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Discussion Starter · #1 ·
Last season I was running an alloy (aluminum) 750 barrel on my Triumph. After only a few months the studs that held the head down to the barrel ripped the threads out of the barrel and that was the end. I am going to helicoil/timesert the barrel this winter so I can try running the barrel again next season.

After some 'research' it seemed this was an issue in the 60's and 70's when these barrels first came out for racing purposes. The barrel and studs would expand at different rates because of their different materials and this was found to be the cause (maybe?). On another board, some older (age wise) members mentioned that 316 grade 80 stainless steel will expand with the barrel at the same time and tehy remember the issue happeneing a long time ago. Unfortuately the memebers aren't around anymore so I cannot pick their brains. Does anybody know anything about this kind of stuff? Where to go to research or who to ask? Thank you
 

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What you're looking for is called the Coefficient of Thermal Expansion. All materials have one. Aluminum is high, as you mentioned- much higher than steel. Stainless steel is between the two.

You'll find charts all over the internet that provide data. The numbers will vary a bit, but you'll get the gist.

If you're looking to match CTE's, just compare the first digits, and make sure you're always looking at the same units (metric, versus US for example.) The chart at the link below should help.

The exact makeup of the material will define the exact CTE, but it does look like a 300 series stainless would be a good place to start.

One thing to keep in mind- it may be tempting to match the bolt and barrel material together, for a perfect CTE match. That's correct in terms of CTE, but if you're using softer materials like stainless steel or aluminum, use of similar materials can promote galling. Ex: a stainless bolt threaded into a stainless barrel. When threading the fastener, a small burr can easily start a snowball effect that will prevent you from ever getting that fastener out. It's not pretty. If you use like materials, just be sure to use a moly or some type of never-seize.

http://www.engineeringtoolbox.com/linear-expansion-coefficients-d_95.html
 

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Many people have run those barrels over the years without trouble, perhaps it was over torqued somewhere along the line. Helicoils would probably fix it. (Think VW engine cases.)
 

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What brand of aluminum jug are we talking about? I have talked with a manufacturer of aluminum jugs and he states that the materials of the 60s and 70s were inferior. Basically that the tensile strength was nerver uniform. He went through many jugs and finally quit making them. Perhaps with todays technology this would not occur.
 

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Discussion Starter · #6 ·
Hello - I was told it was a route jug - now i must also admit that the head was flowed/ported - larger black diamond valves were fitted - a road/race mega cycle cam AND the included 10:1 or 10.5:1 compression pistons were used. Now do I "need" to use it? No - its just for street use, but it would be nice to repair it and not have to get another barrel. This tiem around though I will be running 9:1 hepolites and not using the high comp pistons.
 

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The barrel and studs would expand at different rates because of their different materials and this was found to be the cause (maybe?)
Seriously doubt it. If the thread fit was accurate to start with, there's no way the difference in expansion would cause a 5/16-3/8 diameter hole to get so much larger than the stud that it would pull out. I'd suspect improperly sized studs or improper torque.

Bob
 

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Discussion Starter · #11 ·
Thanks for the replys so far. I honestly don't know what caused the studs to rip the threads out of the jug. The torque settings used were those given in the Triumph workshop manual and set with a torque wrench. I don't recall a name on the base of the barrel but I will look this tomorrow. The mention of improper bolts might be something to look at though. The were bolts that had to be 'sourced' for that barrle now that i think of it. Ill check that tomorrow too. Thank you again
 

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The torque settings used were those given in the Triumph workshop manual and set with a torque wrench.
Maybe that's the problem ?. The Gilardoni 10 stud alloy barrels for the late T140's would pull through if you used the torque settings for the cast iron barrel from the manual (I think they were about half from memory)

I've tried fixing those with helicoils, but you lose so much head gasket surface area its an uphill battle trying to get them to seal afterwards, even with a Harris composite, flame ring type head gasket it was touch and go.
 

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Maybe that's the problem ?. The Gilardoni 10 stud alloy barrels for the late T140's would pull through if you used the torque settings for the cast iron barrel from the manual (I think they were about half from memory)
That's a good point. Steel fasteners in aluminum should have a thread engagement length of approximately twice the major diameter of the fastener in order to fully utilize the tensile capacity of the fastener. That's a rule of thumb I've always used in machine and fabrication work and it seems to be adequate.

Bob
 

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Rouitt jugs were cast iron. They were made to bump the Triumph's up to 750cc for AMA flat track racing. They were offered through the Tri-Cor catalog and deemed legal for competition as they were a manufacturers approved part. They were even marked with the same little triangle at the bottom, just like the stock Triumph's. Some mistake it for the old AMA logo, but it was on the stock Triumph's too. When you turn a Rouitt jug over, there's no webbing in the castings. They are solid.

here were many other aftermarket aluminum and cast cylinders on the market in the 70's. Some good, some not so good.

Currently MAP makes a good quality aluminum cylinder with matching pistons. http://www.mapcycle.com/

I have seen many aluminum cylinders with steel bolts in them. Sometimes there's one heli-coil in them, sometimes they are all heli-coiled. Could be a number of things. I have not seen a heli-coil fail unless it was torqued too much. Like really abusive torque. The ones with a fine thread seem to fail the most. I believe that coarse thread heli-coils are better for aluminum applications (Not 100% sure. I just believe it)

Zilla!
 
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