# 400 stock connecting rod bolt question



## 66CHEEVO (Jan 24, 2021)

Hi everyone, I'm still working on my engine rebuild and had a question about replacing teh stock connecting rod bolts. 
I already had my stock connecting rods resized and balanced by my local machine shop. I had this done using the stock connecting rod bolts. I've been reading that those bolts will be the weak link in my rotating assembly. If I now decide to upgrace to ARP connecting rod bolts will I need to have the rods re-cut and balanced? 

Thank you


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## BearGFR (Aug 25, 2008)

66CHEEVO said:


> Hi everyone, I'm still working on my engine rebuild and had a question about replacing teh stock connecting rod bolts.
> I already had my stock connecting rods resized and balanced by my local machine shop. I had this done using the stock connecting rod bolts. I've been reading that those bolts will be the weak link in my rotating assembly. If I now decide to upgrace to ARP connecting rod bolts will I need to have the rods re-cut and balanced?
> 
> Thank you


This isnt' going to be what you wanted to hear --- now  -- but...

Actually, for what you paid to have those rods resized and balanced you could have had a decent set of aftermarket forged rods --- which would have been a much better approach, and a lot stronger than those factory rods. 

However, to answer your question - if you change bolts now, unless you "get lucky" and the new bolts are exactly the same weight as the old ones -AND- have that weight distributed exactly the same along their length, you'll need to have the rods AND the crank rebalanced, because the difference in weight will change the big end/small end weight distribution on the rods, which affects the bob weights needed to balance the crank.

As to whether they'll need to be resized again - that I'm not sure about - but if the bolt material and torque loading is different, then that might also change the 'roundness' of the rod big end in which case they'll need to be resized again as well.

You might get a better answer from your machinist, BUT be careful there too - a lot of them consider Pontiacs to "not be serious engines" and therefore "it won't be that important". 

A machinist of that ilk is the reason for me having to go through my whole engine AGAIN recently. 

Bear


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## 66CHEEVO (Jan 24, 2021)

BearGFR said:


> This isnt' going to be what you wanted to hear --- now  -- but...
> 
> Actually, for what you paid to have those rods resized and balanced you could have had a decent set of aftermarket forged rods --- which would have been a much better approach, and a lot stronger than those factory rods.
> 
> ...


Bear, thanks for the info. I had a feeling it wasnt going to be as simple as just buying some new bolts. 
When I planned my build I was thinking OEM + on a budget. I wasnt planning on trying to squeeze out as much HP as possible. 
Once again a got a little hastey when buying parts and thought the original connecting rods would suffice. I didnt even think about the rod bolts at the time. 
At the time I was saving up every penny for a nice new set of tripower carbs, pistons and ceramic coated ram air exhaust manifolds. 
As you mentioned the worst part about keeping the stock connecting rods was the fact that it wouldnt have cost much more for aftermarket rods. 
Since I'm not planning on spinning my tires and redlining more than once  I think I'm going to stick with my initial plan. Hopefully I'll get some enjoyment out of the engine before it decides to fail. 
When the time comes I'll hopefully be loaded with cash and wil build the stroker I've been dreaming about. 

Thanks again


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## PontiacJim (Dec 29, 2012)

66CHEEVO said:


> Hi everyone, I'm still working on my engine rebuild and had a question about replacing teh stock connecting rod bolts.
> I already had my stock connecting rods resized and balanced by my local machine shop. I had this done using the stock connecting rod bolts. I've been reading that those bolts will be the weak link in my rotating assembly. If I now decide to upgrace to ARP connecting rod bolts will I need to have the rods re-cut and balanced?
> 
> Thank you


The ARP rod bolts would have been better, and the aftermarket forged rods even better. But, if you are not planning on hammering on the car on a regular basis and keep RPM's reasonable, you should be OK. The cast rods are pretty rugged and hold up well.

The issue most hear about is that some of the rod bolts used in the nid-60's were said to be a "little soft" (not hardened as they should have been). Different suppliers were used on many parts and I believe the rod bolts was one of them. So what happens is that at higher RPM's, the weight of the piston develops mass/force as the crank pushes it up on the intake stroke/compression stroke and it wants to keep going up through the head But then the crank pulls that weight mass down hard on the cranks down stroke just past top dead center. That changeover from moving upward to a pull downward pulls on the rod bolts and wants to rip the lower bearing half from the upper section of the rod. In doing this, if the rod bolts are a little weak, the rod bolts will stretch and as they stretch they tip inward at the edged of the lower cap. When the lower cap tips inward it also tips the edges of the lower bearing inward and the bearing edges act as a scraper and scrape the oil film off the crank journal. At high RPM's and no oil on the crank journal, the bearing heats up real fast and the end result is a damaged bearing that will start that unhappy rod knock sound or worse - damage parts.

I have never had any forged rods as the Pontiacs I had when young were stockers. I am not easy on my cars and I like RPM's, so most of them saw RPM's up at 6,000 RPM's, except the one engine I tried to blow up after an argument with the wife at the time. That poor engine, based on the factory tach, saw 7,000 RPM's and maybe more (and rumor has it a Pontiac will valve float at much lower RPM's) many times going flat out through the gears repeatedly. Got the engine so hot the exhaust manifolds were glowing red in the dark and I boiled the water out of the radiator. Engine never had a hiccup after that and I actually sold the engine out of the car several months later.

My '68 build is a 1972 455 I purchased for it. I got a bad engine from a craigslist seller that flat out lied about the engine. Not a thing was any good on it (but I salvaged the block) and it had been run with no oil for a length of time. Never seen lifters so cupped on the bottoms that I think they were paper thin and there were no lobes on the cam which probably caused it to quit running before it blew up. The engine got so hot that the crank was warped. Every bearing in that engine was just about gone and not a single rod could have been salvaged. I had 1 rod that had no bearing material at all and the crank journal had worn into it. Yet none of the rods came apart or broke at the bolts.

