Head bolt differences

Dr.Jeff

Dr.Jeff

True Classic
In a recent thread (link below), the discussion went a bit off topic to include a lot of information about head bolts. In addition there have been some great discussions about it before, also a bit buried. I thought it might be useful to create a central thread to help bring together some of those discussions and give a easier reference to find later. Hopefully others will please assist in adding links, information, and tags to this thread.

The following is some of the info 'cut and pasted' from multiple posters the this thread:
https://xwebforums.com/forum/index.php?threads/timing-keeps-slipping.37333/
Sorry for not including the poster of each comment, I'm not sure how to capture the reference without importing the whole thing:

On a 14-bolt head, the extra 4 small bolts are not "above" the spark plugs, they're right next to them. Here's a 14-bolt head with the 4 bolts highlighted by green arrows:



Here is a pic of one of my 10 bolt engines. This is the side that would have the extra 4 bolts if it was a 14 bolt version. You can compare it to the pic posted above and to your engine. This is a Calif spec '79 so it has a throttle cable bracket (partially obscuring one of the head bolts) for a carb, a plug on the head (silver looking) where some smog equipment used to be, and a extra temp sensor for the carb cooling fan (possibly other minor differences as well).

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As far as I can tell by interpreting the info in the parts manual, the change (designated #8493) to a 14 bolt head came with motor number 5750182. Apparently there was another change #8702 later on pertaining to the head bolts, starting with motor number 6007752, but from what's in the parts manual, it's hard to tell exactly what the change entailed.

The change to "stretch" bolts was much earlier, I think it was change #6818 starting with motor #4037356. Here's the page from the parts manual, showing the bewildering assortment of head bolt diameters and sizes.
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The "timeline" on these starts at the lower left & goes around clockwise. First studs/nuts, then 12mm bolts, then 10mm bolts with 12mm upper shank, then 10mm bolts. The shorter bolts shown on the upper right are the 4 "extra" bolts that go near the spark plugs on a 14-bolt head.


I have two sets of M10 (1.25) head bolts that have different shanks. The dimensions I'll refer to are actual measurements. In this first picture you can see them side by side; the black arrows are 10mm, the yellow arrows are 9mm. The top bolt is a M10 thread but the shank is actually 9mm all the way up until it reaches near the top, where it becomes 10mm. The bottom bolt is also a M10 thread but has a 10mm shank all the way to the head. You can see the difference immediately where the threads stop and the shank begins, and again near the heads. I assume the top bolt is a TTY ("stretch") type, and the lower one is not?

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This second picture is the same two from a little different angle. Here you see a couple of other differences between the two types. The top bolt (TTY?) is longer overall (red arrows), and has a longer section of threads (blue arrows).

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If the top bolt is a TTY, it is not like others that I have encountered on other (non Fiat) engines. Usually the shank would begin at the threads as 10mm, then reduce to 9mm, then enlarge again to 10mm before the head. Something of a Dolly Parton / Mae West "hourglass" shape. The top bolt in my pictures does not start as a 10mm shank from the threads, but is already 9mm. Compare the two bolts right where the threads stop (blue arrows in second picture).

In direct reference to the factory parts diagram posted, which details the various bolt diameters & bolt lengths. For clarification, diameters "Ø" are underlined in green, lengths "L" are underlined in red:




After reviewing the bolts I pictured earlier (two photos, each with two bolts side by side), and the diagram above closer, I realize one of the bolts I pictured is not a factory head bolt. It is a standard class 10.9 bolt (the one on the bottom with the larger 10mm diameter shank all the way up from the threads). The lengths of these (non Fiat) bolts are 100mm and 80mm, which is different from the factory's 103mm and 85mm (for 10mm head bolts).

Matt sells exactly that combination as an off-the-shelf alternative to Fiat OEM


You don’t have to use the stretch bolts. The only downside is that you if don’t use the stretch bolts you’ll need to retorque them once after a few hundred miles. (This means taking the cam box off if you don’t have the magic Fiat wrenches).
As long as the bolt type matches whatever the gasket spec he bought it (which will likely be std. bolt / retorque required) he will be OK.
 
