[Shiny]

Shot blasting (peening) is a way to increase fatigue life of the rod. The process leaves a layer of compressive stress on the surface. Compressive stress is what usually happens on the inner diameter of a bend so it's a weird thing to picture as a result of shot blasting but it's there.

Fatigue cracks start on surfaces and need tensile stress to initiate and grow. So the idea with shot blasting is to add a "negative" or counter-acting stress to the tensile bending stress that happens when the engine is running. The shot blasting basically increases the stress needed to start a fatigue crack. It would be a good thing to do to any rod.

[Schurkey]

Shot blasting, and shot peening, are different processes.

Shot peening is scientific, expensive, and no-one anywhere near me provides that service. It's an aerospace-level process. Tightly controlled.

Shot blasting on the other hand, is done by pretty-much every automotive machine shop in America. It's a cheap and "environmentally-friendly" way to clean parts. There's little science, and virtually no "control". A big motorized wheel scoops up a bazillion little iron/steel balls and flings them semi-randomly at parts loaded into the machine. If there's any [structural] benefit aside from removal of rust/carbon/varnish, it's purely by accident.

[Shiny]

Thanks Shurkey.

That's too bad there's no easy access to a controlled process.

Do you think shot blasting is aggressive enough to help?

An easy way to know would be to shot-blast a thin steel strip. If it warps, there's enough stress to deform the surface and help fatigue life. Since sand-blasting a body panel can warp it, then maybe?

Mike

[25stevem]

In one of the shops that I worked in we made the mistake of loading in fresh shot and then clamped in two SBC iron heads off early 80s vintage.

After the timer turned the blast cabinet off and I opened up the doors to it my eyes went wide as saucers when I saw that there was 1/8 to 1/4” of cast iron removed from many areas of the two heads.

[Shiny]

ummmm....

that would certainly leave enough residual stress to improve fatigue life

[mgarblik]

Our shot blaster uses chopped stainless steel wire for the media. The wire is .030" in size. It produces a nice uniform surface on iron and aluminum. Doesn't remove much material. It does some very minor surface peening. I say this because the block MUST be line honed once a block has been through it. It closes the bores about .001. Also a lifter will not go through a bore without 3-5 passes with a ball hone. Will it improve the strength of a connecting rod? I really doubt it. Does make it look nicer and doesn't hurt anything. Stronger rod material and better design makes them stronger.

[Cliff R]

Highly UNLIKELY doing anything to the surface of a cast iron rod is going to help it's strength or longevity in one of these engines. Not being a forged steel part you aren't releiving stress risers and or doing much else to it that's going make it any better.

Decades ago before the Internet or this Forum there was a local racer who tried to run a 400 based engine build with RAIV heads in a high RPM combination. I ran into him at the track and struck up a conversation about his combo. It was a manual trans car and running bottom 10's spinning it to and past 7000rpm's. We had a discussion about connecting rods and he said that the life expectacy for cast rods no matter how much "work" was done to them was one or two runs.

He went on to mention that most of the Pontiac forged rods he tried bent into an "S" doing the same thing. He ended up going to some type of forged rod, had them fully prepaired, either the good early SD or later 455 SD's, can't remember which at the moment, but they lived fine for what he was doing at that time..........

[25stevem]

Once you have stronger bolts in and resized the Rods there's not much more you can do to them to keep them happy and on the farm.

The best additional thing you can do to increase longevity is weight reduction.

There's a lot of extra meat that can be taken off the balancing pad on the top of the rod, and this rework along with lighter Pistons and pins will greatly reduce the G force loads on the Rods for any given rpm level.

I don't even think that putting them through a cryogenic tempering sssion would help out.
In fact it may serve to just make them more brittle and failure prone!

[dpoltzer]

Great information everyone, thanks for all of your input.

[Shiny]

There's no way a shot-blasted stock cast rod will have the strength of a forged steel part. That's apples and oranges. OP was talking about a stock rebuild.

I'll stick to believing a shot-blasted stock rod will have higher fatigue strength than one that wasn't.

Would it turn a cast iron rod into a forged steel rod? No. Would it add any value? I think it would if rod failure by fatigue was a dominant fail mode for cast rods. If not, then agree it's low value.

There are lots of ways a rod can fail. I have zero experience to say which stock cast rod fail modes are most common in performance-modified Pontiac engines or at high rpm. This forum suggests bolt fatigue is common as are bearing failures from distortion, dirt, and detonation. How do cast rods fail when overstressed by power or rpm? Do they ever fail in stock applications below the factory red-line?

