MG MGB and MGB GT Tech Talk
After the initial run of the engine after rebuild should the retorquing of the engine be done hot or cold? What about later retorques at 100 mi and 500 mi - hot or cold?
Is it best to back off each bolt a little (~1/4 turn) before torqueing to spec.
I don't know about torquing hot, I aways do it cold. The torquing sequence is important. I do the head gaskets by first torquing everything first to 25 then going up to the final value. The 1/4 turn works good too once you get everything down evenly.
-David
I don't back off before retorque on a new build. I've seen John Twist do this on an older engine to break the tension on the nut in the hope, I suppose, of not snapping a stud.
If you used the proper lubricants on the bolts I don't think it is necessary to back them off before retorquing. As Peter said, it may be different on an old engine (where the bolts may be stuck).
In order to obtain the correct torque you should always back off the nut/bolt. Running torque that is the torque required to keep the nut/bolt moving is less than the torque required to get it moving. If you get a bolt and set it at a certain torque by bringing it up slowly without stopping then stop, the torque required to get it moving a gain will be higher. This doesn't matter if its an old fastener or new.
Russ
I always re-torque cold. And I do back up the nut a little, probably about 1/8th turn rather than 1/4.
As stated, sequence is important.
I have never , and don't ever intend to loosen a bolt before retorquing.If you do that you would ned to go through the whole cycle again (25% 50% 75% 100% 100% ) otherwise you are going to max torque in one shot. Not good for bolts or studs.
You do not release enough torque in 1/8th turn to relieve even 10% of the max torque. In fact, the opposite is true - you may overtorque the nut in simply overcoming the static friction between the nut/washer/head. If you use a "click" type wrench, it will reach the torque setting BEFORE the nut moves. In which case you have not retorqued it at all! Lubricant between the nut and the washer/head will not help - it all gets squeezed out!
In 40 years of building LBC engines I have never had a head gasket problem using this technique.
Incidentally - on high pressure steam turbines, where they use an ultrasonic device to measure the stretch of the stud to measure the correct torque - they also loosen the nut slightly before re-torquing.
FYI I have never had a head gasket fail either,in 37 years of building a variety of high performance motors.
Limey Wrote:
I'm not going to dispute any other methods, just tell you what I've done for years on many, many engines and never had any problems with. First off with the retorque and valve adjustment I don't normally don't it after the running of the motor, maybe the 2 or third heat cycle of the motor, I don't think it really matters if it's the first, third or fouth heat cycle, just as long as you do it. Secondly, I never backed off a fastner to do a retorque, never, ever been a problem, I was always worried more about compromising the seal when backing off a fastner. Oh and I always done this on a warm motor. From the sounds of it, it appears more than one method has proven positive results for many.
Limey typed "Incidentally - on high pressure steam turbines, where they use an ultrasonic device to measure the stretch of the stud to measure the correct torque - they also loosen the nut slightly before re-torquing."
on nuclear weapons systems in (Air Force) this would be a critical error resulting in the decertification of the technician and the weapon, of course steam engines probably have closer tolerances than nukes (lol)
No need to be a smartass Eric , with your little good for you comment.
I agree - same to you!
Oh - and I dont think nuclear weapons go through the heat cycles that a steam turbine, or an auto engine does! Not exactly a "similar" situation is it?
well limey I'll just quote Hap " Secondly, I never backed off a fastner to do a retorque, never, ever been a problem "
No need to be a smartass Keith.
Hap and I do a lot of things differently - as do others. My objection to your comment was that you claimed yours is "the right" way. There is evidence to the contrary, and also evidence that many ways work. I have based my method on what was taught me by a BMC trained mechanic many years ago, and my experience with torque measuring techniques on steam turbines, aircraft engines, and automobile gearboxes using both "bolt stretch" measuring devices, and instrumented torque wrenches. It works for me, just as, I am sure, your method works for you.
Thanks for all the input.
Torque spec is given as a range. I don't have my book here but I believe it is 45 to 50 lb-ft?. Do you set to the higher or lower number?
Can we all agree that the rate of friction between fasteners and the parts being fastened is different in most every circumstance? The only way to measure the true effect of torque when friction is involved is to measure bolt stretch, which apparently none of us do. That makes every other method a slight compromise, all of which are most likely suitable for our situations. Let's face it - how many of us have our torque wrenches calibrated regularely anyway? Are they within 5% of being perfect? Neither is the torquing method we use!
I think using a good gasket makes more difference than being +/- 5 Tq. ft. anyway!
Limey Wrote:
Okay - I'll throw in my two bits.
Modern engines don't use torque specs. Why? Because they don't actually work well. It doesn't matter if you back off, don't back off or use lubricants on the bolts. Modern engines use "torque to yield" head bolts that actually stretch. The "torque" is done in degrees of rotation after the nuts/bolts touch. It is far more accurate and compensates for the compression of the head gasket. However - the bolts used for this style of torquing are special - you can't use this technique on older engines.
So, no matter which method you use there are going to be inaccuracies. Some problems are fixed by backing off, others are created. I think it is as important, or more important, to have all the bolts torqued the same than to hit the exactly correct value. Also, if you use ARP fasteners, you can probably err a little on the high side without any risk of the fasteners failing.
On newly assembled bolts (with lubricant) the stiction will be fairly low. On old head bolts - well, you wouldn't be messing with it unless it was already leaking so it doesn't much matter what you do! :)
