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Discussion Starter #1 (Edited)
There is a difference in the 2 liter rod bolts from the earlier 1600/1750 rod bolts.
It was my understanding that the change over was due to some of the bolts stretching at high rpm and they changed the center section to alleviate this. Interesting to note the differences.

There is a difference of 2.3 grams between the top bolt and the bottom bolt (lighter).

Also about the nuts that go on the bolts. The 1750 bolts had small nuts and used a retainer/lock tab. The 2 liters don't use a retainer and are larger. I have always wondered why this is so? You would imagine over time that the nut would come loose from the heat cycles.

Rather than use a retainer, what about using Locktite green or blue?

These are niggling questions and thought I would put them out for the sake of discussion.
 

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The Shankle catalog shows 3 different bolts, one for 1300, one for 1600 & 1750 and one for 2000.
 

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The pictured bolts work differently Chris. Upper is old 1600 and other engines, and was designed not to stretch under assembly torque. What you had was a tough, but heavy bolt. Those we magnifluxed before re-use. Bottom design is designed to stretch with installation torque, and might be considered more "spring" like. Both work fine. As I understand it, (probably incorrectly) the stretch type may have a lesser re-use life. Some modern versions are not designed to be re-used as the stretch is a one-time-thing.
 

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Gordon:

What is your advice on installing aftermarket bolts (e.g., ARP) when rebuilding? When working on a 70's era 2L engine for street use, my machinist suggested installing ARP bolts to eliminate the possibility of the old bolts failing. The approx. $100 this added to the bill wasn't a big percentage, so I went for it. Admittedly, I have re-used the old bolts in prior rebuilds, and have never had any problems.

 

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Discussion Starter #6
I have 3 complete sets of new rod bolts of the lower pictured items with the smaller nuts. The larger nuts also fit like the ones from the 2 liter engine, and I have enough of those as well. If one is going to the lower rotating mass, then the lower bolt with the small nuts would be the way to go. If one uses a locking tab with the smaller nuts then you wind up with the same weight as the larger nut. In weighing the lock tab and smaller nut vs the larger nut, the lock tab/small nut is heavier by .02 grams. Not a lot. If you subtract the lock tab you save .05 grams for each bolt or .10 gram for each rod.

Another way to look at this is the surface area of the large nut vs the small nut. The larger nut obviously has more surface area, but if you use the small nut with a lock tab, then you probably come out the same. Would the surface area of the small nut without the lock tab be sufficient to use with a Locktite product?

I am only putting this out for discussion, as it raises some topics that people might consider but don't always know the answer.
 

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Discussion Starter #7
Gordon:

What is your advice on installing aftermarket bolts (e.g., ARP) when rebuilding? When working on a 70's era 2L engine for street use, my machinist suggested installing ARP bolts to eliminate the possibility of the old bolts failing. The approx. $100 this added to the bill wasn't a big percentage, so I went for it. Admittedly, I have re-used the old bolts in prior rebuilds, and have never had any problems.

I was on ARP's website last night and they have the bolts that everyone uses for drag racing and high performance engines. These bolts would never see the stress like that in our engines from what I can determine. I was over there reading up on corrosion effects on rod bolts. Interesting topic of conversation. I did post it to my blog spot if anyone wants to read it. (www.alfadoctor.wordpress.com)
 

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The surface area of the nut is not important, the stretch of the bolt is.

The undercut bolt is the better option, it should stretch more uniformly. Its the proper engineered solution.
 

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I personally use new bolts whenever possible. On frighteningly expensive or rare engines with Carillo rods I go with the re-useable "forever" stretchy bolts, but stretch EXACTLY according to supplied use information, and even with those, they get replaced periodically.
 

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A few generic comments on torque and stretching...

Most all steel fasteners develop their maximum tension and reliability (two different things) when they are tightened to just before the point the steel becomes "plastic". The recommended torque is just a stand-in for the targeted tension, based upon a concept called "torque law".

Generally, all designers have a targeted tension for their fasteners. The common way is through setting a torque with a torque wrench and the specified technique (lubrication, re-setting after time, holding the torque during final creep, etc).

The more precise way is to use precision-ground bolts in which the top and tip are ground to a very specific overall length and parallel. During the tightening process, a torque wrench is NOT used. The fastener is tightened slowly until it stretches to a specified length.

In aviation, we often refer to these two approaches as "torque type" and "stretch type". For instance, the O360 engines of 180HP normally use the torque type, while the 200HP IO360 uses the stretch type, presumably for the benefit of slightly greater tension and reliability needed by that extra 5HP per cylinder. The stretch type can be used on the lower horsepower engines, but the torque type are disallowed on the higher horsepower.

