Can anyone tell me what is the max RPM for stock rod bolts and how much abuse they will take, used in a 355 about 375+ HP thanks xyzman

 

Considering that the redline on the tach is at 6000rpm I don't know if I would go much past that for fear of a rod letting loose.

Well, if you are going to a 355 obviously the engine is being rebuilt, why not go with ARP rod bolts, and speak with the engine builder about your concerns.

 

I have revved them to about 6500 before. Upon teardown after 6 months of hard street use one of the rod journals on the crank was starting to wear badly. It wasn't knocking yet, but I don't think it would have lasted a whole lot longer.

 

5500RPm on stock rod bolts i'd say.
ARP rod bolts are cheap. Put 'em in. They're the lifeblood of a high revving engine.

 

I'd be more concerned with the cap ends, rather than the bolts.

I have yet to see a bolt come apart, but the stock rods/rod caps are weak.

ARP's are a good choice if when you're resizing the rods.

What's your goal anyway?

 

Rod cap failure usually comes after rod bolt failure.

I've never seen nor heard of a rod cap primary failure.

 

Ditto, its always the rod bolt that lets go. And boy is it cool when it happens.

 

Every engine I've ever owned or worked on that had that type of failure, it was a rod bolt. Usually they break either right under the head, or at the end of the threads. Either way, like lj said, it's kind of spectacular.... those are the ones that put down oil, water, sheet metal, cast iron, and whatever else.

Although, I did have one motor (a 327 IIRC) that sat in my cast-iron heap for a while, were a rod bolt broke (we found the head laying in the pan), the cap came loose and the other bolt broke in half (again, the pieces laying in the pan), the piston flew up and hit the head, the rod kind of laid over to the side out of the way of the crank, and the motor still ran, although it had a slight miss.... The customer DROVE his truck with all this UP THE DRIVEWAY, pulling a trailer with the motor he wanted rebuilt and put in.

I'd agree, 5500 is about the end of the line for any kind of "regularly occurring" usage. It's not like, hit 5501 once, and it flies apart; but statistically, the "mean time before failure" starts to crater if the motor spends much time up there.

ARP or SPS rod bolts will extend that "knee of the curve" to 6200 RPM or so.

 

My favorite explosion was on a 1996 Chevy blazer with a 4.3. Had a porter at the dealership driving to drop it off, didnt notice that the tech had it in 4wd low range. He called from his cell to say that it was making some noise, the service manager told him to keep going. He told us later that it was all the way to 65mph for almost 25 miles before it blew. When it came in on the hook, there was a wrist pin hanging out the side of the oil pan, no piston. Also a half of a compressio ring, MADE ITS OWN HOLE through the pan and was hanging out. It was a vortec engine and when i pulled the intake I found pieces of piston inside the intake with the spider. Found almost 1/2 of the valves were stuck open, had 2 studs that pulled out of the heads. It was radical. Guy still works there too.

 

Here is one of a few engines that I've seen blow apart at the cap.

Rest assured the other half of the cap was laying in an oil pan with the bolt
clamped tightly to the rod end:


Those of you that measure rod concentricity will see that the big end goes
oval at the cap.

Next time a motor comes into the shop, I'll throw up some measurements
of the rod end just for kicks.

 

Good rod (nice fat cap):

http://img.alibaba.com/photo/1146916...ecting_Rod.jpg

http://www.exoticperformanceplus.com...1113302042.jpg

Not so good rod (weak cap):

http://images.customclassictrucks.co..._smed_05_s.jpg

http://paceperformance.com/ProductIm...s/10108688.gif

 

All rods will eventually pull like that, even on a stocker thats not beat on. What I said was 90% of the time on a rod failure its the bolt(when stock components only are used) that lets go. Unless its like our wonderful toyotas that I work on. Rod slowly bends at the little end, until it starts to wear the bore. The piston slowly rocks more and more until it siezes. Thecool part is when the piston siezes and the rod tears right in 2, leaving the piston in the bore.

 

I guess I haven't seen enough 3rd gen car engines with bolt failures.

 

Purchased the short block rebuilt w/about 2 k miles on it I assume the bolts are stock. Is there any way to remove the stock bolts if they are stock & how do i tell ?, and replace them w/ARP without removeing the pistons?

 

Its not really the 3rd gen engine thing, its more just a SBC thing in general. One of the reasons I always like to laugh when I hear the stories people tell about STOCK 327s pulling 9k rpms. With stock rod bolts, stock valve springs, and stamped rockers.

 

If you really want to upgrade your bolts, Northern Auto Parts sells a set of 8 reconned rods with ARP bolts for like $130.

For not much more they offer these: http://naparts.com/ProductModelDetai...tModelId=17946

In my mind they are very cheap insurance if you plan to rev it high.

 

I can ASSURE you, the wrinkles in the sheet metal that's wrapped around the motor, don't make much difference.

