The Importance of Measuring Connecting Rod Bolt Stretch vs Torque

I had a bit of a slow day today between some family stuff, and waiting on a couple pieces to finish a project, so I found myself semi bored, with a brand new bolt stretch gauge in hand and some data that wasn't making much sense.

To give some back story, I have always torqued rod bolts down via the manufacturers recommendation, in this particular case, Manley would call for 55-65 ft-lbs on an ARP 2000 bolt, which should net you somewhere within their desired bolt stretch of .0058"-.0062". Without a mildly expensive bolt stretch gauge though, you're doing no more than pissing into the breeze hoping nothing sprays back on you. It's a fairly safe gamble, as with these rods, you could say we know the direction of the wind, and chances are we're plenty clear with 60 ft-lbs, which is what I had been using for years with plenty of success. All was well and good until I came across the assembly of this 2.1L Maruha stroker crank for a customer. Bolts appeared as standard 3/8" 1.500 UHL ARP 2000's that you'd find in any Manley rod, and they were, but they were in a Maruha branded rod (that looks eerily identical to a Manley rod, but that's not important here). For once in my life, I was reading the instructions that came with this fairly bespoke and quite expensive crank/pistons/rods package, as it differed from my usual Supertech Pistons, Manley Rods, and OEM crank combination. Better safe than sorry. As I was reading the instructions, it called for a "Recommended Stretch Numerical Value", aka a bolt stretch, with a suggested torque that should put you in that range. The torque suggested was 52-55nm, which works out to 38.35 - 40.57 ft-lbs. Much less than the identical ARP 2000's that come with the Manley rods. This had me curious, so I overnighted an ARP bolt stretch gauge (ARP 100-9942) with the intent to do some digging. I couldn't risk putting this engine together incorrectly, and I was at odds with who to believe.

So for starters, in the hopes that it was the easiest path, I zero'd the gauge, torqued the rod bolts down in the engine to 40 ft-lbs/55nm, and went to see if magically these bolts would stretch more than the ARP's in the Manley's that require at minimum 50% more torque. To nobodies surprise, they didn't. At 40 ft-lbs, I measured 0.0035", 0.0025" under the Maruha recommended 0.006" lower limit. At this point, I decided to set up a bit of an experiment to determine the following:

1. Does the type of lubrication (30W Oil vs ARP Ultra Torque (ARP 100-9911)) matter when torquing/stretching bolts?
2. What stretch is related to what torque value for these 3/8 ARP 2000's?
3. Does rod bolt torque effect the diameter of the big end/rod bearing clearance?

Test Notes:

• Used a brand new set of Manley H-Beam Rods (MAN14011-4) as to not start with potentially pre-stretched or yielded hardware.
• Before increasing torque value during the test, each bolt was re-loosened and torqued in one smooth pull.
• Rods were held in a rod vice around the rod bearing end.
• My torque wrench isn’t professionally calibrated, I just used it out of the box, so YMMV and don’t take my stretch vs torque values as gospel, only a guide that you should verify with your own torque wrench and bolt stretch gauge.

1. Does the type of lubrication matter? Oil vs ARP Ultra-Torque: If you go by Manley’s instructions, you are only supposed to use 30W oil on the threads and nothing else. They give a target bolt stretch of .0058”-.0062” for a torque range of 55-65 ft-lbs. I did a control set of bolts using just 30W oil (see bolts G and H), and even at 65 ft-lbs, I was only able to reach a maximum of .0050” on one bolt, and .0047” on the other. I removed both bolts, applied ARP Ultra-Torque on both the threads and under the head of the bolt, redid the exact same test, and the bolts were able to reach the target and increase stretch by ~.001” or more. The torque wrench/bolt was also able to more freely spin, whereas with 30W oil alone, it had a tendency to “skip” as it got tighter, and wasn’t a very smooth pull. I used ARP Ultra Torque on the other 6 rod bolts and they were all at or just below Manley’s stretch target. Worth noting ARP’s stretch target is .0055-.0060, so by that standard they were in spec when using ARP Ultra Torque. Takeaway here is not to use only oil as it is not as effective. ARP calls out using Ultra Torque on their hardware, and thankfully I have used it every time in the past due to this.

2. Is there a discernable and repeatable correlation between torque and stretch? I think at this point I will use the stretch gauge religiously, but is there a number I can target on my torque wrench that I know should get me to my target window. On my torque wrench, 55-60 ft-lbs was never enough torque to achieve desired strentch during this test. Every bolt needed a minimum of 65 ft-lbs to reach the target. Granted, this could vary wrench to wrench, if mine is out of calibration by a few pounds, yours might be fine at 60 ft-lbs so it’s worth verifying this data on your own. That being said, moving forward, 65 ft-lbs will be my new base starting point for rod bolts, with a verification stretch gauge measurement afterwards.

3. Did I screw myself by setting my bearing clearances with the rod caps torqued to 40 ft-lbs in the rod vice, and now torquing them to 65 ft-lbs? I could pull the entire rotating assembly apart and re-measure, or I could do a standalone test on the bench with a different rod to verify. Rod Bolt Torque vs Rod Bearing Clearance: This one surprised me slightly. I figured as the bolts torqued, there would be *some* force applied to the cap and likely cause some distortion. I started with a new rod at 20 ft-lbs (broke the cap free prior to initial torque just in case it was distorted from previous torque values) and worked my way up in 5# increments. My dial bore gauge was zero’d out at 20 ft-lbs, and never read more than .0003”, and fairly consistently between .0002”-.0003”, which makes me believe it was just noise in my initial zero measurement That being said, I would say there is no meaningful effect on rod bearing clearance across a reasonable torque range.

Long and short of it is, you would probably be ok using a higher end of the torque spec recommended by Manley, but you must use ARP Ultra Torque to get anywhere near the proper stretch. If you're a hobbyist engine builder and you're doing a handful of engines in your lifetime, a bolt stretch gauge is a semi pricey tool to invest in. If you come across something that doesn't seem to make sense, like the Maruha stretch to torque type thing, I would bet that most of your local engine machine shops, if they're worth their salt, have a bolt stretch gauge. You can in theory measure it with a set of micrometers, it's just a bit tricky as the bolt stretch gauge has locating pins to make sure it is repeatable.