[LeMans70Sport]

What is a good technique for measuring rod bolt stretch? In the past I had always just tightened them to the given torque and didn't think about them again. This time around I'm blueprinting my engine and bought a stretch guage. I though the proper technique was to torque the bolts down in increments alternating from side to side. From what I have read, the technique using a stretch guage is to go from 0 to the proper stretch on one side then move ove to the other side. Is that right? Seems you would have uneven loads doing that. I'd like to alternate but I find it is impossible to zero the guage properly when moving from side to side.

Also, is it ok to go to the given stretch and take my measurements, then disassemble the rods and do it again on the engine? The bolt won't keep a "memeory" will it, thus throwing off my final torque?

[Jack Gifford]

Experts? Come on- I want to see the answers.

[Tim john]

I would post this in the SD car section for the Super Duties require from the factory a "stretch" of the rod bolts for correct torque. I am refering to the SD Trans Am's (73/74). The owners of these beasts should be able to provide sufficient information.

Tim john---

[LeMans70Sport]

It doesn't seem like many are going to chime in so I went ahead and got started. For accuracy, I set my stretch gauge to 0 then applied my torque with a torque wrench. I've seen pictures of people trying to apply the torque with a 7/16 box wrench while watching the stretch guage but it is not possible to apply 60-70 foot pounds of torque without using a pipe on the wrench. I then put the stretch gauge back on and tried to place it as carefully as I could to the center of the bolt where I had set it to 0 and took my reading. I then went up from there in 5 lb. increments to get my final torque. I was concerned about torquing then retorquing the nut upon final assembly but the tech info at ARP helped me out with that. You can remove the bolt and measure it and if it has grown more than .001" then you have over torqued it and it is junk. I found that 70 lbs. gave me a stretch of .006 using 8740 cap screws. Hope this helps for anyone following this post.

[GRX]

This page help any?

measuring rod bolt stretch - http://e30m3performance.com/installs...rods/rods5.htm

[Tom Vaught]

If you measure each rod bolt for length PRIOR to installing the rods, you can easily then torque the bolts to slightly below that stretch value (using a given torque value) and then do a check to see if any of the bolts have not stretched an equal amount. This will spot a soft or "atypical" bolt.

You are after bolts that all stretch evenly and exert the same preload on the clamp joint.

Tom V.

[Jack Gifford]

Originally Posted by LeMans70Sport

... From what I have read, the technique using a stretch guage is to go from 0 to the proper stretch on one side then move ove to the other side. Is that right? ...

So... is the answer 'NO'?

[Tom Vaught]

When we use the "Torque to Yield" method we seat both fasteners by using some small number for torque like say 25 lb/ft on each fastener. We then do possibly TWO 90 degree pulls on one fastener and then repeat the same TWO 90 degree pulls on the second fastener. (This would be torqueing a rod fastener set). Stretch to a given value is not the same as torque to yield.

I see no problem with multiple torques on a stretch method. Heads are torqued that way, except the factory specified a torque value vs a stretch value.

Tom V.

Tom V.

[machine shop tom]

Each bolt should be torqued from 0 to full torque in one smooth motion. Creeping up on the torque will result in less stretch. I torque the bolts starting at 10 lbs under what the manufacturer recommends with the lubricant they specify. I then measure how far the bolt has stretched. If it needs more to reach the proper stretch, I loosen the bolt and increase the torque 2-3 ft lbs and check the stretch again. This is repeated until all the bolts have been checked and the individual torques recorded. These figures are then used when the engine is assembled.

BTW, when the proper stretch is reached, the big end bore must be checked for sizing and shape. Most rods that are out-of-the-box need to have the fasteners properly torqued and the big end checked and at least honed before assembly.

tom

[Tom Vaught]

The only comment I would make about your method, MST, is that bolts only have a given number of pulls in their life and if you use up half of the pulls assembling the engine the first time then the engine builder/ buyer/ racer might be buying bolts more often than normal.

If the racer is building a fresh engine with new fasteners every time he builds the engine, fine, but I rarely see people ever change fasteners on used rods that were new at one time due to an engine rebuild (Original owner of the rods).

Tom V.

[Lee]

I've built a dozen+ motors using a stretch gauge.

I install all 8 piston/rods, and "snug" the rod nuts as each is installed.

I loosen one nut at a time, place my LONG box-end wrench onto the nut, install the stretch gauge, and tighten it to spec.

Repeat for the other nut, then go to the next rod.

I've been doing it that way for about 10 years now, and have had no bolt related failures.

Lee

[Karch]

If you don't surpass the yield strength of a bolt, you should be ok for multiple pulls, as long as the threads don't suffer.

[Tom Vaught]

Quote:

"If you don't surpass the yield strength of a bolt, you should be ok for multiple pulls, as long as the threads don't suffer."

Agree on the threads "suffering"

Typically the "work" is done by the first few threads on the bolt or nut. Distortion occurs. Multiple use beyond a certain number of torques with cause the location of the highest force on the threads to move. Not a predictable deal vs changing fasteners on occasion. JMO

Tom V.

[RAIV55]

This is a how I do it reply. First off I engrave the bolt heads for ID. The marking i use are for example, 1I, 1O, 2I, 2O ect. With the bolts at free length I measure all and establish the shortest bolt. On my bolt chart in the rebuild sheet are places for each measurement. I give a true dimension for the shortest bolt, all others are +.0013, +.0057 or whatever the measurement is. This record keeping is for determining when bolt replacement is due in the future, and a quick reference when checking.

The preceedure I use when assembling the engine is to install all rods and snug them with a speedhandle or ratchet. After assembly, loosen each rod bolt and pull down until the required stretch is met. It isn't unusual for the required torque to stretch being quite a bit higher than published torque specs.

If you are going to assemble an engine using the stretch method, make sure all machine was was done with the fasteners tightened in the same manner as you are going to assemble it.