Or, for that matter, whether it's a big block, or a small block. One other such explosion I witnessed was a 396 in a 66 Caprice... same usual deal, a rod bolt broek off under the head. The owner liked to talk about how he would put it in manual 1st (Turbo 400) and just let the governor shift it. He actually wasn't too surprised when it let go, less than 10k miles after a rebuild.

 

The only SBC I ever blew up was in a 69 plane jane that I wish I still had. Flat tops and 64cc heads. Embarassing low end torque but it really turned on at about 2500RPM. Anyway......

I was passing a car and taching it up to about 6000 (as usual) and a piece of one of the pistons let go. A crack had apparently formed in the top from the edge to the depth of the top ring groove. The crack followed along the ring groove for about 20% of the way around. Very impressive when it let go.

It appears that the piston came up and slammed the chunk into the head, cocking the piston to one side splitting the water jacket open. The engine pulverized the aluminum into little pellets that I found in 5 (yep that's right, they crossed over thru the intake and carb somehow) cylinders.

The engine would still start and had enough power to pull the car around. It made lots of horrible noises though. I still have the heads, they were unharmed. Believe it or not.

 

I was spinning my old 383 with stock 400 rods, rod bolts and cast 350 pistons to 7000 rpm for 2 years before the engine finally let go. Hard to say what failed first. A piston either fell apart or a rod bolt let go. The damage was so bad that I pulled the intake and heads off it and threw the rest in the scrap metal dumpster at work. I know the heads have 5 visibly bent valves.

When a piston and rod are being forced down from the burning fuel, the rod is being pushed down onto the crank. When the crank is pushing the piston back up to the top then stops at the top before being pulled back down, all the weight of the piston and rod and being held onto the crank by the rod bolts. While the piston and rod may not feel that heavy, when they're being thrown around inside the engine, they exert even more force than they weigh.

 

The average piston, ring, pin and rod assembly spinning at 6000 RPM will
exert a force of about 1800 lbs./~1150 g's on the rod bolts and rod cap.

That force increases dramatically as RPM increases.

That's no walk in the park for any mechanical component.

I'm not really sure what you mean by this quote, but maybe you can elaborate.

 

I was referring to this comment....Really doesn't matter whether it's a 3rd gen engine, or a 60s Nova engine, or a truck engine, or what. The sheet metal (or even fiberglass!!!) wrinkles don't correlate in any particular way with rod bolt properties or failure modes, is what I'm saying.

The root cause of probably 80-90% of all catastrophic rod failures that I've seen, in all kinds of engines of all ages, has been DIRECTLY traceable to a bolt breaking. I'm talking of course about ROD failures; not ROD BEARING failures. Which, for all I know, could sometimes possibly be traceable to bolts as well, besides the other usual things like debris in the oil, lack of lube, or "ovalling" the rod end from excessive RPMs. Like you point out, the stresses on the bolts are IMMENSE, and concentrated into an area (under the head... where they break) with a very small cross-section and not the most optimum shape for that sort of thing, by necessity. It wouldn't surprise me if it turned out that at least SOME spun rod bearings were due to stretchy bolts allowing the bearings to move around in the rod end. But it's hard to imagine a "smoking gun" any more conclusive than a rod bolt head laying in the bottom of the pan. That's pretty tough to argue with.

 

What I meant by my comment is, I haven't seen many stock bottom ends
come through.

Most of the stuff I'm dealing with has resized rods, and new bolts...so I can't
comment on a stock bottom end, with stock bolts, however based on what
I've seen the cap is usually the culprit.

For those that re-use stock rods, and don't resize them when replacing bearings,
create an instant failure point as the rod end is ovalled promoting a case for
bearings to spin, etc.

Some home/hobby mechanics on budgets don't take the time to properly
spec out a rotating assembly and either cheap out on rods, bearings, etc.
and end up with a blown motor. When you quote them for resizing, or
a good set of connecting rods and light weight pistons for their high power,
high RPM motor...they look at you as if you had two heads.

In other news, I don't know how a stock rod bolt compares to ARP for
instance, but I know that ARP rates their rod bolts for 190,000 PSI of tensile
load which far exceeds most forces on street/strip engines.

Any idea what stock bolts can handle?

 

Right.... stock bolts simply aren't up to the task. People who take a stock bottom end, re-bearing it, and slap it together, and then run it like it was a racing motor, are inviting sudden death. The factory simply didn't prepare their assmeblies for that kind of abuse, and they don't get any stronger after a couple hundred thousand miles.

I don't know offhand what the stock bolt tensile strength is; but I suspect it's ALOT less than the ARPs.

I agree, if there's any ONE upgrade that's wise to make to a stock bottom end, it's upgrading the rod bolts. Which REQUIRES getting the rods re-sized, since the act of removing and installing the bolts, distorts the rod. IMO that's even more important than a steel crank... at least when a crank breaks, it doesn't generally scrap the block, like a broken rod bolt often does.