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There is plenty more that needs to be added to this thread. I started with copying the related bits from another thread. I'll add more later, but please add whatever info you have on the subject of head bolts. Especially helpful will be actual photos to show examples of each type. I am currently pulling the head head off an engine with M12 head bolts/studs and will take some pics of those components soon.
 
Today I removed the head from the '79 engine with M12 head bolts/studs. It was stuck (very stuck), so I had to make a quick 'redneck' version of the head puller tool. I decided to make it very simple and follow the smaller design (only attaches to one side of the head, with the studs) like in this diagram:
Head Removal Tool 1 - Copy.png

Utilizing some scrap angle iron and misc hardware that I had on hand, this is what I came up with:
028.JPG


It worked (had to pull the head ALL the way off the studs). So here are a couple pics of the factory M12 bolts and studs:

Next to a factory M10 head bolt. Notice the shank of the M12 is 12mm (blue arrow) all the way up from the (12mm) threads to the head. Unlike the M10 version that is smaller up the shank until it gets near the head....
020.JPG


Here is a M12 stud with washer and nut. The shank diameter is like on the M12 bolt, 12mm all the way between the threads (blue arrows). Interesting the threaded portion that goes into the head (red arrow) actually has less thread engagement than the M12 bolt....
030.JPG


The thick flat washers for the M12 bolts and studs are much heavier than the ones for the M10 bolts...
025.JPG
 
I would disagree that it makes no difference if you use TTY or a regular bolt.
You will get a higher and more consistent clamping force between head and block with the TTY.
That‘s why they are used as cylinder head fasteners in the first place.

On a stock engine, it may work fine with the regular bolts.
 
carl said:
Matt's instructions for his non-stretch bolts is that no retorquing is needed.
Click to expand...
If Matt says that, then he’s right and I’m wrong.... but I’d expect that you can only skip the retorque with certain types of gasket.
 
Ulix said:
I would disagree that it makes no difference if you use TTY or a regular bolt.
You will get a higher and more consistent clamping force between head and block with the TTY.
Click to expand...
I'm not sure where it was said that it does not matter, but possibly it was meant to say it's more important that the type of bolt (stretch/TTY or not) should match the type of gasket? However I'm sure you are correct about TTY stretch bolts offering more/better clamping force...good to know. Although to a much lesser degree all bolts "stretch" when torqued, but I realize that wasn't what you meant.

I believe this question has been asked before but I don't think it ever received an answer. When you purchase head gaskets from many online sources they do not specify if they are retorque type or not. So how do you tell?

This discussion also brings something else I've wondered about; the use of studs vs bolts. The performance sector prefers studs, especially for head attachment. Studs are not TTY/stretch, and judging from what I saw on my '79 block they have less thread engagement than the bolts (factory studs and bolts combined). But they apparently have greater clamping force?
Regarding the studs on my '79. All 5 of them were very badly corroded and frozen to the head all along the length of the fastener bores. I seriously thought I was warping the head by the tremendous amount of force I had to apply with a head removal tool. But the 5 bolts were not, they came out easily and cleanly. So why would the same size and material of fastener have such a dramatic difference in corrosion in the same head (material)? Is there something inherent about studs (e.g. surface finish?) that allows more degradation?
 
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Some basic observations....

I took quick measurements of the various head bolts/studs I have on hand for a simple comparison of lengths and thread engagement. These are not precise measurements but close enough for this purpose. The head is roughly 60mm high on the short bolt side (the side with the manifolds) and 75mm on the long bolt side (the side with the spark plugs/distributor); this will vary a little with head gasket thickness, head surface milling, manufacturing differences, etc.
All of mine are 10 bolt heads so I do not have any info for the additional 4 'extra-short' bolts on 14 bolt heads.

The lengths of the shank/threaded portions, minus the washer thickness (and nut thickness in the case of the stud) were as follows:

Factory M10 with 9mm shank (I believe these are the TTY/stretch type) =
Short side 83mm long with 21mm of thread engagement.
Long side 98mm long with 23mm thread engagement.