So..... Perhaps this thinking applied to Alfa? There are a few differences between the 1750 and 2000 that suggest Alfa was worried about reliability at the slightly greater power outputs. The cranks of the 2000s were nitrided, for instance.

Note that, on aircraft engines at least, we are instructed to replace all rod bolts in order to log an engine as overhauled or rebuilt. There is some ambiguity when doing a simple repair, but all of the reputable mechanics I know throw away all rod bolts anytime they've been torqued up and operated. The stretch bolts are intentionally taken as close to their plastic limit as the designer wishes to go, and then all bolts suffer cyclic stressing as part of their operation.

This may be somewhat less critical on street-use car engines, as they spend most of their time at relatively low power settings. Aircraft engines spend most of their time up near their maximum rated power.

Still, new rod bolts, as one poster noted, are cheap insurance compared to what happens if one lets go.

And Gordon....

Using the above concepts, if the 2000 rod bolts were intended to be tightened using the "stretch" method in which the beginning and ending lengths must grow by a very specific amount, then they would generally be considered to have been taken to their limit after one go through an engine. Not arguing with your experience with them, as really very few rod bolts actually fail. We're sort of talking theory at this point.
 

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I don't know the relative materials used between Alfa and aircraft rod bolts, but I'd think most non-racing, normal category GA aircraft engines live an easier life than most Alfas. Typical Lycoming and Continental engines turn what, 2700 RPM at most? And hum along for hours at 2400. And they get rebuilt ever 1800/2000 hours.

Certainty and safety margin are desirable where aircraft operation is concerned, but my key point/question is, how does an O-360 rod bolt gets stressed in use compared to an Alfa 2000, which might run to 5800 RPM, or a smaller engine 500 to 1000 more than that? I ask because I truly don't know.

Thanks
Andrew
 

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Hi Anderw,
My friend John Hajduk owns Motorkraft Ltd. in Indiana. For many years he has been one of the top vintage Ferrari engine restoration shops in the USA. Many of the extremely valuable Ferrari race engines are restored from just a damaged block, with other parts made up as required. On most all of the old historically important cars, these engines are built to run, perhaps no longer 24 hours at Le Mans, but at least for the show circuit, and some actually do vintage race.
These engines have custom made crankshafts, Carillo titanium rods with the forever rod bolts I use, custom spun iron liners with modern forged pistons, titanium valves and a whole lot more, not for racing, but stress reduction on original 50-60 year old aluminum castings. These engines may not see 7000 rpm ever again, though some do. Modern internal engine components, superior to what was good in 1950, do not count off in concourse or track competition.
Though vintage Alfa's may not be as valuable, one still does not want to see a rod bolt failure in an early SS, TZ, GTA, or maybe not in your Giulia TI Super or Spider or..
You get the idea.
Modern components have quite a bit to offer vintage engines as far as normal duty service life is concerned.
I recently built a "forever" oil pump for a racing 1300 Spider Normale. The owner did not want to ever worry about oiling again. I understood his concern.
 

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The only "forever" rod bolt is one located in a vehicle that is permanently parked. ;)
 

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Discussion Starter #14
Then the discussion begs the question, how do we know the stretch limit of an Alfa rod bolt? If it is only torqued to 36 ft/lbs +/-, then I could go on the idea that these bolts are not being pulled enough to make any difference, hence they are reusable.
Airplane engines are designed to run at 100% rated power for hours on end, maximizing the stress on all the components. Even at cruise they are running 65-80% rated power even if it is low rpms. If we were to run our Alfa's at max power they wouldn't last very long would be take on this, certainly not 2000 hours. :)
 

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Andrew,

Christopher somewhat answered your question about the comparison between aircraft and Alfas. The bottom line is the very high emphasis on reliability in aircraft engines, with a lower concern about cost. Specific to your questions, however, although an aircraft engine typically lives most of its life at around 2,500 RPM, the pistons and rods are massive compared to cars, so the cycling stresses on the rod and bolts are quite high.

Anecdotally, the experience of trying to use auto engines in aircraft has not been good. Lots of broken stuff, rods in particular. Car engines struggle to live when run full time at 100%. Hence, the frequency of engine failures in racing.

My original point was not to compare the merits of the two types, but to use the aircraft example to better understand all fasteners, and rod bolts in particular.

My first awareness of using the stretch technique on rod bolts was when reading the history of the type 61 Maserati race car. It's not a new technique, but it does raise costs.

I'm going to reserve my opinion of any rod bolt referred to as "forever". I would call that an experiment that has not yet run to completion.


Also....

Although the loads on a street car engine are low compared to other engine types, it is still hot and a reversing cycle. Fatigue builds up.

