Well....This explains a lot!

[moonpup]

My 350 GTO parts bike came with the engine already broken down and stuffed in 2 boxes. Thought I'd go through them a little more today and I'm now pretty sure why the previous owner decided to tear down the engine!  

This picture of the crank explains it all.....(except how does something like this happen?)  

[coxy]

maybe the previous owner wanted to see how fast it went in first gear

[bsracer]

It can happen pretty easily on a 175!

When the bikes go on just one cylinder they will run. But if you keep on em, the crank will snap at the weakest point. i've had three cranks break in this way.

paul

[Sno Jet]

Wow, I never knew they could be so fragile.  Good post.

[moonpup]

Wow, I never knew they could be so fragile. ....snip.....

I agree, it's gotta take some pretty extreme forces to snap a chunk of steel that size! 

[bsracer]

Actually they snap pretty easy under the load of running on one cylinder for a sustained period of time. The above pic was at a race in New Mexico. The bike was running funny and I had even passed a few people. I thought it would clear out so I kept on it until a "snap". Funny thing is it still ran with the crank broken. The center pin for the crank is part of one half of the crank. I believe that the forces of the clutch and transmission against the rotational forces of the crank spinning overcome the size of the pin end. I find it interesting that the press fit of the crank web is tight enough not to rotate but the pin breaks off. I'm no engineer but I would guess it's something with dynamics. After putting another crank in and trying to make up some lost ground resulted in the following...but that is another story.

paul

[ziggy stardust]

At work I've seen 4 inch diameter steel drive shafts snap like carrots. 

I suppose in heavy engineering the same happens only er.... bigger.

Z

[moonpup]

Thought I'd revisit this issue of the broken crank and put a question out to the experts here on the site.

There's been some discussion recently about these and getting them rebuilt and I was curious the see if my broken crank has any "value" to others (or myself) before I consider tossing it.

The bearings & shaft (naturally) are worthless but can the webs/counterweights or connecting rods be of help to someone needing them?

There is/was only light corrosion and staining on them and you can see on the section to the left how much it cleaned up after giving it a light sanding with some wet/dry sandpaper.

Any thoughts?

[BRT-GTR]

Mike ,
           Don't ditch that crank, I may be able to use some of the parts to rebuild mine. Am I right in thinking the male spigot on the drive side has snapped, leaving a broken stub in the left side ?  I will have look at my crank tomorrow and message you.
        Brian.

[moonpup]

Mike ,
           Don't ditch that crank, I may be able to use some of the parts to rebuild mine. Am I right in thinking the male spigot on the drive side has snapped, leaving a broken stub in the left side ?  I will have look at my crank tomorrow and message you.
        Brian.

Haha, you've stumped me there Brian! I gotta admit that I know absolutely nothing about how these things go together.  All this time, I thought it was just one shaft that went through it

Would pic's of each broken side help out?

[slawsonb]

From my look at the pics in this thread, I think the answer to Brian's question is Yes.
...bert

[Gerrit]

Looking more closely at your first pic, moonpup, suggests that the fracture is the result of a hairline crack slowly spreading, though I would, ideally, have to look at the fracture with a magnifying glass to be 100% sure. The top LH side is rusty, while the rest is relatively shiny so separation in that area of the fracture is more recent. I have to press my crank apart, but without measuring my guess is that the radius where the smaller diameter joins the larger on the male inner flywheel  is too small. Sharp corners are stress raisers.
You can probably press the broken-off bit from the female flywheel and fit a new male inner flywheel- perhaps Richard still has a few in stock (assuming, of course, that they were individually available in the first place). If not, a competent machine shop should be able to machine a new flywheel with a bigger radius at the step, and modify the female flywheel by applying a 45 deg bevel at the edge of the bore to accommodate the larger radius. Ideally, I'd redesign the inner flywheels so that the shaft diameters are increased by 2 mm, as Yamaha did on the TZ250/350 crankshaft. This, however, depends on the availability of bearings with 32 x 64 x 16 mm dimensions and I'm not sure they exist. The lab seal would have to be custom-made as well, but this is a minor matter. Unfortunately, Bridgestone chose odd non-standard bearing sizes (64 mm OD rather than 62) for the GTR crankshaft, otherwise we could use TZ inner mains.  

[Gerrit]

There are actually TWO cracks in the shaft- there's also a rusty area at the bottom. These cracks slowly spread over the years/decades until the remaining area was simply too small to take the load any longer. This area failed suddenly, and probably at a roughly 45 deg angle, a sign of a torsional fracture.
Whatever you do, DON'T throw the bits away!  
If/when you get the crank pressed apart, ask for readings for the force necessary (usually in tons).

