Over-stretched rod bolts = disaster?

[Michael]

On the nth rebuild of my big block Chevy (Mark IV, 454 + .030â€), I got distracted and torqued several rod bolts to 65 ft-lbs instead of the correct 50 ft-lbs. These are 3/8ths bolts, ARP PN 135-6002, on stock (resized) rods, originally from a 1978 Suburban. I noticed my mistake about 20 minutes later, backed off the nuts, and retorqued to 50 ft-lbs. My machinist is convinced that the bolts have been over-stretched and need to be replaced, but I’m reluctant to do that… not for the cost (new bolts are only $60), but for the labor of removing the rods/pistons and driving out the old bolts.

 

How safe is it to reuse the current bolts? The application is naturally-aspirated with around 6000 rpm red line.

[Lozer]

youll know that you machinist was right when the rod goes through the side of the block. The real question is are you willing to risk it and how much is at stake if you destroy the block? If it was a junkyard motor I wouldent think twice, but if I stuck some time and money into it I would just suck it up and do it right so I dont lose sleep.

 

just my nickel and dime

[jt1]

You could check the length of the over torqued bolts and compare to some that weren't over torqued. If they're longer, they're definitely scrap. You could also contact ARP, but I have a pretty good idea what their answer will be.

 

If you replace them, you need to resize the big end. Normally they get a little out of round replacing the bolts.

 

I would replace them. A couple hundred bucks isn't much in the entire build.

 

jt

[Michael]

And what happens if the rod big-end is not resized? I just had the crank reground (2nd time) and am on the 4th rebuild. The engine has run a total of 43 miles in 11 years of ownership. So far, with the various rebuilds, my "investment" in this engine is around $7K. I would rather have a spectacular explosion than a further lingering of the project.

[jt1]

And what happens if the rod big-end is not resized? I just had the crank reground (2nd time) and am on the 4th rebuild. The engine has run a total of 43 miles in 11 years of ownership. So far, with the various rebuilds, my "investment" in this engine is around $7K. I would rather have a spectacular explosion than a further lingering of the project.

 

It sounds like rod bolts are the least of your worries. Bolt it together and let it rip, and put a big shot on it for good measure.

 

Be sure and post a vid.

 

jt

[Lozer]

I'm with jt1 if this engine has been that much of a pail. Put it together run a 500 shot of Nitros to it and see what kind of a 1/4 mile time you can put down before it blows. Then go buy a crate motor and call it good

[grumpyvette]

running the torque up to 65lbs may or may not have pushed the bolt past its elastic limits

Measure each the fastener length prior to starting, and monitor overall length during installation., bring the bolt up to torque specs and relax the torque a minimum of three cycles, then measure the bolt length after relaxing the bolt torque the third time, When the bolt has stretched the specified amount, the correct preload, or clamping load, has been applied.

if you maintain a chart of all rod bolts, and copy down the length of the fastener prior to and after installation, youll see any deformation, If there is a permanent increase of .001Ë in length,when the bolts relaxed or if there is obvious deformation, the bolt should be replaced.

 

 

http://www.arp-bolts.com/Tech/TechTorque.html

 

http://forum.grumpysperformance.com/viewtopic.php?f=99&t=3511&p=9253#p9253

Edited September 15, 2010 by grumpyvette

[Michael]

Update: I bought a Jegs rod bolt stretch gauge and checked all of the bolts. I recall that the four rearmost bolts (cylinders 7 and 8) were torqued to 65 ft-lbs, while the others “probably†where not. MSD’s specs for the rod bolts in question are 50 ft-lbs and 0.0063†stretch. Well, when all bolts are torqued to 50 ft-lbs, I obtain the following stretch values:

 

- the four rearmost bolts: stretch between 0.0065 and 0.0075 (approximately)

- the other twelve bolts: stretch between 0.0055 and 0.0060 (also approximately).

 

I say “approximately†because it is difficult to maintain the gauge on the bolt head and shaft dimples in a repeatable manner, and the stretch gauge can not remain installed while the nut is torqued with a socket wrench. Every checking of bolt stretch therefore requires removal of the tool during the tightening sequence. And that introduces possible biases.

 

Based on this, I chose to button everything up and to get the engine running in “limp†mode, which for me means < 4000 rpm and only with the 20+ year old street tires currently installed. If I can obtain a decent tune –a feat never before achieved – I’ll declare victory, mothball the engine over the winter, and in the spring will disassemble it to replace the rod bolts. BTW the top piston rings, as it turns out, were installed backwards (wrong side up) and that has likely resulted in cylinder wear.

 

If the engine grenades even during its planned mild exercises, hopefully I can save the heads, cam and valvetrain, and will replace the bottom end with a 496-type of assembly.

 

Currently I’m looking at a reassembled engine ready to install back in my Z. The only missing piece is an ignition system, about which I’ll be posting shortly.