So I would not lose sleep over your rods unless you plan to really hammer the engine. Yep, they are old. Yep, who knows how many cycles/life have been put on them. But since your machine shop rebuilt them, you gotta have faith he knew what he was doing and didn't want to have to deal with you and a busted engine because of faulty rods.

When you assemble the engine, remember that the rods have a flat edge and chamfered edge. Chamfered edge goes to the crank side/radius. The flat sides go back to back. You also want a little chamfer on the bearing halves (which they should have) that will go up against the crank "fillet" so it does not damage the bearings or worse.


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## 66CHEEVO (Jan 24, 2021)

PontiacJim said:


> The ARP rod bolts would have been better, and the aftermarket forged rods even better. But, if you are not planning on hammering on the car on a regular basis and keep RPM's reasonable, you should be OK. The cast rods are pretty rugged and hold up well.
> 
> The issue most hear about is that some of the rod bolts used in the nid-60's were said to be a "little soft" (not hardened as they should have been). Different suppliers were used on many parts and I believe the rod bolts was one of them. So what happens is that at higher RPM's, the weight of the piston develops mass/force as the crank pushes it up on the intake stroke/compression stroke and it wants to keep going up through the head But then the crank pulls that weight mass down hard on the cranks down stroke just past top dead center. That changeover from moving upward to a pull downward pulls on the rod bolts and wants to rip the lower bearing half from the upper section of the rod. In doing this, if the rod bolts are a little weak, the rod bolts will stretch and as they stretch they tip inward at the edged of the lower cap. When the lower cap tips inward it also tips the edges of the lower bearing inward and the bearing edges act as a scraper and scrape the oil film off the crank journal. At high RPM's and no oil on the crank journal, the bearing heats up real fast and the end result is a damaged bearing that will start that unhappy rod knock sound or worse - damage parts.
> 
> ...


PontiacJim, thank you for the write up. This makes me feel a little better on my decision on not upgrading the rods. I'm going to do my best on not getting in any arguments with my wife while driving my goat and keep those RPMs well below 7K


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## PontiacJim (Dec 29, 2012)

66CHEEVO said:


> PontiacJim, thank you for the write up. This makes me feel a little better on my decision on not upgrading the rods. I'm going to do my best on not getting in any arguments with my wife while driving my goat and keep those RPMs well below 7K


Good deal, then YOU won't become her "ex"! LOL They say "its cheaper to keep her." My experience is that it costs you either way, so you want to make the best of it - which might mean having your own mancave/shop/building separate from the house and out back - with heat/AC a frig for beer and snacks, TV, and a comfortable army cot to sleep on. I like the saying "Less is More." The less I see her the more I am happy. Just sayin'.


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## lust4speed (Jul 5, 2019)

A rev limiter has saved many more engines than good intentions of keeping the RPM's down. Sometimes it isn't even the driver's fault. A rev limiter saved my engine on the way back to California from the Co-Vention in Ohio years ago. Wife and I came out of a restaurant in Tulsa, and I set the choke with one pump of the pedal like I did a thousand times before. It was a Matrix moment of bullet time as the engine screamed skyward while my hand was trying to get back on the key to kill the engine. Rev limiter had been on the car since the initial build and was out of sight and out of mind until it did its job that night. For no reason the tail of the fuel line hose clamp somehow reached out a little farther and locked up the throttle. It was scary enough to hear the engine up against the limiter, but much better than having it continue into the stratosphere followed by a loud bang.

Had a friend that I built an engine for that assured me he was only a show person and a cruiser. Called me up and thanked me for insisting on a rev limiter. Guess he took his car to work and couldn't resist doing a burnout for his buddies. It took him a moment to figure out why the car leveled out in the middle of the burnout. He wasn't used to the new engine and got caught up in the tire smoke.


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## lust4speed (Jul 5, 2019)

A rev limiter has saved many more engines than good intentions of keeping the RPM's down. Sometimes it isn't even the driver's fault. A rev limiter saved my engine on the way back to California from the Co-Vention in Ohio years ago. Wife and I came out of a restaurant in Tulsa, and I set the choke with one pump of the pedal like I did a thousand times before. It was a Matrix moment of bullet time as the engine screamed skyward while my hand was trying to get back on the key to kill the engine. Rev limiter had been on the car since the initial build and was out of sight and out of mind until it did its job that night. For no reason the tail of the fuel line hose clamp somehow reached out a little farther and locked up the throttle. It was scary enough to hear the engine up against the limiter, but much better than having it continue into the stratosphere followed by a loud bang.

Had a friend that I built an engine for that assured me he was only a show person and a cruiser. Called me up and thanked me for insisting on a rev limiter. Guess he took his car to work and couldn't resist doing a burnout for his buddies. It took him a moment to figure out why the car leveled out in the middle of the burnout. He wasn't used to the new engine and got caught up in the tire smoke.


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## geeteeohguy (Feb 2, 2008)

All great info here. I'll make you rest even easier....I have stock rods and bolts in both my GTO's. The 389 was assembled by me in 1981 (a rebuild on a 100k engine) and has seen 50,000 miles since. The 400 in my '67 was rebuilt and assembled by me in 1988. The engine had 173,000 miles on it at the time of the overhaul. Currently, it's at 256,000 miles, so about 85,000 miles since the rebuild. No issues. And I've revved the '67 up a few times tight enough to pump up the lifters. Had the 'good stuff' been available all those years ago, I would have used aftermarket....but back then, it was GM parts or nothing for the average guy. You'll be ok as you are, IMO.


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