[bsracer]

You could always have a straight center pin made and machine (bore holes) the center wheels. That's what I'm having done for the crank on my 175 racer. I had broken the crank before and Oldswartout had some new center pins made however they were stepped. Ended up breaking again.

paul

[Gerrit]

A quick and crude measuring of the top photo suggests that the male spigot of the inner flywheel has an OD of 20 mm, can anyone confirm? It also suggests that the radius in the step, if any, is very small indeed, perhaps no larger than 1 mm. The female area of the press fit shows hardly any sign of a chamfer to accommodate a radius of the male part.
I took a look at the GTR parts catalogue and although the flywheels are all numbered on the drawing- 1, 2, 3 and 4- they are NOT mentioned in the list of part numbers. Did Bridgestone supply them at all, with part numbers only available to Rockford, but not the dealers? Or did Bridgestone only supply complete crankshafts to the importer/distributor?
It might be worth checking the RH inner flywheel for cracks in the step when the crankshaft is apart, at the very least with dye penetrant. I'll certainly be having mine tested. It would be a bit of a bummer to have the crank rebuilt, only for the shaft to snap after a few miles/kms, what?

[bsracer]

Hey Gerrit,

I've never had any 350's. I have an early parts book for the 175 that shows the individual part numbers for each crank wheel along with the seal sleeve/oring for the 28mm type seal. I've never seen those parts listed or for sale anywhere. Rockford had a crank rebuilding program back in the day so maybe that was for them to order like you mentioned. The 175 crank steps from 20mm to 16mm.

paul

p.s. Gerrit

I had a set of cases and cylinders ported by a guy who use to work for Don Vesco. He said he still had a crank or crank parts for one of Don's 350's. I can check and see if you're interested.

[slawsonb]

The lack of chamfer on the female could be the culprit in starting the crack as the small radius contacts the sharp edge during the press operation. Thoughts?
...bert

[BRT-GTR]

   Have a look at the crank cross section on Page23 of the Rockford Service manual. This shows a small gap between the shoulders on each half of the crank, to the left of the centre seal. I've no idea how wide that gap is but presumably it accommodates the radius on the drive side spigot and allows some latitude to obtain the necessary side clearance on the centre seal when it is pressed up.
 But what it does do, is to concentrate all the stress, produced by any bending or flexing of the crank, at that point. It effectively creates a weak point, not what you want. A single 30mm centre section, pressed into each flywheel should be bomb proof but would get close to the big end pins. What did other manufacturers do on twins?
 When my original crank broke, I was informed at Bill Smiths (UK importer) there had been a recall for cranks with faulty radius machining but have never seen this in writing or a service letter.  I have also recently remembered that I did once ride the bike for maybe 20 miles on one cylinder when rushing to catch a ferry, that probably didn't help!!
 Brian

[slawsonb]

That would certainly be better than the scenario I was describing, but does have the effect you describe, Brian. Unfortunately all design efforts are a series of compromises. Who knows what series led to the crank design we know and love.
...bert

I guess the moral of the story is once we all have our 350's running again, we should never push them as hard as we might have in our youth, rats!

[Gerrit]

Kawasaki, Yamaha and Suzuki did two-stroke twins in the 60s. The A1 and A7 crankshaft LH centre flywheels have two bores- a 20 mm bore for the crankpin, and a 25 mm bore for the centre mainshaft, which is in one piece with the centre RH flywheel.
I had removed the outer mains from my crankshaft and checked the radius size where mainshaft joins the flywheel. I'm not impressed. There is a radius, but it's very small- perhaps less than 1 mm. Even early Kawasaki H1 cranks, which sometimes broke, had a larger radius. Bridgestone could easily have made the radius larger- the corner of the inner race of the outer main bearing has a radius of 2 mm and the 0.3 mm shim between flywheel and bearing prevents direct contact, so a 1.5 or 2 mm radius would be no problem. The drawing in the service manual shows small radii at the corners of flywheels and mainshafts, but no radius where the spigot joins the larger diameter mainshaft, suggesting there isn't one, or wasn't one on early crankshafts.
Another thing I don't understand is how the centre flywheels are kept at the correct distance when pressing them together. On Kawasaki cranks there's a steel spacer between the main bearing inner races. There isn't one on the GTR crank according to the drawing in the service manual (or the parts catalogue)! Your thoughts on this?