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MG MGA - Retighten head bolts?
My motor was rebuilt in Feb this year and I was advised not to retighten the head by 3 knowledgeable MG workshop club members. I am now in Perth (17000km since leaving the Gold Coast on an Around Australia Trip) and the head gasket has blown! There is no water in the oil and it did not overheat. A crack test did not show anything so I am putting the head back on and heading home and hoping for the best! Any ideas what may have caused this problem will be appreciated.
I have just been browsing some of the archive articles and most people appear to retorque the head. Here is one thread: http://www2.mgcars.org.uk/cgi-bin/gen5?runprog=mgbbs&access=&mode=archiveth&subject=6&subjectar=6&thread=200406141134045002
In it I also summised how many of us actually torque to the correct ft lbs. i.e. how accurate are our home torque wrenches? How many of us have had them calibrated since they were purchased?
|Either look up the thread I referenced in Steve's linked thread, or send me an email. I can send you the complete explanation, you could then pass it on to the "knowledgeable" chaps.|
|For here at work I use click type torque wrenches, There is a shop near here that recalibrate them every couple of years. For the cost of recallibration you could buy a beam type. For a home use a beam type torque wrench is a better choice. They are much cheaper and if treated properly should never need calibration.|
|R J Brown|
|I've heard that the "click" or dial-type torque wrenches are accurate (if calibrated) to within +/- 5%, the beam type within +/- 15%. I never knew the beam type could be calibrated.|
|I just purchased a click-type wrench this year. It is totally worth it, even if just for the peace of mind knowing that I'm closer to the actual torgue spec.|
|Mark J Michalak|
I was always told to retorque by backing off 1/4" turn and retorqueing, but in this case I used a composite gasket and that was why "they" said DON'T retorque. Does the gasket type change the technique?
|Dave, if you want to borrow my torque wrench you're welcome to. I have the click/dial type and have recently had it recalibrated.|
|Some gaskets are supposed to not require retorque - they fail anyway, it just takes longer. Evidently the idea is that because people don't do it correctly (or at all), it should be designed out, but by the very nature of how gaskets work, that can't really be done. If you read my post on it, you will see what happens and why.|
|I never used to retorque my heads but now everyone seems to! I blew head gaskets in both of my prewar MG's recently, almost certainly entirely due to not retorquing. After only a few miles (ie couple of "warm-ups" my F-type head nuts came down another half a turn or so. Copper/asbestos gaskets do compress, I will pay much more attention to retorquing in the future!|
|The 'don't retorque' style of gasket was really developed for the engines that are really difficult to retorque - valve gear in the way etc.|
In my view you should always retorque - hot on a cast iron head, cold on an alloy one, and cracking the nut maybe a quarter turn to ensure an accurate retorque setting.
No one does it any differently on the race cars if you don't want to be changing gaskets all the time.....
|I ALWAYS retorque. But... you must loosen a quarter turn before you do each one. The reason why is the nut/head needs to be turning/moving as you're reading the torque and coming up on the required value. If you don't loosen, the friction between the nut and the surface may be more than the required torque just to break it loose.|
|I'd say retorque. If the nuts move, then it was needed, if they don't, it wasn't.|
Only if you have the modern HT bolts designed to stretch, do you not need to do it, as these are designed to be turned so much past "snug", not by torque. (Not MG engines)
|How soon should one re torque after changing the cylinder head gasket? The workshop manual says after 500 miles but it will take quite a time for me to do 500 miles so should I do it sooner than that?|
|>Only if you have the modern HT bolts designed to |
>stretch, do you not need to do it, as these are designed
>to be turned so much past "snug", not by torque. (Not MG
Actually, that is true of all bolts. They are designed for a certain amount of stretch, but in general it is not practical to measure that (although it is done in certain aerospace applications). Tightening to a specified torque value just gives an approximation of that ideal amount of stretch, but it is close enough for most applications. The specification may say whether the threads should be dry or lubricated. In general, torque values are specified for clean, dry threads, but not always. The ARP stud set that I used on my MGA engine came with its own chart, and specified different torque values than the factory manual, and listed a particular thread lubricant which was supplied with the set.
I have retorqued after 800km and they pulled up 1/4 turn. (New studs) Old studs may not have pulled up quite so far? RIP is goiing well!
Thanks, Steve, for the offer - we are at Kalgoorlie on our way to Adelaide at thre moment.
A lovely shot of a beautiful car in a magic spot - Kings Park, Perth?
Del, on torque values, a gentleman in this town who had a career in BMC/Leyland/Rover cars told me that the prescribed torque values per manuals were for oiled threads. He said that that lubricated threads were the industry norm - ie 50s/60s. Most commonly used lubricant being engine oil - then frequently a monograde, something like 30W.
Of course, there is a marked difference in the effect achieved between dry and lubricated threads. This was of interest to me at the time because I was replacing the headgasket on my B. Pondering the question of whether to re-torque or not, I was concerned about the potential differential in re-torquing between the four studs that live under the rocker cover (oiled/lubricated threads - particularly if backed off) and those livingg outside this environment.
|"Dry" threads is a meaningless term. The condition of the metal and any coatings is critical, and most if not all high load threads are coated - the coating is a lubricant. The metallurgy of both threaded parts, and the load bearing surfaces, and any applied lubricant interact with the coatings to produce different results. Any threaded part specified to be tightened "dry" is coated, as no "bare" metal will tolerate the heavy loading imparted by tightening without galling. Torque specs only relate to the exact fastener and other conditions under consideration.|
Most MG head studs appear to be black oxide coated, though some appear to be phosphated, possibly waxed. Not all MG manuals say anything specific about lubrication, but the ones that do, specify engine oil, in accordance with Roger's comment.
David- retorque gain in about 1000 miles or a few months at most, then yearly afterwards. Time and number of heat/cool cycles is more important than mileage.
I hear you are going home via Melbourne. Our local MGCC A register look forward to meeting you - your trip will surpass Barney's run from Chicago to Alaska by a few kilometres - around Australia in an MGA
This thread was discussed between 04/09/2007 and 07/09/2007
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