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#859940 10/03/21 8:29 pm
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This is what happens when a flywheel lets go.
The flywheel exited through the cases as he changed up to second while attempting a top speed run at Elvington. He only shut off because it didn't feel right. It was still running and didn't spill any oil
[Linked Image from live.staticflickr.com]241742038_4221889141264210_7683194304958030524_n by Sigma Projects, on Flickr


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Impressive, in a disturbing way.

Looks like money spent on good rods transferred the issue to the flywheel. Along with most of the riders these bikes are not as young as they used to be and ocassionaly that shows up.

Last edited by BrizzoBrit; 10/03/21 11:52 pm.

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Egads !!!
Did he notice it seemed a bit revvier than usual

Trumpy flywheels bolt on to the periphery of the crank, don't they.
So it would only take a bolt to let go for something to come off ???
Or are they equally bolted with 3 bolts, and are fully circular ...

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It looks like the flywheel failed either on one of the bolt holes or balancing holes
[Linked Image from live.staticflickr.com]241826046_1531207773889299_8227173786331557990_n by Sigma Projects, on Flickr


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Eek!

I guess one positive thing might be that at least it's "only" the unnumbered half?
The owner might be able to replace half the cases?
Bugger tho'

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It has probably damaged both halves, the flywheel will have twatted both halves equally..
It makes one think, if racing a Triumph or BSA twin, replacing the cast iron flywheel with a steel flywheel may be a good idea


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Triumph cranks themselves are much more prone to break than the flywheel. In fact most 750 twin cranks come pre-cracked (rarely will the pass a crack test). They are suitable for street use, but break when used in competition. If you are "racing" a 500, 650 or 750 twin a billet crankshaft is a must. The Nourish was the "go to" item. Now the MAP cranks are the way to go.

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3 bolts


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I see a puddle of oil, and if there wasn't some oil spilled when it blew, perhaps that's WHY it blew!


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Hi GrandPaul; I thought the same.

Hi Andy Higham; happened to me in my pre unit 500 but was not the crank but a connecting rod that after thousands oc cycles hitting the barrels; yes the machinist put a larger one so after a while the conrod just cut off the small end and broke that same part of the cases included the pointy part of the timing cover (pre units have that part to hold the dynamo bearing) I had another half; a needed to match it and then filled the timing cover then mirror polished and the bike still kicking.

Hi Mr Healy; they only has the 76º not a 360 then the RO DI ones do not have sludge trap etc. I tried.

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Originally Posted by reverb
Hi Mr Healy; they only has the 76º not a 360 then the RO DI ones do not have sludge trap etc. I tried.

Many cranks don’t have sludge traps. They run for well over 100,000 miles (if not triple that in some cases) with never having a sludge trap. Just decent oil and a good filter.


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Exactly the reason I put a Nourish crank and Carillo steel rods in my 750 Triumph road racer— I added the crank at John Healy’s recommendation, and the steel rods later after breaking an aluminum race rod. Racing with stock parts leaves only one question—not if, just when, something will break.

Dave Nourish told me his race cranks didn’t have sludge traps because they weaken the crank and were unnecessary if you add a modern external oil filter.

Last edited by linker48x; 10/06/21 2:05 am.
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My 650 Triumph land speed racer has a stock but nitrited crankshaft, 7400 rpm at 133 mph...Alp,the racer who has hit 175 mph with a naked frame 650 running 90% nitromethane claims he uses stock cranks...What works for the engine held wide open for a few miles might not work for the high rpm cycling of road racing
I believe if a guy wants a billet crank, get a 270 crank and find the extra money for the cams and ignition...


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Hi Tony; after the nitriding process is necessary to regrind the journals?

--the shape of a billet crank is different and would be a hassle to let all things right if you do not have an specialized shop in your Country. Even that; when I bought a new head last year; I sent it to an specialized shop to do another 2 holes for the dual head plugs; happened that they did not do it exactly right; incredible PO in the original head did it right without precise equipment...

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The nitriding is done after the machine work.The crank journals increase about .0002 from the process..