Factory M12 with studs on the short side and bolts on the long side (non stretch) =
Short side 83mm long less about 7-8mm extending above the nut (effective length 75mm) with 15mm thread engagement.
Long side 99mm long with 24mm thread engagement.

Aftermarket M10 class 10.9 standard bolts =
Short side 75mm long with 15mm thread engagement.
Long side 95mm long with 20mm thread engagement.

The blind holes in my 1500 block are 26mm thread depth (plus an addition few mm of non threaded depth), which is deeper than the thread engagement for all of them (i.e. no bottoming of the threads at the distal end).

All of the bolts have threads that reach up the shank longer than the engaged thread depth, so no bottoming of the bolt shank with the surface of the head. This also means all of the bolts have exposed threads above the surface of the head. Some have considerably more thread length than others, meaning they have a lot of exposed threads.

The studs (M12 short side) do bottom against the surface of the head, and have 15mm of threads (therefore the 15mm thread engagement). These and the standard class 10.9 short side bolts (also 15mm thread engagement) have considerably less thread engagement than any of the others. I recall reading somewhere that generally speaking a bolt should have more thread engagement than its diameter. For example the M12 factory studs need more than 12mm of thread engagement, which they have (but not by much). I'm surprised the factory did not make these studs a bit longer in this aspect. The other end (where the nut clamps against the top of the head) has plenty of threads, and the overall length of the stud allows plenty of extension beyond the nut. It is just the lower threaded portion that seems a little short. Likewise I would think using 85mm (instead of 80mm) class 10.9 standard bolts might be best for the short side (allowing for 20mm of engagement). For that matter, the long side could also be 5mm longer (105mm instead of 100mm), offering more engagement without fouling anything.
 
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Stretch (TTY) vs non stretch (retorque) bolts.

Looking at the page from the factory manual (posted previously, from the related thread) with the various head bolt part numbers (for FI engines). It appears all of the M10 bolts are stretch (TTY non-retorque) type? The M12 are not TTY. There were no M10 non TTY (stretch) factory bolts? Note I said 'factory', I'm not referring to the aftermarket class 10.9 standard bolts in M10.

The accompanying comment about factory changes seems to support that:
"The change to "stretch" bolts was much earlier (than the change to 14 bolt heads), I think it was change #6818 starting with motor #4037356."
Any idea when motor #4037356 was made? Does that coincide with the introduction of FI? My '79 1500 carb 10 bolt head had M12 (non stretch) bolts and studs. My '80 FI and both '85 FI engines have 10 bolt heads with M10 TTY (I believe) bolts. Well, one of the '85 was switched to standard class 10.9 bolts, but it was originally the same as the other '85 (which I believe are TTY; M10 threads, with M9 shank increasing to M10 near the head).
 
A normal bolt or stud is torqued so that it stretches in the elastic (I had written "plastic" previously) range.
There are standard torque values per thread size that can easily be googled.
The clamping force can be calculated from that and the material properties.
If you need more clamping force, you go to a larger bolt.

Then at one point, TTY was invented. Basically you just torque the bolt much more than you normally would. If you do this to a normal bolt or stud, it starts to really stretch at its thinnest cross sectional area. This is at the threaded part. It starts doing that at the top of the hole. This will pull out the top threads of the hole, then the next ones and the next.
We don't want that. So we make the shank thinner than the threaded part. This will let the shank stretch and the threaded part will remain pretty much unstretched! Bingo.

So now you can get the same clamping force with a thinner bolt or stud. Very nice.
Also the bolt can stretch quite a bit more without changing the clamping force.
A very stable system.

So Fiat used M12 regular steel head fasteners and later went to M10 TTY bolts. Normal technical evolution. Both probably have similar clamping force.
A thrid way to get the high clamping force is to use a higher quality material than can transmit more clamping force than a regular steel one. That is what ARP is doing.

Does a regular steel M10 head fastener make sense in this scenario?
I don't think so. It could not generate the same clamping force as the M12 or the M10 TTY.

I don't know what kind of bolts Matt is selling and if his specified torque exceeds the one listed for regular steel bolts, but that could easily be googled.
 
