383 stroker , back with photos

OK, back in December I started a dumb post called EX RACE CAR MOTOR< WILL IT BLOW ON THE STREET?, or some such dumbness. I finally dug it out today from under the work bench and I want to see what you guys think and test your Chevy/GM parts knowledge. More on that in a bit.
I Bought this motor in 2010 off Craigslist, from the son of the man that raced it. 350 bucks as I recall. Dad was gone and the son wanted to get rid of it. It was built for dads race car , a 67 Coronet, ( not a drag car, some kind of circular motion class). He raced it here in Oregon. This was built in the Late 70s or early 80s, before they had stroker kits for Mopar, so they welded up and then offset ground the rod journals on the crank, to stroke it by 1/4 inch, from .3.375 to 3.625. Main journals are normal 383 size of ( 2.625). This is where it gets weird. Chevy/GM guts! The rods are clearly stamped GM, but I cant figure out exactly what they are from. One buddy said they looked like Chevy straight six rods. They measure 6.75 inches in length and have a big end bore of 2 and 3/16. They obviously were polished and smoothed. The pistons are marked ROSS 425R, but I see no overbore marks on them.
Its kooky as it only measure out to 419 inches. Hardly seems worth the bother. Everything looks real good in it except that one piston, which the son did by trying to drive it on the streets for a week on pump gas, The compression was pretty high as it had seriously shaved closed chambered heads. I'm going to call Ross piston on Monday and see what they think and if they have an old piston laying around. The bore looks really good and zero lip on it so I think this motor was pretty fresh at one time. Pistons look good too except the one in question.
I took it to a guy here and he just wanted to start over with a stroker kit. I get it , he doesn't want to be liable, but that's absolutely what I don't want to do. I want to get this thing running in its old school glory and throw it in the pictured Coronet that's been languishing behind the garage for too long.
You guys have any idea on the rods and Pistons?

Heres the rods and Pistons up close

Oh yeah I forgot, it had a Fluidampr on it. I don't know if those can be run on the street or not

And lastly, here's the offender, detonation is my guess, from the dumb-dumb trying to street drive the thing on pump gas. Luckily the adjacent cylinder wall looks fine

The Fluid unit would be fine to run. Good unit. Welded/offset stroker was common back in the day to keep cost down. Someone was really thinking on parts combos for a workable solution. I wonder if there was a displacement limit for whatever class they were running. See what a piston will run from Ross, if they do singles. Then you have to decide if a stroker package would be a better choice since you don't know the age of the rods, how many cycles they've run, is one or more going to break sometime down the road due to some internal flaw. Stuff to stew on.

What is the piston pin diameter?
What is the crankshaft rod journal diameter?
Those answers will help.
I would normally have guessed a long bbc rod, but it sure looks factory GM , and they never made a long rat rod. And I don't think bbc rods in mopars was common that many years ago.
I'm thinking Buick or Olds, maybe a six cylinder chevy.

Edit: I'd be tempted to see what a good welding shop could do with that piston.

Did you measure deck height before disassembly?
What is the head cc?
It'd be nice to know the actual cr, better than "high:.

piston diameter is 4.25, crank journal diameter is unknown because I don't have a mic big enough, but I did put down the big end rod size as 2 &3/16, so add bearing thicknesses and it should be around2.25, maybe slightly smaller. The rods are clearly factory as they are marked GM and some other numbers as well. I searched for GM engines with a 6.75 rod length and came up with 292 Chevy, and Cadillac 500. BUT the big end size doesnt match. A search for engines with a 4.25 bore came up with Chevy 427,454, AMC 401, and Mopar 383 and 426.
The block is bored at about 4.28, so I'm guessing a .020 or.030 over 383.

Scratch the Chevy straight six rods idea, according to the internet they never made a 6.75 inch rod for straight six. That just leaves Cadillac

How accurate is your con rod length measurement?
Buick 455 rods are 6.6".
Again, what is the piston PIN diameter? (They look pretty thin-wall to me)

....and it appears that the Caddy rods are CAST, not forged. Not an upgrade in my opinion.

Pretty sure the R425 on the Ross piston is just the basic forging number, before any machining was done. You'd need the job number for the pistons, which you don't have, (unless you have paper work from Ross), for Ross to be of any help.

Neal Zimmerman said:

piston diameter is 4.25,
The block is bored at about 4.28, so I'm guessing a .020 or.030 over 383.

Click to expand...

Bit of a mismatch there.

Something else I thought of earlier. Since this was an old school build and you mentioned that the heads had been cut quite a bit to increase the compression, I wonder if they angle milled the heads. It was a common way to get the compression up on a wedge type chamber by removing more deck on the exhaust side of the head compared to the intake side. The kicker is that the intake manifold has to get cut to match the angle change. May want to compare the existing heads and manifold to known stocker units. If they have been angle milled, all 3 pieces would need to be replaced if you change from existing.

I got rid of the heads, as I could tell they were not street friendly, and the intake had been seriously cut down to match.
The pin diameter is 29/32.
As to my measurements, remember I don't have mics , so this is just ball park, probably off by a bit

If the rod measurement is actually 6.75, then it's 10 thou shorter than a 440 rod. Strange

Are you sure its 29/32 for the pin diameter? Really need to get a cheap micrometer... can't get pistons until you know what you have. A tape measure isn't going to work. Also need the compression height.

A Ten dollar plastic dial caliper is good enough for lots of measurements in an engine, probably within a couple thousanths. Better, splurge for a $20 metal one. I did lots of work within a half thousandth with a $50 one. (If your gonna splurge, get an eight inch. I have and use a 4", 6" 8" and 13" I like and use the 4" the most, but an 8" is much more versatile).

RemCharger said:

If the rod measurement is actually 6.75, then it's 10 thou shorter than a 440 rod. Strange

Click to expand...

I think they might have been looking for a similar length rod, that could use a smaller rod bearing.(due to the offset ground stroker crank). Op can't measure the rod throws, or at least, hasn't told us. Might be bushed down to a sbc pin too. That piston and pin look light!

Yeah I haven't found a small block Chevy connecting rod anywhere near 6.75. Pontiac 400/455 rods come close at 6.67 or something like that. When I find one rod close in length, then the two ends don't match up