When setting the tension on a "stretch" bolt, they lengthen only a few thousandths of an inch. For amusement, go grab a 10mm bolt and nut and run them up through a piece of steel small enough to get a micrometer around. Torque it to the Alfa torque setting and compare the before and after lengths. Might raise an eyebrow or two.

Lastly, the stretch method eliminates torque wrench and torquing technique errors. This is what lets the designer run high performance rod bolts at the upper end of the allowed tension range.
 

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Thanks Don. Yes, and I guess aircraft engines, even if not at high rpm, are at a good percent of rated power a good amount of the time. Edit: duh, wrote my answer before I read Chris's response, said the same thing. Duh again.

Andrew
 

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Had a friend who raced a recammed 59 Giulietta 1300 Veloce Coupe for several years in the early sixties. When in the heat of a race, he normally shifted at ~9000 rpm, unless trying to get those last few feet at the finish line, then he would take it to ~9500 rpm. Never had a problem with the stock rods or bolts in those several seasons. Those materials used were evidently quite good. Did have to replace the clutch between every race though, lol.
 

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I have never pulled an Alfa engine down to see why it had a bottom end failure and said: "Rod bolt failure!"

I once put an 2000 engined 105 into 1st instead of third on a hillclimb and must have revved it to 8000 rpm. the only damage was to my undies.

When I was doing my Mech Eng studies, I took these two different rod bolts types to show the lecturer. He said that the necked bolt would be stronger for two reasons. One was there would be a more even stress over the length of the bolt. The early bolt would have two main stress raisers: under the head, and the start of the thread. The other was that the threads would be more evenly loaded along the bolt as the necked part of the bolt was the same diameter as the root of the thread.

From what I understand, all pretty much all Alfa Nord cranks are nitrided from about the time the 2000 came out.

I was talking to a colleague the other day who had worked for GM at Fishermans Bend. He was telling me that GM had a group of engineers whose sole job was to find ways to cut costs of manufacturing engines. Maybe Alfa worked the other way and this is why their engines are so good (and they never made money).

Off topic somewhat, but: I worked once in an area of Government involved with internal fraud. A middle level civil servant was given the responsibility to organise disposal of a fair number of turbine blades. He thought this would be a good way to make some money, so he sold then to a place in the US that dealt in this sort of thing. Problem was, they arrived without usage certificates. The dealer (who now was out of pocket a few million $) made enquiries and found where they actually had come from (well, who else in Australia would use this type of aircraft and made some calls). Not sure what became of the chap, but I can't imagine he is sailing around the Great Barrier Reef now.
 

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Perhaps relevant, perhaps not...

Back in the 70's when the Japanese were starting to bring in remarkably reliable cars, and Detroit was making crap...

It was observed that the quality control in Detroit would measure their transmission gears, and if they got a reasonable bell-curve of tolerances, that was good enough. Of course, if one gear at one end of the bell curve was mated with another gear at the opposite end of its curve, binding or non-shifting would occur.

My lady friend, who worked for the Muncie transmission plant, was a QC inspector, and her testing tool was quite literally a brass hammer of a certain weight. If it could make a transmission go into gear, it passed.

The Japanese gears, when measured, did not show a bell curve around the intended dimensions. They showed a "spike". All gears were dead on the specification. All transmission shifted perfectly one to the next.

It is meaningless to consider one, or even a few data points. That a person has never observed a failed Alfa engine due to a rod bolt failure is irrelevant. How many failures have their been over 100,000 engines?

More relevant to this thread is how many failures of rod bolts have there been in non-factory overhauls in which the rod bolts were re-used?

This data, I'm quite sure, is not available. Worse, if someone had an engine blow up not long after rebuilding it with used bolts, they are much less likely to admit it than an original manufacturer looking to improve their overall reliability.

I recommend that people simply adopt what the world knows about fasteners....

1. They fatigue.
2. When tightened to their optimum tension, they are just below the point of going plastic.
3. Once a fastener has been taken to the optimum tension, and run hard with cycling stresses, it can be assumed that some fatigue has set it.
4. Rod bolts are cheap compared to an engine failure.
5. If you rely on your buddies telling you that none of their reused rod bolts have failed, remember you are probably not hearing from your buddies who blew up engines but don't want to admit what happened.

Yes - I have reused rod bolts. So far, none of them have failed.

BY LAW, I am not allowed to reuse rod bolts in an aircraft engine overhaul, so I don't. I wonder why they made that law?
 

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Engine rebuilds are difficult at 20,000+ feet. Ask how I know. They are also difficult when coming out of a corner at 8100 rpm.
 
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