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Torquing rod bolts - drama, yet again

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Joined: 29 Jul 2001
Posts: 8328
Location: Royal Oak, MI, USA

PostPosted: Sun Sep 15, 2019 10:57 pm    Post subject: Torquing rod bolts - drama, yet again Reply with quote

Friday the 13th hit me pretty good this week.

Was in the process of reassembly of the motor for the racecar (#77 924 NA), which got new pistons and some minor refresh machining.

Decided to use my newer Craftsman digital click torque wrench for improved accuracy (vs the old analog click one I've had for 20 years now). The result I got was exactly the opposite.

After doing 3 rods OK, the bolts on the 4th rod wouldn't take the torque, just kept turning. A check with my ARP rod bolt stretch gauge (measures the length of the bolts before and after install) verified that one was starting to stretch, and the other was starting to strip.

These were stock rod bolts, not ARP, FYI. Next time...

After swapping out for another good pair of rod bolts, salvaged from spare rods kicking around, I threw the rod in a vise, no bearing, to torque the bolts to spec and verify that they wouldn't stretch, before going to the trouble of installing.

I also use a digital torque readout (installs like a socket extension, provides a peak torque readout) inline between the socket and the digital torque wrench.

I was shocked to see the torque wrench was SO far off calibration that it was giving at least 10 ft-lbs over the reading! No wonder the bolts failed.

I had set it to 50 based on previous experience... stock torque is 45, but because racecar... so I was getting at least 60 ft-lbs, sometimes more, and poor consistency.

I switched back to my trusty old analog click wrench, set it to 50, and got a consistent 45. Bumped it to 55 indicated, now getting the 50 ft-lbs I wanted.

I then un-torqued all the rods and mains in the engine, and re-torqued them using the analog wrench with the digital indicator just to be sure.

On with reassembly...

Next off-season task: get all my torque wrenches professionally calibrated!!!

This is the device I was using to check torque settings:

Note that it also has a "Set" mode, which can be used in place of a torque wrench to make noise when a desired torque value is reached. If it looks to be solid, I may have to go get the more expensive one that goes up to 250 ft-lbs (for some of the bigger fasteners on my other racecar)...
Vaughan Scott
'79 924 #77 ITB racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype
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Joined: 29 Sep 2013
Posts: 2008
Location: MI

PostPosted: Mon Sep 16, 2019 12:07 am    Post subject: Reply with quote

Craftsman ain't what it used to be.. A lot of tools ain't what they used to be..
80 Turbo - Slightly Modified
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Joined: 04 Apr 2010
Posts: 755
Location: Central-ish Arizona, USA

PostPosted: Mon Sep 16, 2019 1:06 am    Post subject: Reply with quote

I have 4 torque wrenches, two of them being Snap-On "click" types, (1 high ft/lb and 1 little inch/lb), a newer Craftsman click type low scale ft/lb and my ancient Craftsman beam style ft/lb, (Anyone remember even seeing one of those, haha).
I trust the newer Craftsman the least. I bought a few years ago primarily to properly torque oil pan drain plugs and oil filter canisters as I'm a believer that a torque wrench isn't as accurate at the extremes of their scales and this one fit the bill for what I wanted. Unfortunately I trust it the least as it "feels" like I'm always over-torqueing the fastener.
The Snap-on wrenches have been very reliable but the Craftsman beam is still my favorite even though it's not always the easiest to use. It just "feels" right and there's that connectivity between the "feel" and "eyeing" the deflection in the beam when I'm tightening the fastener.
'95 BMW 318i/5 ..."Pearl"
'80 Porsche 924 Turbo..."Pogo"
'81 Porsche 924 Turbo...Parting!
'87 Porsche 944 NA... "Liebchen"
'02 Porsche Boxster..."Sunbeam"

Still on the Prowl!
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