Commando Head stud thread.. (2016)

[oldmikew]

Ok will admit have never tried but IF there is room , depending which stud it is ,my inclination would be to go for an oversize unc thread and make up stepped stud with the original bsf on the narrow step..Do make sure the stepped stud bottoms out in the hole

 

[oldmikew]

I believe it is a356 or similar. Similar spec to the original casting -as good as you can get for the purpose.

The casting methods on the new heads have improved so porosity and voids are no longer a problem -but they are still gravity cast aluminum. Jim

Well the original heads are sand cast whether it was before use of freeazing agent I do not know.I am not up to speed with changes in foundry technology , but there is nothing wrong with gravity casting surely - provided the moulder lays adequate risers and feeders

 

[texasSlick]

Wow.. How long or how many miles do you think it burn the groove? I road it less than 50 miles after I noticed the oil leak and tried retorquing the bolts. It must've been this long before I knew about it. So glad to have someone like you in our camp. Thanks for doing your magic on it. Glenn T. Suneson

The gasses erode rather than burn the groove. Similar effects can be observed in the seat of a water faucet that has been allowed to drip for a time.

If a microscopically small channel opens up in the seal, the gas flow thru the channel can reach very high velocity.

Slick

 

[jseng1]

Timeserts are the strongest over the counter solution and you can switch to the stronger 16 threads per inch. Not many people know but there is also a "bigsert" available with a larger OD that allows you to repair a helicoil that has pulled out. The best option is the triangulated ARP studs that allow the stud to stretch and stay tight without pulling out the threads - the triangular shape will still provide proper head alignment with the cylinders. Spring washers are also a good idea.

The vid below shows how to install the timeserts yourself

 

[comnoz]

Why would the three studs need to stretch more than standard. The stud passes through a cast iron barrel so the stud and barrel expand at the same rate.

And the waisted stud would make no change in the load applied to the threads. The load will be whatever is supplied by the torque when assembled. Since the steel and iron expansion rates are the same then the load on the threads will stay the same....

Nice ad though.

 

[lcrken]

Why would the three studs need to stretch more than standard. The stud passes through a cast iron barrel so the stud and barrel expand at the same rate.

And the waisted stud would make no change in the load applied to the threads. The load will be whatever is supplied by the torque when assembled. Since the steel and iron expansion rates are the same then the load on the threads will stay the same....

Nice ad though.

Maybe for reliability, rather than coping with different expansion rates? With the standard studs, there will be more stretch in the threaded part of the stud than in the body, because of the reduced diameter at the thread root. With the waisted studs, the stretch will be more evenly distributed along the length. That should extend the fatigue life of the stud. Not that I think we are anywhere near the fatigue limits of the stock studs. I've seen a lot of them that pulled out of the head, but have never seen one break.

Ken

 

[jseng1]

I suggest the waisted shanks for the head studs because they are better (for the same reason that rods bolts have waisted shanks). A waisted shank provides a wider "forgiveness range" - the stud will stretch a little - it will spread out the load and will not change the load as much as a non-waisted shank - less likely to pull out the threads. A typical Norton owner won't be using a torque wrench on these hard to get to studs so he could use a little "forgiving" stretch. The studs in the photo below are for Maney alum cylinders that expand - I like using the same type stud for iron cylinders - plus you can spin them in/out with a crecent wrench on the shank.

Installing time serts allows you to use higher torque loads. Without the waisted shank its more likely that a stud (or bolt) will break at the thread root as in the photo below. As Ken says - its not likely to happen with a head stud but it doesn't hurt to have the extra insurance.

For those who want to do it yourself and don't care if the stud is waisted or not you can get the ARP studs from Summit racing part ARP AJ2.310-1LB TRIM COARSE END ONLY TO 2.2" OVERALL SO THREADS BOTTOM IN TIMESERT

You can also use .080" thick belleville washers from McMasterCall part # 9294K61

For the long nuts you can make your own out of 1/2" hex stock McMasterCarr part # 86755k87 heat treatable 416SS. The threads are so long on the fine thread side of the stud/nut that you don't have to worry about threads pulling out.

For the timeserts you can get the numbers off the video.

 

[comnoz]

Maybe for reliability, rather than coping with different expansion rates? With the standard studs, there will be more stretch in the threaded part of the stud than in the body, because of the reduced diameter at the thread root. With the waisted studs, the stretch will be more evenly distributed along the length. That should extend the fatigue life of the stud. Not that I think we are anywhere near the fatigue limits of the stock studs. I've seen a lot of them that pulled out of the head, but have never seen one break.

Ken

But in cast iron there is virtually no stretch obtained or needed since there is no expansion to deal with.
They are needed with aluminum cylinders.

 

[jseng1]

A waisted shank bolt/stud is superior to a non-waisted shank bolt. I prefer the superior item.

