A tale of two connecting rods

[Duke W.]

I think it's worth starting a third connecting rod thread.

Way back when I rebuilt my '63 340 HP engine I remember the look my Magnaflux guy gave me when I picked up the rods. "Boy are you lucky", he said. The #7 rod had a crack at both the crotch of a bolt seat and the corresponding crothc of the nut seat on the cap. He took me into the lab and reran the test on that rod to show me the cracks. I breathed a sign of relief as I knew I had dodged a bullet. I don't think the engine would have taken many more trips to 6500 before the rod let go.

Higher strength aftermarket rods were scarce and expensive then, so after consulting with several knowledgeable people about replacing all the rods, I decided to just replace the one bad rod, and found out when I picked up the new rod that the later design was improved with the hump of metal adjacent to the bolt seat. I had the new rod Magnafluxed and it checked out okay. I then ground down the forging flash and lightened all the rods IAW the Chevy Power Manual. Following this work I had them 100 percent shotpeened. Chevrolet did shotpeen some later hi-perf rods (and the Cosworth Vega rods), but my understanding is that only the bolt and nut seat areas were shotpeened, not the entire rod, but the bolt and nut seats are the most failure prone areas.

Someone with more materials science knowledge/experience than me might want to chime in on this, but fatigue failures usually start from small surface flaws. These flaws will then propogate as a crack and the part can fail if it is not caught during some kind of service inspection. Since the surface is the critical factor, if you rework a flaw free rod surface by shotpeening, you will essentially turn the fatigue clock back to zero, which is why I elected to use the seven rods that passed Magnaflux. With my rework, I'm confident that the rods will last, since I don't plan on doing any racing, but I will engage in an occasional blast to 6500.

High quality aftermarket rods are cheaper and more readily available today, and labor is higher, so if I were to rebuilt a SHP/FI engine today, I would just throw the OE rods away and buy a set of Crower Sportsmans. I consider this to be a no-brainer decision and cheap insurance. A lot of high revving 327s, especially in racing, blew up because these early rods were the engines' Achilles heel.

I learned some new information from Joe's post in the thread with the photos. First I didn't know that the 283 rods were even more spindly than the early 327 rods, and I didn't know that the early 327 SHP/FI rods were heat treated to a higher hardness, but just the same I would never resuse them today.

If you have a medium performance 283 or 327 (redline 5500 or less) I would dispose of any rods earlier than the 881s and replace them with a new set of 881s, which are not expensive. If you have a medium performance engine with 881s, I would have them Magnafluxed, and if okay, replace the bolts and nuts with higher strength aftermarket bolts and have them resized. Of course, you could also just replace them with a new set, too, as the additional cost over Magnaflux, new bolts, and resizing may not be much. Even new rods can be suspect because I don't believe they are Magnafluxed in production, so there's a chance that a new rod could have a potentially fatal surface flaw, which is why I'm okay with used 881s on a medium performance engine as long as they pass Magnaflux.

The L-79 is in a gray area. A Magnaflux inspected used set of 881s with new bolts is probably okay. Then again, considering the cost of replicating a numbers matching 870 block the Sportsmans are cheap insurance!

Duke

[Clem Z.]

most cracks start from

sharp corners left from machine work. this is called a stress riser and even new rods should be deburred and all sharp edges and corners removed. the new PM rods sold by GM are really better than the old forged ones. the PM rod were tested in a 550 HP SBC at 6000 RPM for 45 hours but they do not come in the 2.000 crank pin size to be used in the older SBC. if you have a crank with the 2.100 crank pin diameter i would use the new GM PM rods before spending any money on the old forged ones. you can buy 8 rods for less than $250

[Tracy C.]

Re: A tale of two connecting rods

I've looked in the Jegs Catalog and on the net, but small journal rods aren't readily availible.

Where might I look for a source on the Crower Sportsman or others replacment rods? An occasional rip through the gears is all I'm after. No heavy duty usage.

thanks,
tc

[Duke W.]

Re: A tale of two connecting rods

Google to the Crower web site and look at page 147 and 151 of the catalog. You can call Crower to find out how to buy them. Maybe they will sell direct, but you might actually get a better price through a dealer.

Note that there are two big end sizes, 2.215" and 2.225". The 2.215" big end rods are the "small bearing" type. These dimensions are for the big end bore, NOT the diameter when the bearing inserts are installed! The through bolt type are more than adequate for mechanical lifter cam SHP/FI engines, and I think they're a bit less expensive than the cap screw type. You can buy them with the small end machined for pressed or floating pins. For a street engine rebuild, I would go with pressed pins.

I'm not surprised that an outfit like Jegs doesn't sell them. They sell mostly hotrod stuff, not real high quality racing hardware.

Duke

[Joe L.]

Re: A tale of two connecting rods

Duke, et al-----

A little more research reveals that there was another 2.00" journal connecting rod. This rod was GM #3927145. It was manufactured from the same forging as the GM #3864881. However, it was manufactured from selected forgings, it was hardened to a greater Rockwell hardness than the 3864881, it was fully shot-peened, and 100% magnaflux inspected.

