odds and ends for my "junk parts" Hemi and I'm at a crossroads.
I FINALLY found a cheap set of .060 forged pistons for my beat up old block. The compression height is 1.765" and the dome looks to be 75cc. The main snag is the wrist pins are 1.094".
Three routes I'm considering:
(A) Chinese 4.15 crank, used 6.86" street Hemi rods, machine the small end out to 1.094" with new bushings + resize big ends.
-484ci, 10.4:1 CR
(B) Used 3.75 street hemi crank, NOS 7.065" nascar rods. Both are on ebay now, the rods are already 1.094" pin bore. I bet this combo would be expensive to balance because of the rod weight.
-438ci, 9.6:1 CR
(C) Chinese 4.25 crank, used 6.76 "six pack" rods, resize big ends + new ARP bolts. Rods are on a shelf collecting dust, low miles on them. Not in love with this option, but it would certainly be cheap and yield the most cubes.
-496ci, 9.6:1 CR
I'll do my best to answer any questions as needed. Thanks!
With B , decent rod ratio & best compression , but I believe your CI is incorrect stock 3,75 crank with .0606 overbore will not get you too 484 CI
Can you make the 4.25 crank work with the 6.68 rods ?
I didn't do the math but I think you reversed the CI on a and b. How heavy are these pistons? If they are stock Hemi weight I don't like the 3/8" rod bolts of a B-rod.
Also: Are you going to have enough skirt to counterweight clearance if you use these pistons in a stroker?
Whoops, you guys are right, I flip flopped A & B. FIXED.
Chryco, with the 4.25 crank and a 6.86 rod, the piston ends up way too far out of the hole.
RUNCHARGER, I didn't get any paperwork with the pistons, & had to physically measure to get the dimensions. I have a set of standard bore street hemi pistons on hand to get a direct comparison if needed, but for all intents and purposes, these pistons look to be about .2" shorter than stock.
As far as skirt to counterweight clearance, looks like there are plenty of 4.15 stroke Hemis around with standard Chrysler journals. The 4.25 would be pushing it I bet.
What do you guys think about re-bushing the small end of a street hemi rod from 1.031" up to 1.094"?
Cost Comparison:
484ci
Molnar Crank=$8xx, 6.86 rods=$0 (already have a set with ARP bolts), resize big ends $, re-bush small ends $, balance rotating assy $.
438ci
Street Hemi Crank=$5xx, NOS 7.065 rods=$550, mag crank $, balance rotating assy $$. (all U.S. made!)
496ci
Molnar Crank=$8xx, 6.76 rods=$0 (already have a set), resize big ends $, install ARP bolts $, balance rotating assy $.
Well if they are .2 shorter they are likely designed for a 4.15. Is the measurement from the pin to the bottom of the skirt (underneath the pin opening) .2 shorter than a stock piston as well?
Can't say how the skirts compare, no question the pistons are designed for a 4.15 stroke though.
I found some new info. The nascar rods I had quoted at 7.065" are actually 7.061". On Arias's website, I found an extremely similar std bore piston and the specs show 1.740" compression height, and an 80.5cc dome. I'm thinking my pistons are likely based on these, and I trust their measurements more than my crude dial calipers.
Plugging the new numbers in my calculator yields a nice 10.3:1 CR for the 484ci combo.
Here are the numbers I used:
4.31" bore, 4.15" stroke, 6.86" rod, 10.725" deck ht, 1.74" compression ht, 170cc chamber vol, 4.38" gasket bore, .020" compressed thickness, 80.5cc dome vol.
Some variables to consider:
The block has definitely been decked, and the heads have been surfaced in the past. I figured if it is enough to be a problem, I'd just stack steel shim gaskets to make up the difference. Sort of a poor-man's MLS, lol.
OK so here is a wild thought use the 425 crank & the 6.86 rods which you are going to bush & bush them off center to bring them down to 6.76