Parts are usually made as cheap as possible to be "good enough" in the application…. Pontiac's cast rods fit that model and were apparently good enough for a stock engine. Are 50-yr old cast rods more likely to fail in a stock engine? I'd speculate only if they were deformed or routinely run beyond red-line.

It seems the economics and experience say if you're worried about it, buy new forged steel rods. I understand and would personally follow this advice. But if I chose to keep stock rods in a stock engine and shot blasting was cheap, I'd do it.

Mike

[Tom Vaught]

Originally Posted by 25stevem

The rocker ratio would be 1.5 and the push rod lenght stock was 9.130 to 1.935" or so.

Note that the ideal length for the push rod due to geometry changes from milling the heads or block may need to change.

Also a valve job being done can change the length of the push Rods needed.

Think you have a typo error here 25stevem. I believe you were trying to post: "the push rod length stock was 9.130" to 9.135" or so."

Sometimes I have to reread my posts 3 times when I am in a hurry with the info.

Tom V.

[25stevem]

Your 100% correct there Tom, and thanks!

What’s the problem, you have never seen a 7.2” tall lifter in a Pontiac before? Lol!

[Sirrotica]

The factory deemed the cast rods sufficient to get past warranty on the 3.75 stroke RA IV, but with the same red line (6000 RPM) they deemed forged rods with 7/16 rod bolts needed in the 4.21 stroke 455. I'm sure they knew much more about the engineering, and stress evaluation than most people reading this.

I've done autopsies on dozens of failed Stratostreak Pontiac V8s, the ones that tossed rods, spun bearings before the rod left it's connection with the crank throw.

Pontiac also used .001 over rod bearings in the RA IV engine to get more bearing clearance, and a better oil wedge with the cast rods. Any engines I ever built for use on a dirt track with cast rods, I always traded bearings to get .002 clearance. With tight factory clearances it was a given that in one/two nights you'd spin a rod bearing. Loosen the rods up to .002 you cold run a full season with no bottom end failures. When a 2.25 journal cast crank gets hot, it grows in diameter. Without that extra clearance to begin with, it will force the oil out of that area, spins the bearing, you can almost count on it. Running heavier oil won't band aid it because there is no space for the oil wedge to survive. The 400 performance engines also used full grooved mains, that was also another thing I made sure my engines had. After blowing up a lot of engines, I listened to an old timer, and solved the spun bearing/tossed rod problem. Now I'm the old timer, time to pass the knowledge on.

If the engine was originally a 2 bbl, you also needed to go with the big pickup 60 PSI pump. It also helped to run 7 quarts in the engine to keep the pickup covered under hard cornering. The later pan with the baffle, also helped keep the pickup covered.

Now this was low buck racing, so even with stock rod bolts, and not reconditioning the rods, as long as you didn't run it too hot, and changed oil frequently, the engines stayed together. There's a lot of experience in what I just posted, no one has to believe what I posted, but I have no reason to tell tales. I got tired of hunting used engines that would only last a week or two, all summer long for my race car. Spending a little money, and taking some time to clearance the bottom end saved me a bunch of work/money down the road, and kept me racing every week.

[Shiny]

Originally Posted by Sirrotica

...
I've done autopsies on dozens of failed Stratostreak Pontiac V8s, the ones that tossed rods, spun bearings before the rod left it's connection with the crank throw. ...

Great info, thanks for sharing your experience.

Spun bearing feels consistent with Cliff's description of a bent rod after failure but when stuff turns loose, I guess carnage happens.

[Sirrotica]

Originally Posted by Shiny

Great info, thanks for sharing your experience.

Spun bearing feels consistent with Cliff's description of a bent rod after failure but when stuff turns loose, I guess carnage happens.

[Skip Fix]

Didn't Chevy shot peen their high perf rods? I want to think in one of my old Chevy Power manuals they even had shot size and spray pattern listed.

[25stevem]

Yes. Chevy did, as did Ford and Chrysler for there top performance motors .

[dpoltzer]

Are stock pushrods for a RA3, 5/16 diameter?

[25stevem]

Just keep in mind that the early production non SD rods just like the old HO racing Assembly manual stated will stretch .010” when subject to enough rpm / load / G force.
Ask yourself how long could a rod possibly last if it can stretch and or go out of round by let’s say even .006”?

[SD455DJ]

Originally Posted by dpoltzer

Are stock pushrods for a RA3, 5/16 diameter?

Yes.

Dennis