502nova3

So I have one piston that is sitting. 020 lower than the rest the rest are..020 in the Hole and this one sits about .040 swapped piston to a different hole with different bearings still sits .020 lower than rest had my heads at the machine shop and asked them to check that one for bent rod but they said they dont want to because there scared to brake my piston as it's cast tho i beleve its forged . It's doest look bent by eye just wondering if I should send it or not. I believe compression was ok in that cyl before I took appart is a mark iv 502 bbc marine the one with the tag is the one thats .020 lower

Smitty275

Unless it also has a twist in it, it's hard to determine if it's bent that small of an amount. It could've been reconditioned because of a spun bearing and the others are still original length. To many variables to make an accurate guess without it in hand.

articfriends

Those speed pro pistons arent real expensive. Id replace the rod with a reconditioned one, if they break the piston, replace it but their is a fixture like goodson sells that supports the pin boss just for pressing pistons off with thought of reusing again. I assume those are 7/16 dimple rods? I have some loose that I would part with one if needed, Smitty

Brad Christy

502Nova,

I just did a quick and dirty sketch on AutoCad, to represent what a bend that yielded a .020" reduction in C-C length of a 6.385" rod would look like. It requires a 1/4" deflection at the midpoint of the rod beam, assuming a single point bend and straight chords between rod ends and the bend. You should be able to see that from space, so to speak. I'm no engine guru, but my math tells me you can rule out a bent rod.

Thanks. Brad.

BillK

The one rod on the bottom left in your picture definitely looks twisted to me ?? That looks like a forged piston but they can still be hard to remove without damaging them unless you have the OTC fixture that supports the rod rather than the piston to press it apart. To be honest with you they should be able to check that close enough with a 12" set of dial calipers. Or stand it up and use a tall dial indicator to the big end of the rod and compare it to one of the others.

Where are you located ?

RSCHAP1

With them all out and setting there, there would be a lot of ways to get .020" to show up.
Would be accurate within .001", but just using the depth rod on a pair of calipers from the inside of the big end bore, run along the length of the rod, to the pin.
Compare the suspect set up to another.
Wouldn't do something like that to find real precise measurement, but a difference that big should be noticeable.

502nova3

so I just did that and used the rod in question as 0 and checked 3 others agants it and it went like this the other 3 were + .015, +0.22 and +.012 so so it is shorter than the rest of em but than again there's one thats .10 longer than the other good ones to so idk could just be carbon on the piston but I measured a couple times and that's what I got + - a few thousands of a inch as my dial indicator is just a cheap one

502nova3

Also measured the thickness of the rod cap in the center and got between .325 and .405 the thinest one being on the rod thats .020 lowere tho I thought the thiner rod cap would rase the piston. Cyl idk
if this means aanything or maybe maybe just a shabby rod resize job done on last rebuild possible?

Xcomunic8d

Lots of ways to measure your rods as have been discussed.

If I were you, I’d disassemble those pistons completely from rods and wrist pins. making sure to keep a tray for each rod, pin, piston, and retainer so I kept the parts the same and in same cylinder.

then I’d look hard at those rods (read replace) and probably replace the pistons (don’t know the hours but those skirts look worn). Pistons are fairly cheap and new pistons and rings are cheap insurance and nice peace of mind. Of course now you need the bob weight and crank balanced but again you’re already there. I tried to reuse some once on a recent LS build. They were way out of tolerances. I suspect you’ll find the same on those.

I get trying to keep a budget but it’s way cheaper to spend a few hundred now than a whole bunch more later…not to mention downtime and having to go back in this far again. Get it done right the first time. Good luck!

BillK

Could be a crappy recon job if the rod was real bad and they had to cut the heck out of it to get it back to size. But the other thing is a lot of people dont realize how much the stock rods varied also.

I am going to agree with Xc. The pistons look pretty worn out. Its going to be hard to get them off the rods without hurting them without the right fixture which very few shops have. I would buy a set of Eagle I beam rods and a new set of pistons and be done with it. By the time you find a decent set of used rods and have them resized you will almost spend as much and still have a set of 30 year old rods. Only downnside is you might have to rebalance the crank depending on what the new rods weigh. The pistons should be pretty close to the same.