SHP big block connecting rod durability

[Duke W.]

Consider a restoration L-72 (less than 60K original miles - parked in a garage for over 30 years, engine currently seized) that will not be beat on, but the owner should not be concerned about buzzing it to 6500 when opportunities arise.

Assuming the rods pass Magnaflux inspection and better than OE bolts/nuts are installed, are they durable enough?

There's some cost tradeoff here, but durability is key. I haven't checked prices, but figure about 500 bucks for a set of Crower Sportsmans as a baseline upgrade.

Magnafluxing the rods might be "free" since a crank alone or a crank and eight rods may still be within the minimum "lot charge" of most shops.

Also resizing rods shortens their center to center rod length (which increases deck clearance and equal amount), which can be an additional method of managing as-built compression ratio.

Anyone know how much can be ground off the rod and cap for resizing - .010"? .020"? - before strength/durability are comprmised.

Duke

[Clem Z.]

Re: SHP big block connecting rod durability

you should not need more than .002 off the cap and rods to recondition them. install new bolts for sure. you can check the big ends with a dial bore gauge and if within specs i would not even recondition the rods. you can change the bolts by installing a snug fitting steel plug in the rods big end torque up the bolts and remove and install the new rod bolts one at a time,retorquing as you change them. i have done this many times with no problems because back then the rod bolts were the weak part. check the rod for alignment between the big end and the pin. good shops have a fixture to do this and if any are out of alignmnet they can be twisted back into alignment,very very inportant so the piston is true in the bore.

[Duke W.]

Re: SHP big block connecting rod durability

Thanks, Clem. I'm keeping you busy today!

Sounds like the rods are basically an okay design, so the preliminary plan will be to replace the rod bolts with the best high strength replacements available and have the rods resized with minimum material removal to a tight (.001") range of center to center length for all eight and proper alignment verified.

BTW, I am assuming that the L-72 rod bolts are 7/16". Correct? Any recommendations?

I don't have the equipment and tools to the work as you do, so we'll have to have the machine shop do the work (assuming we can find a machine shop that we can manage and control to do only those operations that are ordered, which is no small task).

The CR is so high that using rod resizing to increase deck clearance enough to yield a meaningful drop is probably not worthwhile.

Not having to replace rods (assuming they pass Mag) frees up some budget for trick pistons!

Duke

[Richard F.]

Re: SHP big block connecting rod durability

Duke, There were no 7/16 rods until 1969. I took out the 3/8 dimple rods in my 68, and put in later 7/16 just out of neurosis. I believe Clem has said he always used 3/8 because they were lighter, and were strong enough. I'm sorry now that I didn't reuse the 3/8 rods. They sit in a box with the 12:1 pistons.

[Clem Z.]

Re: SHP big block connecting rod durability

1966 BBC 427 would still have 3/8" rod bolts. the 7/16" bolts did not show up till the L-88 engines

[Clem Z.]

in drag race engines we used the 3/8' bolt

rods because the were lighter than the 7/16" bolt rods. we never had a failure even turn 7500+ RPMs thru the lights.

[Richard F.]

Re: in drag race engines we used the 3/8' bolt

Apropos to nothing, years ago I bought a crank from a road racer who ascribed to the theory that because nitriding can put a bend in a crank, you go to the dealer and order six cranks, hoping to get one that is perfectly straight. I bought one of his rejects, because I ascribe to the theory that you hit the crank with a big hammer until it is straight. His telltale tach showed that he was hitting 8500 RPM. I, of course, don't do that.

[Duke W.]

Re: in drag race engines we used the 3/8' bolt

It's supposed to be a real art "peening" one one of those babies back straight. You must be pretty good! The few I checked were all okay. My '63 L-76 crank was dead straight and all the journal diameters were right in the middle of the OE tolerance.

Considering I ran that puppy pretty hard on occasion over 115K miles is testimony to its durability.

But the connecting rods were ANOTHER MATTER; #7 was cracked across a bolt seat. Who knows how many more trips to 6500 it would have survived.

Chevy cranks, especially the forged, Nitrided SHP types are amazingly durable, but some of the connecting rods are downright scary!

Duke

[Duke W.]

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[Clem Z.]

ARP if they still make them.

the 3/8" rods have not been used for 40 years so the bolts could be a problem

[Duke W.]

Re: in drag race engines we used the 3/8' bolt

Thanks guys. The information you provided leads me to conclude that a high strength aftermarket 3/8" replacement rod bolt is the way to go for this project.

Any recommendations on a specific manufacturer/brand and part number?

Duke

[Richard F.]

Re: in drag race engines we used the 3/8' bolt

Duke, A last couple of thoughts: 1) When I put 7/16 rods in my March,68 5439 block, I had to grind extra clearance for the rods. Just another reason to stay with the 3/8's. 2) You can get ARP 3/8 bolts from many sources, including Jeg's and Summit.

[Mark #28455]

Scat rods or Eagle

You can get the pressed pin Scat rods for about $220 a set, I just bought them for my 454 about 2 mos ago. In addition, the Eagle I beams go for less than $300 a set.

I have a March '68 439 block that has the small cast in reliefs. With the ARP 12 point nuts there apppears to be just enough clearance for a 427 crank in my block. I also had a May '68 block that had larger cast in reliefs that cleared a 454 crank with the 3/8" rods.

All the Scat and Eagle rods use the 12 point ARP bolts for max clearance. My current machinist pretty much doesn't rebuild Chevy rods anymore as the aftermarket stuff is so cheap, uses a better steel alloy, comes pre-balanced to within about 1 gram, is magnafluxed already, and has no history of prior abuse. It's your choice.

Mark

[Duke W.]

Re: in drag race engines we used the 3/8' bolt

ARP's catalog can be downloaded in PDF, and they have an 8740 replacement for the BB 3/8" bolts, but I'm starting to take a look at aftermarket replacement rods, as discussed further down.

Duke

[Duke W.]

Re: Scat rods or Eagle

I'm beginning to come around on this issue. It appears that both Scat and Eagle have some new product lines, and the weights are closer to OE. In the past what I didn't like about there rods was that most were signficantly heavier than OE, which can complicate cranktrain balancing in addition to increasing inertia load.

The Scat "Pro Stock Premium" is an upgrade from their OE replacement rod, and the Eagle "SIR" series appears to be very similar - in fact, maybe virtually identical, and they conincidently have very similar part numbers. The Scats are claimed 4340 and the Eagles claim 5140, but they could both be produced on the same tooling - no doubt in China.

I did a quick price check and they are in the range of 250-275 dollars, but at the rate the dollar is plunging relative to the Yuan, they are probably going up monthly!

None the less, these rods with 7/16" 8740 cap screws are probably more durable than the OE rod with replacement 3/8" 8740 bolts. The bolt set is about $60 and though I haven't costed it out, the qualification and machining work will probably put the total up to at least $200, so maybe the OE rods are better off as paper weights.

Being as how Scat is about 3 miles due east of me in Redondo Beach, I will likely be paying them a visit.

Duke