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These crankshafts have got a few years on them now, it's possible at some point in the past the flywheel has been removed without using a 5 ton press and heating up to 100 deg C to replace it, as it should be done according to the factory manual.. If the cast iron flywheel has been hit off and back on again with a hammer, then that could also be a cause.


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Hey Tony; that is why I questioned; so what happens with that increment on the journals? is not a .020 too big? I am a bit lost with the inches...

Thanks

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Nitriding is considered to be dimensionally stable so in most cases the parts are machined to final dimensions then nitrided.
If done properly they go in bright & shinny & come out the same way

.0002" ~ 0.0051mm


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Nitrided shafts with loaded bearing should have the bearing surfaces lightly ground after nitriding to true them up. If you look at any nitride hardened Triumph camshaft the bearing surfaces have all been ground after Nitriding, but not the cam forms. Cosworth had a problem with nitrided DFV cranks developing cracks in corners of the bearing journals that was traced back to small differences in the grinding wheel corner radius between the pre-hardening and the post hardening grinding, they called up for a slightly bigger corner radius for post nitriding grinding and the problem went away.


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I have had one crankshaft ferritic nitro carburized and three cranks done with a similar process called Truff Triding. All four of the cranks came out of treatment with a dark finish that was easily removed from the journals with 1200 grit abrasive paper. The crank journals "grew" about .0002"(.0051mm) and none were out of round as far as hand held measuring equipment can detect..The shops I use have extensive experience building auto race engines and did not recommend machine work after the heat treatment process..
As it was explained to me, nitriding requires very precise control over the treatment process... Nitriding does increase surface strength but draws "elements" from the crank inner structure and may slightly weaken it..
I am no expert but my statements are from actual experience...Different heat treating methods may give different results....


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Tuftride is the trade name for ferritic nitro carburizing, it's a salt bath process done for a relatively short time and the parts are generally not machined afterwards due to its shallow depth, we have a customer who we take a very light grind off their tuftrided parts, but that's unusual. I only have experience of gas nitriding and not plasma, when you book something in for nitriding you specify the depth you want and that will dictate the time in the furnace, which can be days long!
Triumph twin crankshafts nitro carburize (Tuftride) well, although I've had no success with B25 cranks to date, that come back just the same. Nitriding is even more fussy as there's a relatively small number of steels that will nitride compared to Tuftride.
Here's a really good guide from a company in Small Heath Birmingham I use, comparing the two processes https://www.wallworkht.co.uk/content/nitride_and_nitrocarburise/


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Tufftride here in the USA is one type of several carburizing processes from what I know....I am not an expert but did listen to what the tech man told me at the heat treating plant...
I believe the steel used in Triumph cranks has no exact equivalent here in the USA...


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Hi; sorry I forgot to add a few words. I was referring to the big end shells at .020 more. Seems too much for me or is that after the process you guys still use the STD shells?

Thanks

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Originally Posted by reverb
Hi; sorry I forgot to add a few words. I was referring to the big end shells at .020 more. Seems too much for me or is that after the process you guys still use the STD shells?

Thanks
Yes the machinist can take off a few 10 thousands more than needed. Heat treating is no guarantee but the crank should last longer....My bike crank suffered no damage when metal debris lodged in the bearing shell. The journal hardening provided protection.In the US it costs about $100


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In both cases nitrogen is introduced into the metal crystals ( grains if you like ) .
The nitrogen atoms migrate into holes in the lattice.
When the metal starts to crack, the nitrogen atoms act like a dowel pin and prevent the layers of atoms sliding over each other
As such it reduces the crack sensitivity of the steel or increases the fracture toughness.
Thus stresses that would cause the crankshaft to crack & fall apart will not cause the nitrided crank to fall apart .
However it does not make much of a difference to the ultimate strength or hardness.

Carbo nitriding is very much the same as the case hardening we are all familiar with with stuff like Casenite powder.
The only difference is both carbon & nitrogen are introduced into the structure
The carbon cause a phase change so dimensions and hardness increase while the nitrogen acts the same as it does in plain nitriding
So the crux is
Nitriding = tougher usually done in a controlled atmosphere furnace
Carbo-nitriding = tougher & harder usually done in a salt bath


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