Wasted waisted head bolts & studs

I have followed the development of ideas regarding the waisting of head fasteners with interest. I'm not sure whose idea it was originally to apply this proven technology to Norton heads (Comnoz, jseng1, ???) but it really seems to be a great idea. Indeed, I plan to use these ideas on the rebuild of my 650SS - should save retorquing and leaks.

I have, however, recently seen proponants of waisting all head fixing bolts and studs.
I believe the logic in this approach is flawed.
There is significant evidence that the 4 outside bolts (3/8") will see benefit from waisting (apparently to an effective 5/16") they are 3,4,5 & 6 in the diagram below. Their grip length on my 650 is about 44mm and this is the length over which the detrimental differential expansion occurs. The waisting increases their "springiness" to a lower load within the compressive strength of a hot aluminium cylinder head hence stopping the aluminium "creeping" and losing bolt tension.



I believe 3/8" studs 2, 7 & 8 (grips 28mm, 63mm & 63mm) will certainly not see any benefit from waisting as they clamp through cast iron barrel material. There is no appreciable differential expansion so no extra "stretch" required.

5/16" bolt and studs 1, 9 & 10 (grips 25mm, 32mm & 32mm) are already at the diameter apparently proven to give the required stretch. Yes the "effective" length of the bolt/studs is shorter but then again so is the length over which differential expansion occurs. I would argue that if 5/16'' is good enough for the 4 outside bolts then is is good enough for these 3.

So, at the risk of upsetting anybody, I would suggest there is only benefit in waisting the 4 x 3/8" outside bolts (3, 4, 5 & 6)
Regards
Rob

One thing that might enter into the equation is the difference in clamping force between waisted and non-waisted fasteners and will a mix-and-match of both kinds cause any uneven clamping at the gasket? When the waisted bolts stretch and expand and the non-waisted items do not, is there a potential problem?

Danno
If only the 4 outside bolts are waisted then all 7 that clamp aluminium are of the same diameter (5/16") in the stretch zone.
The 3 X 3/8" studs that clamp the barrel will not go through stretch cycles due to thermal effects so will only be affected by the torque applied
So... I don't think eneven forces should be a problem

robs ss said:

I have followed the development of ideas regarding the waisting of head fasteners with interest. I'm not sure whose idea it was originally to apply this proven technology to Norton heads (Comnoz, jseng1, ???) but it really seems to be a great idea. Indeed, I plan to use these ideas on the rebuild of my 650SS - should save retorquing and leaks.

I have, however, recently seen proponants of waisting all head fixing bolts and studs.
I believe the logic in this approach is flawed.
There is significant evidence that the 4 outside bolts (3/8") will see benefit from waisting (apparently to an effective 5/16") they are 3,4,5 & 6 in the diagram below. Their grip length on my 650 is about 44mm and this is the length over which the detrimental differential expansion occurs. The waisting increases their "springiness" to a lower load within the compressive strength of a hot aluminium cylinder head hence stopping the aluminium "creeping" and losing bolt tension.

View attachment 3991

I believe 3/8" studs 2, 7 & 8 (grips 28mm, 63mm & 63mm) will certainly not see any benefit from waisting as they clamp through cast iron barrel material. There is no appreciable differential expansion so no extra "stretch" required.

5/16" bolt and studs 1, 9 & 10 (grips 25mm, 32mm & 32mm) are already at the diameter apparently proven to give the required stretch. Yes the "effective" length of the bolt/studs is shorter but then again so is the length over which differential expansion occurs. I would argue that if 5/16'' is good enough for the 4 outside bolts then is is good enough for these 3.

So, at the risk of upsetting anybody, I would suggest there is only benefit in waisting the 4 x 3/8" outside bolts (3, 4, 5 & 6)
Regards
Rob

Click to expand...

What you are saying is true except for the center bolt #1 which is 3/8th inch on a 750 or 850 and does benefit from waisting it. Jim

PS, the three 3/8th studs benefit from waisting when they are used with an aluminum barrel.

Since the waisted bolts are designed to provide some elasticity, could a mix of waisted and non-waisted fasteners cause uneven clamping force and possible head warping? I would be hesitant to mix the two for no good reason.

Danno said:

Since the waisted bolts are designed to provide some elasticity, could a mix of waisted and non-waisted fasteners cause uneven clamping force and possible head warping? I would be hesitant to mix the two for no good reason.

