Torque wrench spec strips threads

Ross Nicol

GT40s Supporter
I've built or rebuilt a few 302 Ford Windsor engines now (blown up the odd one or two) and being a stickler for doing things by the book, I adhere to torque wrench specifications almost exclusively. Ok so bearing caps, rod bolts and head studs etc I will always tighten using a high quality Torque wrench and spec from a trusted source. The source I use is 'How to build small block Ford engines' by Tom Monroe. Now using this procedure has been ok with one major exception -The inlet manifold, Monroe describes a 2 step procedure and sequence for torquing these bolts 1/ 15 to 17 ftlb 2/ 23 to 25 ftlb Now the first time I tried to use these settings I was fitting my DC&O 8 barrel (stack) inlet manifold to a 302 fitted with Ford Racing alloy heads. Got down to 15 to 17 ftlb step ok but stripped a thread in one of the heads as I tried to go to 23 to 25 ftlb. Had to fit a heli coil to repair the head. Now since that day I have reverted to Ross' feel spec which means going through the sequence a lot of times, tightening in small increments, until Ross feels it is enough. Has worked brilliantly and I've not had any leaks or problems. Fast forward to the latest 302 I bought as a complete engine then realized an inlet manifold change was required. This engine has the original steel heads and my natural instinct told me I should be able to use the Monroe spec on the inlet manifold bolts. Wrong!!!! again I could feel one of the bolts begin to strip. Now my senses were telling me the tension was a bit high and unnecessarily so, but you press on thinking how could the guy have got it so wrong. Well from now on I'll trust my senses on these manifold bolts. I firmly believe now Monroe would have to be referring to steel heads only with this torque spec, and that 15 to 17 ftlb would be plenty enough for them. Ross
 
Correct. Alloy heads are a totaly different spec than the iron lumps. Many aluminum heads end up helicoiled due to "gorilla" torque used on them.
 

Cliff Beer

Supporter
An experienced mechanic should certainly feel free to use the "intuition" torque spec for a bolt.

Torque specs are highly dependent on a number of factors which are typically not considered (or not specified) such as different material (steel v. alloy v. ?) of the bore thread, dry v. lubed application, condition of the bore and bolt thread (easy to change the bolt...), heat cycles, local vibration, etc. Torque specs are far, far, far from static/fixed settings. My experience here is based upon running a company for a few years which produced (among other products) the most precise thread gauges in the world, and the most respected sub-surface inspection tools (magnetic, sonic, IR, multi-mode) for detecting cracks, which happen to most commonly start at a bolt thread. The US Air Force was our biggest customer, and torque specs were a major area of consulting expertise (by our propeller heads, not me).

If you know what you're doing in the garage, go with you instinct every time.
 

Terry Oxandale

Skinny Man
Supporter
When I went to the Alan Root (and later the AFR) heads, I selected longer bolts that ensured I had more threads engaged in the head than the OEM manifold bolt length offered. When I saw how much of the female threads were not being utilized, I started looking for longer bolts and combinations of lock and flat washers to ensure full use of all the threads while avoiding shank interference. I can't remember the difference, but think is was at least 1/2" or more engagement. It did make a big difference in my confidence in not stripping threads. Then of course, there's always the anti-gall compound as well.
 
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Alan Watkins

Lifetime Supporter
Correct. Alloy heads are a totaly different spec than the iron lumps. Many aluminum heads end up helicoiled due to "gorilla" torque used on them.
Sure, but isn't Ross saying that in the second instance he stripped a steel head once he went past 17 ft-lb?

If so, let's look at the fundamentals so we can sanity check Monroe's figures: what was the thread size of the bolt, about how deep was thread engagement into the head and what lubricant was used (if any)?
 
Sure, but isn't Ross saying that in the second instance he stripped a steel head once he went past 17 ft-lb?

If so, let's look at the fundamentals so we can sanity check Monroe's figures: what was the thread size of the bolt, about how deep was thread engagement into the head and what lubricant was used (if any)?
Alan,

Yes, he is talking about the iron heads. however many factors are in play, thread condition, lube, etc. I personally am a lousy engine builder but I learned to "pre-fit" fasteners to assure that the female surfaces are "up to snuff". Most all threads need to be chased or cleaned after disassembly/tanking/machine work, etc. If the fastener will not thread in by finger wrench there is likely a problem.

