connecting rod bolts with loctite? 5

[torks]

hello,
I have point I am trying to make to a friend , but I'd like to be a litle more sure of my answer than just , my opinion and the way I do things, and have a knowledgable answer to back it up.

A friend who happens to be a heavy equipment mechanic, claims that in every engine he's ever built, he uses blue loctite on the connecting rod bolts.

I not only don't do this, but am of the mind that this could really give a false torque reding , even hinder the ability to get proper torgue with a stretch gauge if he is using one.

I was taught to use oil or even the moly type lube (I think) that ships with quality aftermarket bolts such as ARP, etc., or you can't get the proper tension on the bolts.

Can anyone tell me if I am correct here, and explain so that I might inform this person why this is not a good thing? (if I am am correct that is)

Thanks

 

[Majik]

http://www.chevyhiperformance.com/techarticles/49258_rod_bolt_torquing_stretch_info/

4th result in google for "arp rod bolt torque" the other 3 from arp also contain information.

In the case of your friend old habits die hard and I'm sure some where that will be atributed to another smokey yunick secret trick. The locktite should not hinder the bolts ability to stretch. Should hinder the ability to torque it with a torque wrench alone.

 

[MikeHalloran]

Loctite can't affect the stretch gauge. It could affect the torque required to get a given stretch, but so what?

I, personally, would use oil on rod bolts only with French locks.

Mostly, like your friend, I use Loctite of some kind. In recent decades, I've been using Loctite PST, which feels better in the torque wrench than blue Loctite.





Mike Halloran
Pembroke Pines, FL, USA

 

[patprimmer]

With aftermarket rods or bolts, I follow the manufacturers recommendation.

If no recommendations are available, or on engines with stock rods I use Loctite instead of oil on the threads, but a drop of oil under the head of the bolt.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[strokersix]

If the torque to achieve a given stretch is greater with loctite then won't the residual torsional deflection in the fastener also be higher? Seems to me that even when using a stretch gage you still will get more consistent fastener tension with lower thread friction.

 

[EngJW]

Mike, what is a French lock?

 

[MikeHalloran]

A French lock is a piece of sheet metal that positively locks a bolt head or nut. It usually comprises a hole that fits around the fastener shank, and two ears that are bent up and down respectively, so the fastener flats are constrained from rotating relative to the substrate. Sort of like a Stage 8, but with fewer parts.



Mike Halloran
Pembroke Pines, FL, USA

 

[EngJW]

Seen them used all over, but never heard them called French locks before!

 

[Metalguy]

>"I was taught to use oil or even the moly type lube (I think) that ships with quality aftermarket bolts such as ARP, etc., or you can't get the proper tension on the bolts."<

I'd use a GOOD moly paste with a lot of moly, not a moly chassis lube. ARP moly or Dow Corning GN paste is fine. It is the PRELOAD and only the preload that prevents fatigue cycles and fractures on something like a rod bolt.

Loctite is good for non-rigid connections, like soft gasketed joints. I use it between the rod/main bearings and the rod/block to help avoid spun bearings--along with the bearing crush on the high side.

Naturally if a journal bearing is starved for oil it can seize and spin no matter what you do.


"I'm that dog who saw a rainbow, only none of the other dogs believed me." from "Kate and Leopold"

 

[patprimmer]

If the person who specified the correct tension specifies oil, use oil. If they specify "XYZ" lube, use XYZ lube. A change of lube changes the stretch for the torque applied, so it is important to replicate the friction as used for development of the spec.

With no spec, oil is the most likely the presumed method. Loctite still works as a reasonable lube until it starts to set.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[evelrod]

I guess my two cents---I seldom use a chem lock or safety wire on any fastener system that has a specified torque. Perhaps I'm a bit anal in that respect, but I use only the ARP moly lube on all my bottom end and cyl head bolts and studs (all are ARP these days with the one exception, my Carillos use SPS). I have used 'red' on my crank pully bolt in place of the 'French lock", but generally confine Loctite and safety wire use to internal and external brackets, hangers, motor mounts, etc.
I have used the 'stretch' of my rod bolts as a guage to determine what the 'torque' should be, it can vary from specs a bit, but I have not needed the info for quick repairs in the field lately (knock wood). Rod bolt stretch is still my main method, oddly (or perhaps not), the ARP and SPS specify the same .006" in my case (two totally different engines). Maybe I'm a little old fashioned, but if I have fasteners that tene to loosen frequently I generally search out some alternative to "just using Loctite". I tend to use it, but only as a last resort (which does happen). On the other hand, I use it extensively on all my chassis and suspension bolts and nuts, along with safety wire when applicable. I use AN flat washers only, no split lock washers...hate them thangs!