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Regarding studs, modern blocks are made of aluminium.
With aluminium being much softer than the fastener, studs rather than bolts are normally specified.
This explains why today studs tend to be used.
I would suspect that for our cast iron block it makes no difference.
The bolts are easier to remove because they can be turned with a wrench, breaking any corrosion that may have formed between it and the head.

I have also read that using a stud will apply the load at different places along the fastener than a bolt and is therefore better.
This, I would argue, makes absolutely no difference.
 
The "elastic" range is torque values that stretch the bolt, but on release of tension the bolt returns to it's normal length.

The "plastic" range is when the bolt / fastener stretches but on release of the load, the fastener remains elongated... this is a TTY bolt.

The TTY bolts don't stretch at the thinned section of the shank, they stretch the unseated / un-engaged threads above the block...easy to pick a stretched bolt by holding a thread gauge alongside the threaded portion, if it's stretched, then the thread gauge won't line up along the whole length.

In the pictures above you can pretty clearly see the TYY bolt has been stretched along the upper threaded portion.

SteveC
 
Steve, yes, that was a typo, elastic/plastic. Duh.

I have seen different designs of TTY bolts. Some even have threads all the way up to the head, which I assume work the way you describe.
But I have also seen (and read the description) bolts that are designed such that the threaded part remains intact by thinning the shank.
 
Just for fun I will check the threads on my used M10 bolts for elongation (I think in part it might be a bit of illusion in my earlier photo due to the angle and camera focus). But indeed those bolts have a much longer threaded length on the shank than on the M12 ones. So that must be where these stretch like Steve said.

In the past the TTY bolts I've seen were like the ones in the pic Ulix last posted; they have a shank that was much narrower (for the bolt size) as compared to the Fiat ones (9mm shank on a 10mm bolt). So it wasn't so obvious to me if these Fiat bolts were TTY or not. But after all of the info you guys have given it is clear to me now.
 
With this clearer understanding of stretch bolts and the options for our heads, another thought arises.

If the M10 bolts were replaced with new non-stretch ones (e.g. standard class 10.9, not factory type), [Note: I'm not inferring if it is wise to use this type or not, just discussing what if they were used]. Then as Ulix suggests it seems a different amount of torque should be used when installing them. I haven't looked up the torque recommendations for any of the factory bolts yet, but what might the difference be when converting to non TTY? Given they are smaller than the original non TTY (which were M12), and they do not offer the elastic property of the M10 TTY, I'd assume a greater amount of torque is required to get a similar clamping force? Are grade 10.9 strong enough for that torque? If they are, then perhaps it would be best to have a greater thread engagement in the block to avoid possible thread damage to the block? In which case I'd think they should be at least as long - probably longer - than the stock bolts. So maybe 90 mm on the short side (25mm of thread engagement) and 105mm on the long side (25mm engagement)? Based on my quick measurements it looks like the factory bolts typically have about 20mm engagement, with almost 25mm in some cases. This still leaves plenty of room in the block's blind holes for no bottoming of the bolts.

Just a mental exercise out of general interest. As the saying goes, "curious minds want to know". I have converted my current project engine (M12, 10 bolt head) to ARP studs, mostly due to finding them at a huge discount that I could not resist. ;)
 
I think Matt specifies a torque value for his non-TTY bolts of 58 ft-lbs.
That is in line with the values usually listed: https://floorjacked.com/wp-content/...que-Chart-Class-8.8-Class-10.9-Class-12.9.jpg

So you cannot tighten this bolt any more to gain more clamping load. That's it.
When using the TTY bolts and the torque-angle method, I tried to estimate the torque that this resulted in by using my torque wrench on the tightened bolt and increasing the setting by 5 ft-lbs each time.
This isn't entirely accurate because of the difference in static/dynamic friction, but I ended up at around 88 ft-lbs (vs. the 58 ft-lbs of the non-TTY bolt).
This is why I will be continuing to use the TTY bolts.
- cheap
- higher clamping force
- clamping force is constant within a certain range --> if the head gasket sets, clamping force won't change --> no retorque necessary
- torque - angle method of tightening is more accurate because it does not depend on the friction between bolt head and cylinder head
 
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