Threadlocker and engine assembly

Having just gone through the unpleasant experience of throwing a rod through the crankcase on a well maintained, quiet running and unabused engine (with 150,000 miles on it) has caused me to wonder why the application of thread locker or "Locktite" is not done on rod and main bearing cap nuts. (The Permatex package recommends use on the following engine parts only: "Cylinder block and rocker arm studs...camshaft sprocket bolts, crankshaft bolts, idler bearings..." (?). Does anyone have any insight as to the pros and cons of its use for this purpose? Thanks.
Orin Meyer

[ February 01, 2003: Message edited by: Blue Oval Blood ]
 
I think the reason is twofold:

1) A properly torqued fastener doesn't need threadlocker.

2) Assembly of critical fasteners such as rod and main bearing screws or bolts requires lubrication of the fastener in order to get the most accurate torque reading possible. I would think that the use of threadlocker would interfere with measuring a good torque value.
 
Orin,

Further to Marks comments, I have also been told in the past (by a very experienced engine builder) that the dramatic loading/unloading that the road/main bearing studs/bolts cope with requires as close as attainable to 100% contact area between bolt and thread in order to maintain integrity. Any kind of thread lock reduces this contact area, and hence, actually makes failure more likely. It is also why he suggested using the best quality studs you could buy, being fanatical about thread cleanliness before final assembly, and being very very careful to not knock studs on anything before assembling. It is also why re-using previously torqued studs is a bad idea, because they stretch and hence have non optimal contact patches....

Now, I'm not sure how much of this is over exuberance, or perhaps self delusion, but it seemed to make sense to me at the time....

Rgds

Neil
 
I'd be very interested in seeing some data or studies on this subject. I have worked with Loctite's technical department many times on various (non automotive) applications (both threadlockers and adhesives), and perhaps they can be a resource for such info. Generally, threadlockers help prevent gauling, do add a lubriciuous thread interface for torquing, and of course help in high vibration retention applications. I personally like to use a low retention threadlockers in most frequent dis-assembly/assembly applications where a steel fastener in used in aluminum to help prevent gauling. It really extends the life of the aluminum threaded component.

Andy
 
Tightening bolts to a certain torque is meant to stretch the bolt slightly, not past the point of permanently distorting the bolt, but up to the point the bolt will return to normal length when loosened. Using a bolt, a stud with a nut, using oil on the threads, using a moly lube on the threads, using nothing on the threads, all effect the amount the bolt or stud stretches with a certain torque put on it.

In other words, it takes more torque to stretch a bolt put in dry to give it the same stretch that it has if put in with moly lube or oil on it.

In the case of rod bolts, it is important to stretch the bolt to the manufactures recommendation so the rod cap doesn't actually try to pull away from the rod at high rpm.
The best way to tighten rod bolts is to measure the stretch and pay no attention to the torque required to get that amount of stetch. If properly stretched, the bolts won't come loose.

See ARP's excellent article at
http://www.arp-bolts.com/pages/tech/fastener.html

I made a stretch gauge using a dial indicator when I did my engine. Worked great.

Also see
http://www.arp-bolts.com/media/pdf_files/RB_16-18.pdf

In ARPs old catalog, they say about rod bolts:


"If a bolt is installed without sufficient preload (or pre-stretch), every revolution of the crankshaft will cause a separation between the connecting rod and rod cap. This imposes additional stretch in the bolt. The stretch disappears when the load is removed on each revolution, or cycle. This cycle stretching and relaxing can cause the bolt to fail due to fatigue, just like a paper clip that is bent back and forth by hand. To prevent this condition, the bolt's preload must be greater than the load caused by engine operation."

As for thread lockers, they are excellent and I use them on lots of things, but usually not rod bolts. I've never had one come loose.
 
Hi Guys - Thanks to all for the very informative responses. FYI I made the same inquiry to the Loctite tech contact via their web page and received the following response:

"Orin,

We have no test data on the use of Loctite on rod and main bearing cap nuts.
We'd suggest trying Loctite 242. It will provide a serviceable lock. If you need
something a little stronger, try 262 (removable with heat and force).

Regards,

Cathy"

Just goes to show once again that the members of the GT40 Forum are indeed a very knowledgeable group, but I am somewhat surprised that Loctite has no test data but still recommends the use of their product in this application. I am confused.

Best regards, Orin Meyer
 
I saw a cheap bolt stretch gauge from Lunati on TV last weekend.
Forgot which show it was on.
Either Horsepower TV or 2-Guys Garage.

Anyway they certainly thought it was nifty.

MikeD
 
I would assume that all quoted torque rates are determined using a particular standard. I would also assume that details of this standard would be available to the general public.

Anybody care to remark on what the standards might be or where one might find them?
 
Blue, I feel for you.

I borrowed a road car to do some club racing 2 years ago. During the 5th lap, I heard a loud BANG followed by an enormouse vibration. On investgation, we found half a main bearing cap sitting in the sump. Sitting next to it was one of its retaining bolts... in perfect condition!

On further inspection, we discovered ALL bearing cap bolts to be little more than finger tight! Here is the clincher... this engine was known to have traveled at least 20,000 km since its last re-build
shocked.gif


Oh well... I guess we all have a spare $1000 or so lying around to perform the odd engine rebuild... NOT!

[ February 06, 2003: Message edited by: Chris L ]
 
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