My short block is now completed, here are the specs.

'04 LM7 iron block. Bored to 3.903", decked, & line honed at HKE.
Balanced rotating assembly from TSP: Wiseco pistons, Eagle rods, K1 4" crank. Clevite H & HX bearings. At this point I'm not planning boost or nitrous, however I'm planning to get some nice heads.

While I'm waiting to decide what heads to get, I'm playing around with the rod bolt stretch gauge, measuring stretch, researching stretch vs torque, and crunching some numbers.

Here is where I'm at today:

The spec sheet with my rods (ARP 7/16" 8740 bolts) specifies 63 lb-ft OR ~.006" stretch. With generous amounts of ARP lube, and a few torque cycles to 63 lb-ft, my stretch numbers vary from .0030" to .0035".

Both the Eagle tech help line, and my engine builder source say "just use the torque method and you'll be fine". Not that I don't trust this, however I'm just curious about the physics behind the issue, and I have some time to sort it out. Now I'm down to crunching some numbers:

At .003" of stretch, the preload in the bolt (assuming neck diameter of .38" and E=30Msi) is about 5,700lb. Also note that stress at this amount of stretch is only around 50ksi, which is less than 30% of yield.

Now to calculate the max force the rod bolts will see:

Component Mass:
Piston = 427.36g
Rod Big End = 445.01g
Pin = 106.81g
Spiro locks = 3.80g
Rings = 33.90g

Total mass = 1020.20g, plus a 10% contingency factor and I get 1.122kg.

Peak acceleration of the piston (assuming 7500 rpm for extra contingency) at TDC is 41568 m/s^2.

F=ma, so the force shared by the rod bolts is about 10,500 lb.

So each rod bolt has 5,700 lb of clamping force, while the rod separation force is 10,500 lb. This results in an extremely low safety factor, and something I'm not comfortable with.

I have independently verified that my stretch gauge is accurate, so that is not the problem.

Should I torque the bolts down further to get more stretch? I'm open to anything, what do you think?

Thanks!

REZ

 

I would try another lube first and see what happens.

 

When you torque a bolt, thats exactly what you are doing..."stretching the bolt" creating the clamping force. Are you using a proper strectch guage?

This is the exact flaw with "torquing" bolts. Friction and a torque wrench that may not be accurate will produce the undesired results.

If you are going to stretch, get the proper tool or else just torque to spec and hope your torque wrench is accurate.

 

any recommendations? engine oil? anti-sieze?

exactly, so I don't know why my numbers are so far off. seems like I can just keep torquing until I get the preload I want. I would think to hit about 10,000 lb in each bolt to be safe.

I'm using an old click-type snap on. It was given to me and I don't know how old it is. maybe I should have it calibrated?

thanks for the replies.

 

Like stated in this post already try a different lube and re-torque then check stretch again. If its still not where it should be I would torque them until I got proper stretch.

 

yeah, I'm going to try this tomorrow after work.

the bigger question is, of the 2 methods listed on the spec sheet, why are the results so different? which one is correct?

Interested to hear if anyone else has run into this same issue.

 

Everything I've seen/read from ARP and engine builders on here ,,,,,,,,etc all say that the best method to use is the stretch/measure method ,,,,,,,,having said that if it were My build (I am building an LM7 .030" over and I don't have money 4 a stretch gauge so I using a caliper with ARP sportsman bolts 10mm torqued to 58lb./ft. was able to measure roughly between .0002" and .0003" of stretch so since that torque # is higher than the recommended torque using that method which I believe is 45 lb./ft. am going with that) I would use the stretch/measure method and probably stretch them to .0005 . They say if any 1 bolt has been stretched to .001" replace it . Maybe crunch the #'s for .005" and .006" of stretch and see what they say and decide . Good luck for sure ,,,,,,sweet ride BTW .

 

i would defiantly get your torque wrench calibrated, its only $40 bucks and can make the difference between life and death. I have snap on click style torque wrenches, i have them calibrated every 4-6 months but then again i use them each and every day. P.S. if your torque wrench is calibrated properly you should be good with the LBS torque specs but i always use a stretch gauge to verify results. good luck and make sure you use alot of ARP assembly lube on those bolts

 

where do you have your torque wrenches calibrated? somewhere here in Houston?

 

This is the correct lube....high pressure, moly lube.

 

The torque wrench calibration is an excellent idea and really something I am ashamed to have neglected to think of.

 

im in cali and haven't lived in Houston for 7 years. i use a tool repair shop down the street from my spot. Google up torque wrench calibration for Houston, im sure you will come up with more than a few. if you can't find someone local there are also several company's that you can send out your torque wrenches to be calibrated... runs anywhere from $25+ depending on if the wrench need service or not and what the shop charges. FWIW i pay $40 a wrench for calibration

 

Calibration is $75 here in SE TX...

 

Update:

I have not done the calibration yet.

I re-torqued the #1 cylinder rod bolts to 80 lb-ft and measured another .0015" of stretch (.0045" total). After this, I removed the bolts and re-measured the free standing length to ensure the bolts had not yielded.

With .0045" of stretch, the bolts now have about 8700 lb of preload each. Max piston force is 10,500 lb, so I'm not quite to 2:1 safety factor, but I'm getting closer...