Rod

 

[WilliamH]

I agree with Metalguy. I am assuming the subject rod bolt is automotive and not a lawn mower.

Friction keeps the bolt or nut tight and not a thread locking compound. Friction on threads and nut/bolt base is key to non-rotation during engine operation. If sufficient stretch is applied to the fastener so that the assembly never "feels" the loading which is proper design, then the fastener will not "un-screw".

As far a these lock plate devices on any high load fastener--------bad idea. In order for the lock tab to bend it needs to be ductile----soft. If soft and used under a rod bolt or nut, it will compress over time and the bolt stretch will diminish. BANG/CRASH/BOOOOM!

 

[JWaterstreet]

I'm not sure I agree with Metalguy about Moly and especially not about the use of Loctite on bearing shells. There are two factors at work here, the preload or "stretch" of the fastener, and the friction of the nut/thread interface to keep it at that spot. Ideally, you want to get the fastener to the proper stretch, and then have a high degree of friction at nut/thread interface to keep it there at that spot. It also depends upon the loading factors of the fastener as well as the strength of the fastener. Moly or other high shear strength lubes could cause you to overtorque/stretch AND cause you to lose friction of the nut/thread interface, leading to fastener failure or loosening.

I agree with Patprimmer; if the spec says oil, use oil, if not, go dry. To me, loctite is OK, and won't hurt anything, so why not use it?

I think loctite on the shells is bad news, as it could affect heat transfer between shell and rod/block.

Jeff

 

[MikeHalloran]

I suspect that loctite behind bearing shells could affect the running clearance. If Plastigage can be believed, a film of oil behind the bearing shell reduces the clearance.



Mike Halloran
Pembroke Pines, FL, USA

 

[Majik]

Yes it will but as long as you machine for it... it won't be a problem.. although the bearing's crush will keep it held in there better than locktite. Most people don't realize the tab's are just for locating and not for holding the bearing from spinning. It's the surface area of the bearing and the pressure holding the halfs in that generates the resistance to spinning. If you have a rod that has inadiquate clearances to where it might spin you have other problems than trying to use locktite to "prevent the spinning". bandaid on a bullet wound. "running the bearing crush on the high side". What are you doing when you do that? Machining the housing diameter small and then honing out a babited bearing to size? Still don't see how that helps anything because if it was set with proper clearance then it should just be riding on the wedge. Anyone want to try and figure out the drag caused by an avg journal and see if we can then turn that into a force onto the bearings in the housing to see how much force we're really talking about here.

 

[strokersix]

Am I totally off base with my concern for residual torsional strain in the fastener? I recall reading that torsional strain relieves itself in service, further tightening if threads slip or loosening if nut face slips. Either way should result in less consistent preload in service. Did I just dream this or is it real?

 

[Tmoose]

Once upon a time I knew some folks that put a drop of loctite (unknown/forgotten grade) behind the inserts in some "race" engine. AIR the clearance decreased , maybe 0.0005 inch diametral.

With steel rods I'd first take pains (and measurements) to get the bores round and straight and repeatable, the right size and surface finish (finish for HD use used to be in the TRW and Sealed Power bearing books, and the Chevy power book), and forbid the balance shop from reducing the "footprint" at the parting line or any area that reduced Big end stiffness.

I never did the experiments, but I'd kind of expect heat transfer from shell to rod thru loctite might just be better than thru all that "air" in a typical interference joint anyway.

Oil flow is a powerful tool to cool bearings of many types. Clearance has a direct influence on oil flow thru a journal bearing.

http://www.chevyhiperformance.com/techarticles/4380_bearing_clearance_info/photo_02.html

 

[BXRStang]

With steel rods I'd first take pains (and measurements) to get the bores round and straight and repeatable, the right size and surface finish (finish for HD use used to be in the TRW and Sealed Power bearing books, and the Chevy power book), and forbid the balance shop from reducing the "footprint" at the parting line or any area that reduced Big end stiffness.

Not wanting to start a fight, but I have read and have practiced sizing my con rods at the parting line 0.0005"- 0.0007" larger than my vertical measurements. The theory being at high rpm/load the rods stretch vertically and tighten the clearances at the parting lines. Have I been missinformed?

By "reducing the footprint at the parting line" do you mean removing material from the outside of the rod where the bolts pass through?

Carl

 

[patprimmer]

The correct bearings for your intended use will take care of rod distortion, so don't double up on it.

Footprint is the total contact area at the parting line.

Regards

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