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MG MGA - Cylinder head - Torque ??
|Haynes says 70 ft/lbs. The MGB only 50. There must be a reason. What do the experts think ?|
|Tell us more about your engine.|
|45 ft lb for 3/8 fine threads. 70 would strip i think|
|Roger. I think it best to consult the factory workshop manual rather than an aftermarket manual or the "internet experts". (Who defines what an "expert" is?) My copy of the factory, MGA, workshop manual gives a value of 50 ftlb of torque for the cylinder head nuts. Just below it, it gives a value of 70 ftlb for the main bearing nuts. This is on page "General Data 3". |
|50 sounds correct to me. I wouldn't go over 60 or under 45.|
|Yes 50 it is in my original workshop manual. I think I might have found the cause of the confusion. I have a 1968 third part party manual by 'Autopress' and that quotes 50 for pushrod MGAs and MGBs BUT it quotes 70 for Twin Cams.|
By the way there are examples of wrong information in the official workshop manuals so beware. These are probably caused by the fact they would have been written almost theoretically for the launch of the car. Who has ever removed transmission tunnel and floor and then lift out with gearbox!
|She's a standard 1500 bored out to 1588 (apparently). The head lifted under pressure (55 uphill) so now she has a new gasket. And 50 ft/lbs it is.|
Very many thanks for your prompt help.
... and Les, it just goes to prove that you can't manage without a factory manual. Thanks again
What Paul says is correct. It is 70 ft/lbs for the mga twin cam head studs and that is where Haynes probably got the incorrect information for the pushrod head studs.
However, the Twin Cam head studs are 7/16" not 3/8" as with the pushrod engine.
|M F Anderson|
|My MGB manual quotes 45 to 50 lb/ft for all 1800 engines. This equates to 61 to 68 NM (Newton Metres).|
|It depends on what studs you have. Unmarked studs can take one torque, studs (IIRC) marked with a "22" or a dot can take a little more. Also, there are bogus repro studs out there. When one of my "new" studs broke before reaching torque, I ended up using the ARP stud set that is available, which included it's own set of torque specifications.|
|Isn't the real issue, how much bolt tension is required to stop the head gasket from blowing, rather than how much the studs will take?|
|Studs stretch, so what the studs will take, and how much torque is needed to allow for that is at issue also.|
Studs at factory spec will "need" a certain torque. Different specs will require different torque. Finding the studs that need the torque that the threads in the block can accommodate is important.
Studs do not "need" a certain torque.
What is needed is a certain tension in the stud to hold the head down.
That tension is a function of thread pitch and diameter and friction on the thread and under the nut.
It has nothing to do with anything else.
|You are both correct. In the end, the object is to keep the head gasket from blowing. All else being equal, more stud tension should make the gasket less likely to blow, so you want as much as the studs will safely take. Therefore, stronger studs that can take more tension are desirable, and if you have a stronger stud, it's counterproductive to use a lower torque spec meant for a weaker stud.|
In an ideal world, we would tighten fasteners until the desired amount of tension is achieved. In the world we actually inhabit, tightening to a specified torque is much easier than measuring the amount of tension in a fastener. It's merely an approximation of what we are trying to achieve, but it's more than close enough for the vast majority of applications.
In some aerospace applications, special calibrated studs are used with precision measuring equipment to measure fastener stretch. I don't recall having personally encountered a job that called for any method but a torque wrench, however.
|Also, it's important to note that while most torque specifications assume clean, dry threads, others will call for a lubricant, and the numbers can't be directly compared to a torque with dry threads. The head stud set I used with my 1500 engine included their own torque specifications and a lubricant for the threads. So, I used the stud manufacturer's specs (and the lube) rather than the specification out of the manual.|
|Absolutely right Del. Friction is the biggest variable by far in limiting the tension achieved.|
In some high pressure vessels they use hydraulic stud tensioners and almost no torque on the nut. This eliminates the friction issue entirely.
|Should standard studs be dry, or lubricated?|
|Dave O'Neill 2|
|I don't recall if the manual specifies, but if it does not, then I would torque them dry. It was touched on lightly earlier, but it's also important to verify the condition of the threads in the block. There is a video on Youtube where John Twist demonstrates cleaning the threads with a tap, and then chamfering them slightly with a countersink as part of his block prep process.|
|An interesting read:|
|It's a very good summary. It points to the hazard of torquing dry, and highlights the effect of the nut area.|
A good example of this is the bolt on the front end of the crank, holding the pulley. It has a large head for the thread size and is often difficult to remove.
This thread was discussed between 02/09/2015 and 05/09/2015
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