[jonathanks]

I have been looking through the site and other places and really haven't found a solid answer to this question, so here goes.

I am planning on reusing my bolts throughout my new sbc 355, the original build was advertised at 140 HP and I know the entire history of the engine back to '73. So I know this is the first time the bolts have been touched. My machine shop and performance parts store (Sure some of you have heard of Dick Harrels on the drag strip...) recommend using the ARP moly grease. But because it is a higher performance lube than the 30 wt that the GM engineers set the torque values to, I am told to knock the torque values down 10 ft-lbs across the board. This does not entirely make sense to me, because there are vast ranges of torques for the various components...

So I log into the ARP web site and see what they have to say. I look up what seems to be an appropriate strength spec and compare the 30 wt to ARP moly grease torque values. They difer by a relatively consistent 33%. So I look up the fatory torque spec and set the wrench to the new value. 54 ft-lbs compared to the spec 70 ft-lbs. Put all my plastigage down just to check for good measure, but 54 pounds seems awful loose... And my journal clearances all appear to be at the upper limits.

So after all this, what can you all tell me about adjusting torque values based on the lubricant used?

(And yes I will consider buying the new bolts, but for now the old ones stay...)

[ianhealey]

if it sounds to good to be true it probably is
if it were my engine i'd doit by the book settings
the factory spend lots of dollars on R/D
and some jazzy grease anit going to make those bolts
screw in much further be sides if the grease is as good as they tell you it is whats to say it wont make your bolts come lose or id them to wind back out do them up properly

[k-star]

What holds a bolt in place is stretch.... Think of the bolt as a spring, as you tighten it you stretch the spring.
The kind of lubricant that you put on any bolt will effect how much torque it takes to get the proper stretch. As your turning that bolt in you have friction areas in the threads and where the bolt head contacts what ever you bolting together. So if you have dirty threads,dry threads, moly,oil ect this will all change the amount of torque needed to pull a bolt up to a set amount of stretch.

I just spent about 10 hours doing some test's on rod bolts in my shop. You can take the same bolt and get totally different stretch depending on what kind of lube you put on the threads and under the head of the nut.When it come to rod bolts the only way to get the proper stretch is with a stretch gage. If you are building a stock or close to stock motor you will be fine just using the factory spec's on torque on factor bolts (i believe all the manuals recommend oil on the threads)

Try this simple test. Take a block of steel drill a hole through it and put a bolt and nut together thru it dry. Take a set of mic's and check the length of the bolt. Now start torquing the bolt in like 10 ft/lb increments. Check the length after every pull. Take it apart and put some molly lube on it and try again. Record the lengths per ft/lb dry and with molly you'll be surprised!!!! If you really want to get wild try it with different grade bolts...

keith

[woodz428]

I use ARP hardware and do agree with the lube comments mentioned, there is a difference. I would, however, use stock torques if using stock hardware.

[engineguy]

There is no advantage at all in using a moly (or any other "slippery stuff") on fasteners. Best procedure is to use 30# motor oil and make sure that the threads are absolutely clean. For cleaning the threads, use only a thread chasing set. Do not use a tap and die set to clean the threads as these will actually remove metal from the threads. The amount of metal removed by tap & die set is very minute, but it will affect the clamping force of the fastener.

[willys36@aol.com]

Also, you mentioned that you are using bearing clearance as an indicator of bolt tension. DON'T! Clearance has NOTHING to do with bolt tension, rather it is solely a function of the machining process. Joints are a necessary evil to allow assembly of parts in machinery but engineers strive to create bolted joints that come as close to a solid structure as possible. Bolt size, spacing, and pre-stress (stretch or torque) are all designed so no single component is overstressed by normal operating loads, plus a healthy safety factor. The engineer determines the tensile, compressive, and shear loads on a joint then sizes the bolt material, diameter, and tension so the bolt takes all of the loads, protecting the joint yet the bolt itself is not overloaded.

[jonathanks]

I am not using the journal clearance as a guage of bolt stretch. However, if a 33% reduction in torques if not appropriate, meaning significantly off, will make a differences in structure deformation. The static load that I am putting on the journals for a gage test will not be affected very much from a few lbs difference in pressure exerted on them. Unless it is too low to bring the caps into full design deformation.

Maybe I just need to revert back to using 30 wt with the oem hardware, but it would be nice to know if there are any definitive rules for changing lubricants.

Rod bolt stretch can be measured, but cap bolts are another story. The higher performance lubes have definite advantages, but only if they are matched with the corresponding torque values.

At this point the engineer in me is probably just rearing its ugly head. It seems that there has to be a definitive way to translate torque values based of the kinetic friction coeeficients of the various metals and lubricating agents.

Thanks to all who have thrown in their two cents so far!

[willys36@aol.com]

There is no deformation anticipated in cap design, especially since it depends on very imprecise torque specification which would result in indeterminate bearing clearances which is unacceptable. Torque is used as the bolt tensioning standard simply because it is easiest for the average technician to measure. As you are finding out it is a pretty bad way to do precision assemble. It is close enough for government work but for very high demand applications, there is no substitute for measuring bolt stretch which is the base design parameter. Torque is simply an easily measured secondary indicator. For highly stressed industrial applications technicians use instrumentation to measure bolt stretch only and aren't concerned with the torque level. Chilton and other assembly manuals specify torque levels that assume a certain thread condition and lubrication that yield a known range of bolt stretch. Change the lube or thread condition and the torque needs to be re-calibrated to the desired stretch. You are getting vague answers because there is no 100% way to tell what the effect on bolt stretch without measuring it directly. Best bet is to defer to the bolt or lube suppliers who have hopefully done the lab work to calibrate torque spec to bolt stretch.