Click to expand...

The idea of the waisted bolts has been to equalize the clamping force.

With a non waisted bolt that is in a position where it travels through a larger section of aluminum, the growth of the aluminum when it gets hot makes the clamping pressure too high. This results in displacement and warpage of the aluminum in the area of the bolt.

Studs that run through cast iron do not suffer from this since the iron expands very little when heated. [iron has about the same expansion rate as the bolt] Jim

I have looked at the CNW head mounting kits.

https://coloradonortonworks.net/part-categories/cnw-hardware-fasteners.html

They are absolutely brilliant, but also expensive for those of us who are more budget conscious. Is there a source for the five 3/8" - 26 TPI bolts that are waisted and rated for such use, stainless or not? I'd love to plunk down the money for a complete CNW bolt kit, but I would have to give up eating or heaven forbid drinking to do it.

Russ

rvich said:

I have looked at the CNW head mounting kits.

https://coloradonortonworks.net/part-categories/cnw-hardware-fasteners.html

They are absolutely brilliant, but also expensive for those of us who are more budget conscious. Is there a source for the five 3/8" - 26 TPI bolts that are waisted and rated for such use, stainless or not? I'd love to plunk down the money for a complete CNW bolt kit, but I would have to give up eating or heaven forbid drinking to do it.

Russ

Click to expand...

So, you’ll lose weight, give your liver a rest and have a new bolt kit.

That’s a win-win-win !

Maybe we should use alu-bronze bolts on our heads.

Does waisting help mitigate the problem of the three head studs pulling out of their aluminum threads, necessitating helicoils or time-serts?

Danno said:

Does waisting help mitigate the problem of the three head studs pulling out of their aluminum threads, necessitating helicoils or time-serts?

Click to expand...

The way I see it, shank waisting mainly does just that. The tension and elongation in the bolt or stud is transferred to the waisted shank. In the non-waisted fastener, the tension concentrates in the root area of the first two or three threads engaging in the female part. The thread root is a de-facto "waist" of very short length. Such concentration leads to the pulling of the female threads, and in the extreme, can lead to the bolt breaking off at the first or second thread root should a stress riser be present.

Slick

Danno said:

Does waisting help mitigate the problem of the three head studs pulling out of their aluminum threads, necessitating helicoils or time-serts?

Click to expand...

On the CNW kit the new studs have a different thread from the Norton studs, so installing helicoils is part of the fitting.

It has been discussed that by taking old head bolts and custom making studs from them that you can...
A.) Put inserts into the head to accept a 3/8-16 thread
B.) Get a couple more threads into the aluminum
C.) Make them a little bit longer so that you get more thread into the nuts
D.) Get a lot closer to putting the required torque on them without pulling them out.

https://www.accessnorton.com/NortonCommando/commando-head-to-barrels-studs-too-short.10388/

FWIW, I was sort of serious about the alu-bronze fasteners. As I understand it the expansion rate is closer to aluminum and would put less stress on the aluminum head. It seems kind of simple so there must be something really wrong with the idea.

Interesting idea. I think part of the reason you don't see them is that even high strength aluminum bronze fasteners are not as strong as typical steel head bolts, and certainly not up to the 160 - 170 ksi of the ARP stainless bolts or 8740 steel bolts. The highest strength aluminum bronze fasteners I've seen are in the 110 - 130 ksi range, and would probably have to be custom made to get sizes needed for our applications. That looks like a fairly expensive exercise. The material also has less fatigue resistance than most steel fastener alloys, although I don't know if that is as significant in our application. I haven't looked at its crack sensitivity compared to the steels we now use, but that's something you'd want to evaluate before trying them. Doesn't mean it couldn't be done, but it looks like maybe not enough benefit for the cost and effort.

The ideal material would seem to be something like Dilavar, the high expansion steel alloy that Porsche (and some Italian engine builders) used for some of it's race car cylinder studs. It has a thermal expansion rate right in the middle of the range for the common aluminum engine parts alloys. That was some time ago, and I don't know if you can still get fasteners made in it, or a similar alloy.

Regardless of the material used, a waisted design fastener is still superior in terms of consistent clamping force and fatigue life. That's why you see it used so much in high end engine fasteners like con rod bolts and head bolts and studs.

Ken