And I have seen all too often a set of fasteners purchased for "XYZ" Speed Supply for intakes that are too long or too short, both of which cause severe problems.
 

Alan Watkins

Lifetime Supporter
Alan,

Yes, he is talking about the iron heads. however many factors are in play, thread condition, lube, etc. I personally am a lousy engine builder but I learned to "pre-fit" fasteners to assure that the female surfaces are "up to snuff". Most all threads need to be chased or cleaned after disassembly/tanking/machine work, etc. If the fastener will not thread in by finger wrench there is likely a problem.

And I have seen all too often a set of fasteners purchased for "XYZ" Speed Supply for intakes that are too long or too short, both of which cause severe problems.
Yes there are a lot of pitfalls. But the critical question here is
whether Monroe's torque figure is incorrect for an iron head. So can somebody tell me the thread size of the bolt we're talking about? Ross?

BTW Tom Monroe has posted here before; it sure would be neat to get him in on this conversation.
 

Ross Nicol

GT40s Supporter
Bolt threads are 5/32" and 18tpi. Manufactured by ARP. The bolts are long enough to place plenty of thread in the heads.
Ross:thumbsup:
 

Ross Nicol

GT40s Supporter
Sorry forgot to say I don't use any lube on the threads although ARP bolts have a nice flat washer under their head.
Ross
 
Bolt threads are 5/32" and 18tpi. Manufactured by ARP. The bolts are long enough to place plenty of thread in the heads.
Ross:thumbsup:
No wonder your stripping them if they are 5/32"...you aussies must have different stuff to everyone else in the world, the hundreds of windsor engines I have messed with were all 5/16" dia bolts on the intake manifold...:heart::drunk::lipsrsealed:
 

Cliff Beer

Supporter
If you think about that for just a moment, you'll realize it's a contradiction in terms.
Alan, I'm sure you're right. My brain is so friend right now it has me going around in circles!

Next time I'm in SD I'll look you up for a chat. I have a house up in Oceanside and I'm always looking for something GT-40 related to do whilst I'm there.

Cheers!
 

Trevor Booth

Lifetime Supporter
Ross,

how long is it since you had your torque wrench calibrated

Check the thread of the bolt, plated bolt can have irregularities in the plating finish, lightly buff on a wire buff it need be

always use a lubricant on the threads, old fashioned machine oil is suffice for small bolts
 

Cliff Beer

Supporter
Bolt threads are 5/32" and 18tpi. Manufactured by ARP. The bolts are long enough to place plenty of thread in the heads.
Ross:thumbsup:
Ross,

5/32 seems awfully small in diameter.....and at 18tpi that's a very steep thread angle.....

Your ARP bolts are good if they're full length of engagement in the head. The bolt threads aren't the problem as those ARPs are roughly the equivalent of grade 8 (very strong). Keep in mind, the ARP have a self locking cap which, in itself, changes the CoF of the bolt and therefore the torque setting.

It's your bore threads in the head that are weak - not surprising considering the material. What kind of seals are you using at the front and back end of the manifold? Cork? Good to use something which is compressible w/o ten thousand pounds of pressure being applied via the manifold bolts.

Best to start in the middle of the manifold and gently work outwards, maybe just a 1/8th of a turn per round once the bolts meet the initial load up. Just go to perhaps 13-14 ft/lbs at first, heat cycle it a couple of times and then go a bit more (18-19ft/lbs?). Be sure your gaskets are good, and come with a pressed in countour shape (Felpro is pretty good) rather than just flat with a bit of goo squirted on in a circle.... This helps with preserving the integrity of the gasket while heat cycling at the lower torque setting.

Good luck!
 

Ross Nicol

GT40s Supporter
oops red face here. Of course the bolts are 5/16", I'd like to say that was a typo but have to admit it was really, a recently attained 60year olds senior moment. Ross
 

Alan Watkins

Lifetime Supporter
oops red face here. Of course the bolts are 5/16", I'd like to say that was a typo but have to admit it was really, a recently attained 60year olds senior moment. Ross
Well this is interesting... choosing one of the more trustworthy tables
Proper Bolt Torque - by Zero Fasteners

For Grade 5 fastener, zinc plated but not lubricated we are told 21. If waxed the figure is 10. Even for Grade 8 the waxed figure is 13.

