re-torque LO206 head bolts

I had the head on an LO206 done over the winter by the local kart shop. How often should the head bolts be re-rotorqued? When they get retorqued do I just the the wrench to the low end of the range and try to tighten it? Or do the head bolts need to be backed out a partial turn first?

It does seem like a fair amount of stuff needs to be taken off the engine to expose the head bolts ...
 

Outrider

Member
For checking or re-torqueing it is neither necessary or desirable to first loosen the head bolts any before applying the chosen torque with your torque wrench. We would choose a torque no lower than the middle of the specified range IF you can't find out from the local kart shop what they use for an assembly torque.

Fasteners that have been tightened with a torque wrench using a proper procedure (more below on that) still suffer from a phenomenon known as short term relaxation (5 to 20 minutes after final pass), and depending on a number of variables, that can be considerable (5% on a really good day, often more, using the correct procedure), plus the long term relaxation due to vibration and other forces in service, so don't be surprised if your chosen torque results in slight rotation of any or all the head bolts as you check the torque.

Now, proper procedure for actual assembly, absent specific instructions from a tech manual. First, decide how many passes you wish to use, 3 or 4 (one third of the total torque on each pass for 3, one quarter of the total torque for 4 passes). Apply the torque for each pass in the pattern specified for the head. The reason for multiple passes is that if you tightened bolt #1 to the total torque in one pass, you would apply a severe prying load to that fastener when you tightened the second fastener, which is on the opposite side from the first. Wait a few seconds between each pass, then go on to the next pass, until you have completed all 3 or 4 passes using the specified pattern. Now comes a useful little trick; having achieved full torque on the final pass, using the specified pattern, forget the pattern and go around the bolt circle clockwise or counter clockwise, mechanic's choice, applying the final torque again in the check pass; start by applying the torque to the #1 fastener and finish by applying the torque to the #1 fastener again. While it is unlikely on an OHV Briggs head, doing the check pass around the installation clockwise or counter clockwise assures that if you missed a fastener on the last pass, it gets caught. Another thing that helps to minimize short term relaxation is to walk away from the installation 5 or 10 minutes before applying the check pass; it may be necessary to apply the check pass 2 or 3 times before rotation ceases, or maybe only once.

That's the short version of the book.
 

Jimbo

Premium User
The torque used to tighten a bolt is consumed by overcoming the friction between nut and washer (about 60%), overcoming the friction between bolt thread and nut threads (about 30%), and providing energy to elongate the bolt (about 10%).

Newton's First Law of Motion states that a body at rest will remain at rest unless an outside force acts on it.
Once the fastener is in a state of rest (not turning) you can see that a very large % of the energy needed to start the fastener moving again is consumed by friction. So you may try tightening a fastener that has taken a set and you may even exceed the specification before it breaks loose. Experience tell us that the longer the fastener is allowed to stay at rest the more energy will be required to break it loose. A rusted bolts is an extreme example.
A good way to determine if a head bolt has lost it's clamping load is to put a paint mark on the bolt and another one on the head. Loosen the bolt 1/4 turn and then re torque it to the original value. If the paint marks line up again things were good. If the paint mark on the bolt goes past the one on the head the bolt had lost clamping power for some reason.
Try that on all 4 head bolts of a LO 206 that has been run for season.
 

flattop1

Dawg 89
typical error rates. when tightening fasteners.


 Operator "Feel" +/- 35% Torque

 Wrench +/- 25%

 Angle Torquing (Turn of nut) +/- 15%

 Load Indicating Washer +/- 10%

 Measuring Bolt elongation +/- 5%

 Hydraulic Bolt pretension +/- (1% to 10%)

 Strain Gauges / Ultrasonics +/- 1%

 Superbolt multi-jackbolt tensioners +/- 5%
 

Outrider

Member
You've done the homework well, flattop1, although research projects I funded for my employer (about a quarter of a million dollars worth over a 10 year period) indicate that the number for use of a torque wrench is often a
bit higher than that and the number for angular turn error is usually a bit lower than those numbers (but not much, in either case). We always expressed ours as the % difference between the highest and the lowest preload in the sample. Angular turn is definitely the most accurate way to establish consistent preload around a bolt circle for the average person IF you have a source for the correct angle for the application (we had a couple of methods available), and ultasonics, if you can afford the equipment, are definitely the way with the best accuracy, but usually cost prohibitive outside of the major production/repair environment because of the price of the equipment, including the necessary shop floor calibration standards.