Some sources indicate that this rod was used for 1967 Z-28. However, GM sources don't indicate that and I don't think that it was ever used in PRODUCTION for that application. I believe that this rod was released in 1968 as a SERVICE-only piece and, primarily, for use on 1967 Z-28 applications in order to improve durability.

Unfortunately, this rod was discontinued from SERVICE in May, 1985 and I don't think you'll find many NOS examples around. However, it should be possible to use a GM #3864881 rod (no longer available from GM but not that hard to obtain) and "create" from it a rod with nearly the same attributes as the 3927145. I still think that going aftermarket is the best bet, though.

[Duke W.]

More engineering insight

Connecting rod loading and material stress increases with the SQUARE of engine revs, so (6500/5500)**2 = 1.397. What this says is that rod stress in a mechanical lifter SHP/FI engine at 6500 is FORTY PERCENT greater than a medium performance engine at 5500!!!!

Steel has well characterized fatigue properties and there is a stress value known as the "endurance limit" such that if the part is not stressed above this level in service and has no significant surface flaws, the fatigue life is considered essentially infinite. I'm not sure what this value is for a connecting rod because there are also some geometry considerations (like corners), but given the higher stress that a SHP/FI engine will see, it definitely needs a higher quality rod!

Heat treating a connecting rod increases its hardness and the higher the hardness the higher the stress limits - ultimate, yield, and fatigue, which is why some of the OE rods are heat treated to a higher hardneses than others. There is also a direct correlation between hardness and endurance limit, but it's not linear.

Grinding surfaces smooth will eliminate most microscopic surface flaws that can initiate cracks and propogate with cyclic loading. Also, shotpeening "work hardens" the surface to make it even stronger than the interior material, and since cracks usually begin on the surface, a harder surface will reduce the propensity for microscopic flaws to propogate as a cracks, which means shot peening will increase the endurance limit.

Duke

[Michael H.]

Re: A tale of two connecting rods

Joe,

You're exactly right. The 145 rod was a service only part and not used in production. It was just exactly as you described it. These were released for the 67 Trans Am 302 Camaro some time in early 1967. The rods were completely coated in a dark green semi transparent color. We used a lot of these in the late 60's and early 70's and found them to be excellent as far as durability and quality.

[Mark #28455]

Re: More engineering insight

The biggest advantage of aftermarket rods comes in the alloy of steel used in the rod. 4130 and 4340 are significantly stronger even with the exact same forging shape - even the stock LS6 and L88 rods used the improved alloy. I think the basic Crower rods are 5140 and the high end rods are the 4000 series alloys.

[Duke W.]

Re: More engineering insight

It's not that simple. The ability to harden/strengthen steel is a function of carbon content, and carbon content on the order of 0.4 percent with proper heat treatment generally yields high hardness with good ductility to give excellent fatigue endurance. Higher carbon content can yield higher yield and ultimate tensile strength, but high carbon steels are brittle and are not suitable for applications where fatigue is an issue.

The problem with plain carbon steels is that the heat treat is not uniform in sections over about half an inch thickness. Alloy steels achieve the same hardness/strength to a much greater depth and are an advantage when section thickness is over an inch. Connecting rods sections are thin enough that alloy steels probably don't provide much of an advatage.

What is critically important is design geometry, and this is the problem with the early small bearing rods. The combination of a thin section and the corner at the bolt seat creates an area of high stress, and the primary improvement in later OE and aftermarket rod designs is greater material in this area to reduce unit strees.

For a low volume application 4340 and 5140 alloy steels are wonderful materials, which can be uniformly hardened to great depth in order achieve high strength and endurance limit, but Chevrolet's decision to use plain carbon steels was not the problem because the section sizes of connecting rods are not large enough to justify alloy steel in a high volume application.

The design geometry around the bolt seat is the reason why these rods are weak, not because of material specification or processing deficiency.

Duke

[James F.]

Re: A tale of two connecting rods-Tracy

Tracy,
You can ontact Crower (619) 433-1191 and order SBC Sportsman 5.7" small journal, pressed fit rods. Part no. SP93200-PF. Cap Screw rods will require four to six week wait, Bolt rods on the shelf. Regards,

[Brian Monticello]

Re: A tale of two connecting rods-Tracy

Scat makes a nice set of small journal press fit rods - have found them for about $230 shipped. Rated for 7500 RPM and up to 500HP if my memory is correct.
Brian

[Clem Z.]

you have to be careful of the weight of the rod

because if they are lot heavier than the stock rod it will make it hard to balance

[Brian Monticello]

Re: you have to be careful of the weight of the ro

Clem,
My builder said he was pleasently surprised how well the SCAT rods were balanced....although they are a bit on the heavy side. HE did, however, at the same time complain about the pistons (stock domed) and the crank. He had to take a bunch of material off the pistons but add a bunch to the crank.

Brian