 

[lcrken]

But in cast iron there is virtually no stretch obtained or needed since there is no expansion to deal with.
They are needed with aluminum cylinders.

Agreed, that's why I wondered if the main benefit in such situations might just be additional fatigue life, which probably doesn't mean much in case of the Norton studs. I was thinking of the normal stretch you get from torqueing enough to get good clamping force, plus enough to handle the dynamic load (cylinder combustion pressure on the head, etc.), while still staying below the yield point of the fastener. It seems to me that having the entire stud with the same cross section as the thread root would distribute the tensile stress more uniformly, and the cyclical fatigue life will be increased.

But that's just my opinion, and I could be wrong. I've done some clamped joint analysis in the past, but never looking at this particular question, and that's what it would take to see if there is any benefit. Just not a high enough priority in my life right now. In any case, there doesn't seem to be any drawback from using waisted studs, except the additional cost.

On the other hand, if you're using alloy cylinders, the original argument for using waisted fasteners (different material expansion rates) should still apply.

And, of course, don't forget the bling factor.

Ken

 

[comnoz]

Agreed, that's why I wondered if the main benefit in such situations might just be additional fatigue life, which probably doesn't mean much in case of the Norton studs. I was thinking of the normal stretch you get from torqueing enough to get good clamping force, plus enough to handle the dynamic load (cylinder combustion pressure on the head, etc.), while still staying below the yield point of the fastener. It seems to me that having the entire stud with the same cross section as the thread root would distribute the tensile stress more uniformly, and the cyclical fatigue life will be increased.

But that's just my opinion, and I could be wrong. I've done some clamped joint analysis in the past, but never looking at this particular question, and that's what it would take to see if there is any benefit. Just not a high enough priority in my life right now. In any case, there doesn't seem to be any drawback from using waisted studs, except the additional cost.

On the other hand, if you're using alloy cylinders, the original argument for using waisted fasteners (different material expansion rates) should still apply.

And, of course, don't forget the bling factor. Ken

Of course the bling factor helps.

And I would go for a waisted bolt if this were a high load situation, but 25 ft lbs on a 3/8th inch bolt with no expansion differential to account for [on an iron barrel] is not exactly a high load situation.

Now 45 ft lbs on a 5/16th inch rod bolt is a different thing...

 

[jseng1]

With a non-waisted stud that does not stretch - you torque it down but then after the head gasket or whatever settles in and the stud loses its tension - then you can have an oil leak near the pushrod tunnels.

A waisted or fluted stud is stretched when it's torqued down. When the head gasket starts to crush and settle in, there is still some tension in the stud because it was stretched to begin with and this way you can avoid an oil leak.

Of course even a waisted bolt can become too loose and need re-torquing. But it won't need as much attention as a non-waisted bolt because it has more elasticity and a wider range of forgiveness. A non waisted bolt that doesn't stretch has a narrower more critical range of tension - any change in dimension will have a more dramatic effect.

Whether the head is on an iron cylinder or an aluminum cylinder - its still going to loosen up - its just going to get looser faster with a stud that doesn't stretch.

 

[wrecks]

But it won't need as much attention as a non-waisted bolt because it has more elasticity and a wider range of forgiveness.

Do waisted bolts really "spring back" and have elasticity after they stretch?

 

[comnoz]

Do waisted bolts really "spring back" and have elasticity after they stretch?

Yes, as long as the material is correct and the machining does not cause stress risers and fractures.
Carrillo rod bolts are normally torqued using a bolt stretch gauge. They stretch 5 to 7 thousandths when tightened correctly and go right back to original length when they are loosened. Jim

 

[Dances with Shrapnel]

Wrecks, as JimC says above.

Steel has roughly three zones, elastic, plastic (or non elastic deformation) and rupture. Most tendons or bolts are designed to stay in the elastic range of the stress-strain curve. A spring is an example of something that can stress and strain allllll day llllong and return to the original dimension. Once you exceed the elastic limit of a bolt it yields. You can relax it but it will be a tad longer. Generally (but not advisable) you can continue the stress and elastic strain relationship after yield, just with a longer and somewhat altered bolt. Then there is pulling a bolt to rupture where there is no going back - nuff said there.

Pretty much all steels have near identical modulus of elasticity so their stress/strain relationships are for all intents and purposes, identical.

A waisted bolt is basically “more springy” and this is what JimS is referring to. A waisted bolt at the same thread pitch and torque should be more forgiving and more accommodating (in this application) for head gasket crushing and settling.

So if we test a scenario where a waisted bolt of (of cross section area 0.5) identical material is torqued the same as a non waisted bolt (of cross section 1.0), the tension will be identical. Say there is settling of a distance X to the point where the non waisted bolt loses all of it’s tension, the waisted bolt at settled distance X will maintain 1/2 the tension.