We had a lot of fun working with Superbolt over the years, both before and after we retired and became a consultant. It a marvelous solution for large fasteners that would otherwise require huge (by engine building standards) torques - we had joints that were in the 1200ft/lb to 4000+ft/lb range for normal torquing, so Superbolt was a big improvement for both new construction and life cycle maintenance.

Jimbo, I have no argument with anything you said, although my comments in my original post assumes that the situation has nothing to do with rusted fasteners. Our experience has been that between the effects of short term relaxation and long term (in service) relaxation, the fastener would move with application of a torque toward the high end of the range specified for the joint. Certainly, if one wishes to loosen the fastener by backing off a quarter turn, there is nothing wrong with adding that step. We will say that marking the joint as you proposed gives an indication that something has changed if they don't line up after retorque, but note that friction coefficient between the threads of the head bolt and the head improves (drops) with each loosening and tightening (you don't want to know how much it cost to find that out), so often what you see with that test will be both an indication of preload loss and reduced friction; that doesn't mean that test isn't proper and useful, just that what is going on can be influenced by more than preload loss due to short term and long term relaxation.

Your emphasis on friction between the threads of the bolt and the head and the bearing surface friction between
the underside of the bolt and the head hits on the most important aspect of establishing preload as consistent as possible among the fasteners in any bolt pattern, not just a cylinder head. Some people get fanatical about buying the most accurate torque wrench they can, spending more than they can afford; it's money well spent, but given the big fastener to faster variation in preload when torquing a joint, they need to know that the portion attributable to torque wrench accuracy is down in the weeds, essentially something they can ignore. Friction coefficient variation from fastener to fastener is the big culprit. On the best day of an experienced technicians life, the difference in preload between the highest preload and the lowest preload in the fasteners in a joint, all tightened to the same torque by the same mechanic using the same wrench, is 33% or worse, 50% variation is not uncommon, and if you throw in a shift change, so you have two mechanics and maybe also 2 wrenches on a joint with 30-50 or more bolts and preload variation can get even higher. With an inexperienced tech who didn't follow the proper tightening procedure closely, we have measured 100% preload variation ( the highest preload being twice the lowest preload) around the bolt circle. But far and away the biggest contributor to preload variation is fastener to fastener friction coefficient variation.

To give a specific example, among many tests we conducted, we did two different friction coefficient tests with alloy steel bolts and within any individual test sequence, all the bolts came from the same manufacturer's lot. The thing is, friction coefficient of fasteners is dependent on fastener surface finish at a level that can take anywhere from 10x to 100x magnification to detect. So we tested fasteners from one specific manufacturer's lot unlubricated and established a high, a low, and a mean friction for unlubed alloy steel fasteners from 1/4 to 1.5 inches. Then during another series of tests we wanted to test in the same size range lubed with motor oil. From a friction coefficient standpoint only, motor oil is not a great lube at the thread psi loads encountered when a joint is fully torqued (the pressure additives in motor oil aren't the type necessary for thread lubrication) but even we were surprised by the results. The fasteners used in the motor oil test were from a different manufacturer's lot than the unlubed test, and while they looked the same to the naked eye, the mean friction coefficient developed for THAT BATCH of alloy steel fasteners lubed with motor oil was actually slightly HIGHER than in our previous test of unlubed fasteners.

So Jimbo's emphasis on friction cannot be overstated - it is the single biggest factor by far in determing actual preload with application of a specific torque.

Still didn't write a book, but this is the start of a chapter, lol.
 
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