Question(s) about Torque-to-yield bolts (engine rod bolts)
I understand the theory of Torque-to-Yield bolts, but I am confused about some points I have begun to consider in regards to a FORD factory service manual and my understanding of TTY bolts. Maybe someone can help me out??
Some info
The issue is the connecting rod cap bolts & nuts
1. FSM says tighten to 50 lb/ft and turn more +90 degrees.
2. FSM says bolts are not reusable if rods are disassembled new bolts are required
3. New FORD bolts are $20 each $240 total.
4. The rod bolt is a somewhat normal bolt with a "T" shaped head and conventional nut & thread and a raised knurling on the shank to help guide/center the cap during assembly.
5. The bolt is approx 5/16" x 24 x 2"(it may be metric but very similar) and assuming every 90 degree of turning the nut past snug stretches the bolt approx .001" for arguments sake (1/96")
6. General rule of thumb for engine rebuilding says if rod cap bolts are removed and replaced, the rod 'big end' must be re-sized/re-machined.(which requires removal of engine, dis-assembly, re-assembly, and re-installation, + many new expensive parts, gaskets, seals, bearings, etc)
My Goal is to not spend lots of money repairing my possible mistake in disassembling the rods to inspect the bearings.
My questions:
1. If I mark the bolt & nut, remove & replace the same nut to the same bolt and align the marks, has anything really changed. Assuming that no additional material has been worn away from the thread engagement of the nut & bolt, and no wear from the nut face turning into the rod cap, was disassembling & reassembling to the exact original position, harmful to the bolt? Has the clamping force changed?
2. If I reassemble with the torque wrench set to the recommended 50lb/ft and turn +"some number of degrees" method, I currently see only approx 45 degrees additional turning necessary to realign to my original marks.
.a. I believe the bolt has a permanent size distortion of +.000125 in length.
.b. If I stop tightening at the original marks, is the clamping force just as it was before it was loosened & removed?
.c. If I goto +46 degrees(1 degrees more) or +50 degrees (5 degrees more)I believe I will begin stretching the bolt in the yield zone of the bolt again, is there any reason to believe +.0000014"or +.00007" extra stretch will harm the bolt?
.d. I thought TTY bolts were supposed to be measured with a micrometer and tightened until a specific amount of stretch was measured/achieved, not calculated by turning the nut, can i reliably believe the first use of the bolt totally maxed out the available 'stretch'?
3. Does anyone have any idea about the rated tensile strength of any factory FORD connecting rod cap bolt?
.a. Is there any reasonable reason to believe FORD uses(or doesn't use) the same grade of bolt across their entire production line except for specials like the SHO v6 or the Shelby /Mustang V8? Somehow I think 140,000 psi comes to mind for stock bolts and high performance aftermarket bolts are 180,000 psi rated.
.b. Can someone provide a formula for calculating the stretch force being applied to 2 of these bolts when the estimated rod+piston weight of 8.5oz, is being pulled down by a 3" throw on a crank shaft spinning at some number of RPM's with +1 - +10psi atmospheric pressure applied to the bottom of the piston 12.6 sq in surface area, (2"-20" of vacuum on the top of the piston)? At what RPM is the clamping force of the 2 bolts being exceeded by the "weight at xRPM's" of the moving piston+rod, and the cap-to-rod connection beginning to come apart?
Most of these questions should end with a Yes/No/Why question for follow-up. Can you explain??
I know there is a lot of "question" here, and probably there is not enough info to answer with 100% accuracy, but hopefully someone or multiple someone's can make some good generalizations with typical types of bolts found in the general automotive industry, and help clear some of this up for me. (Maybe I will be lucky and a retired FORD engineer is here on this list?)
Thanks in advance
Byghtn