Now, I'm used to seeing oiled (as opposed to waxed) specs "de-rated" less than the ~50% shown here. This table:

BOLTS AND TORQUE SPECS

Illustrates that, but again taking the worst case (Grade 8) the oiled figure is 18. So, if Ross had any lubricant at all on the threads, and especially given that the head is cast iron (right? nobody "normal" makes heads out of steel do they?) it's not very surprising to me that at least one of them stripped.


But it also shows that the figures that Tom Monroe supplied are at least plausible but only if the male and female threads are clean and dry. And this is my biggest complaint about most shop manuals in this area: they are not specific about the lubraction conditions for the torque figures they supply, or they hide that stipulation up in front or as a footnote somewhere. I think this is one of the reasons that torque wrench use gets a bad rap.

So the moral I would take away from this is that unless a torque spec specifies some lubricant, you'd better make the threaded surfaces clean and try, or else verify for sure that the spec assumes llubrication.

I'm traveling so don't have access to my copy of the book, but could someone give it a look to see if he says anywhere how to prepare the threads to go with his torque figures?

Now, if Ross tells me he cleaned and dried the threads before he did this, I'm going to kill myself. :)
 
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Alan Watkins

Lifetime Supporter
Alan, I'm sure you're right. My brain is so friend right now it has me going around in circles!

Next time I'm in SD I'll look you up for a chat.
Cliff --Naah, that's just repartee, they'res notihing wring with your brain. Please give me a shout when you're next in SoCal.
 

Cliff Beer

Supporter
Here's a little tid bit about torque specs that's frequently overlooked......

The torque spec of off-the-shelf generic tables is for the bolt only! It does not take into account any sub-par conditions which may be present associated with the bore thread. In other words, the spec assumes the bore thread and material grade is an absolute. Said yet another way, it assumes the bore thread is of infinite strength and infinite perfection in terms of fit and surface condition. Here, obviously, it is the bore thread that is the weak link (ie. aluminum)....not the ARP fastener. The ARP fastener (being roughly equivalent to Grade 8 (relatively close to 12.9)) will strip those aluminum threads out out w/o even breaking a sweat.

Thus, logically, what we're left with is what is the torque spec for an aluminum 5/16th fastener with respect to the thread integrity (not torsional strength)?

Now that's a bit of a brain teaser.....
 

Alan Watkins

Lifetime Supporter
Here's a little tid bit about torque specs that's frequently overlooked......

The torque spec of off-the-shelf generic tables is for the bolt only! It does not take into account any sub-par conditions which may be present associated with the bore thread. In other words, the spec assumes the bore thread and material grade is an absolute. Said yet another way, it assumes the bore thread is of infinite strength and infinite perfection in terms of fit and surface condition. Here, obviously, it is the bore thread that is the weak link (ie. aluminum)....not the ARP fastener. The ARP fastener (being roughly equivalent to Grade 8 (relatively close to 12.9)) will strip those aluminum threads out out w/o even breaking a sweat.

Thus, logically, what we're left with is what is the torque spec for an aluminum 5/16th fastener with respect to the thread integrity (not torsional strength)?

Now that's a bit of a brain teaser.....

Right, the torque spec is telling you how hard you have to turn the fastener to get (something like) 80% of the maximum longitudinal force possible with that fastener before pastic deformation of the fastener.

But Cliff -- remember, this is a cast iron head with a lot of thread engagement, so the torque table figures are probably reasonable under the specified conditions. And my point really is the same as yours, namely, if you put an oiled grade 8 screw into this hole and torque it to 25 lb, it's kinda guaranteed either to strip the hole or break the screw, or at least to come very close to doing so.

So what I'd like to know is:

  1. where the Monroe book's figure came from originally (presumably Tom didn't test a whole bunch of bolts to write the book). For example, does the Ford shop manual of that era say the same thing?
  2. whether it was documented as "use clean and dry threads", and
  3. whether there was residual oil on Ross's bolt or in his head (the engine head, that is :) ).
 

Cliff Beer

Supporter
Alan,

Right - brain fade here, had somehow got it in my head we were speaking of an alloy head.....duh...

But still, wouldn't you think a cast iron head is still going to be a fair bit cheesier (technical term....) than a grade 8 equivalent bolt? In other words, still the weakest link?

However, you're right, probably the stock charts work just fine with a cast iron bore thread.....
 
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