Is my crank bad?

[Sci85]

Ok, I finally got around to splitting the cases on my 550 motor in anticipation of a 650 conversion. I had been noticing a sound coming from what I thought was the lower end this summer. I thought I could feel it when placing my hand on the cases. So, I wanted to take a close look at the lower end when I opened it up. I can't see anything that pops out at me visibly like major scoring or something. But, when I spin the crank in the upper engine case, it spins freely for about half the rotation then kinds "grabs" for a 1/3 rotation before freeing up again. I did notice that one of the flywheel weights next to the stator end is real close to the case but I don't think it's rubbing. I assume the crank should spin nice and free while sitting in the case correct? What should I be measuring to determine what's going on?

Oh yeah, I'm pretty sure the big end clearance is off too. Do these bikes run this much side to side clearance? I have this on the two rods on the stator side of the engine. The ignition side rods do not have anywhere this much clearance.


Thanks for the help!

[Nessism]

Crank time.

Good news: ebay has a bunch of cranks.
Bad news: you need one.

[mike_of_bbg]

Yeah, looks like new crank time.

Just make sure whatever crank you get has the correct output gear. Some of the very early GS550 cranks used gears with flat teeth. Yours probably has slanted teeth.

Straight tooth example:

http://www.ebay.com/itm/77-78-79-Suzuki-GS550-GS-550-Engine-Crankshaft-Crank-Shaft-Pistons-/230917077189?pt=Motorcycles_Parts_Accessories&hash=item35c3baf8c5&vxp=mtr

Slanted:

http://www.ebay.com/itm/SUZUKI-GS550-CRANKSHAFT-CRANK-MOTOR-ENGINE-/310397493748?pt=Motorcycles_Parts_Accessories&hash=item484521e1f4&vxp=mtr

[Wester Cooley]

The slanted gears are called Helical Cut Gears.

The straight gears are called Straight Cut Gears or Spur type gears.

The helical cut gears are quieter than straight cut gears.

Straight cut gears consume less horsepower.

[Sci85]

Crap. That's what I was afraid of. Well I appreciate the input even if it is bad news. Why can't it ever be simple, easy and cheap? The 2nd gear dogs are rounded off too so gotta replace that as well. Might as well just find a new engine at this point.

One other quick question - with the crank looking like that, is it safe to assume the crank bearing journals are still good or would they be suspect? Short of boring to make them straight, is it possible to measure or check to make sure the case journals are good? Don't want to go through the hassle of installing a new crank if the cases are wonked.

Thanks!

[Nessism]

You should inspect the cases for signs of damage. Assuming that crank wasn't rubbing on the bearing support webs or similar, the cases should be fine.

[Sci85]

Thanks Ed. I'll lift the crank out and post up some pics.

[Sci85]

Ok, here's what the webs look like. It is evident the crank at one time rubbed against the outside support. I noticed blue silicon when I split the cases so it's been open before so I can't say for sure if it was this crank or another. I couldn't see any damage on the crank throw but not sure if I even would.

What would cause this btw?

So the question comes down to are the cases good or would you pay the money and replace the case halves?

[TeamDar]

Where are the dowels that go in the holes on the bearing saddles to hold the bearings in place?

[Sci85]

Haha, I already removed them when I took the pic. That would be a big problem. I should have noted that. Good catch!

[Nessism]

Looks like just that one web? Not sure how the side thrust is managed on this engine. The scuffing shouldn't be a problem, unless that web is a thrust surface (but don't think it is).

[Sci85]

Good question Ed. And yes, just that outside saddle that's scuffed. That's the stator side. Same scuffing on the top and bottom cases. What's weird is that if you look close, there is no scuffing on either side of the case where the surfaces mate together. The scuffing is only below that. I'm at a loss as to how that can be. Seems to me that there would be scuffing completely around the circumference?

It's a mystery at this point...

[gearhead13]

One of the things I did with my 1100 pressed crank was used plastic gauge to see how tightly the cases were to the crank main bearings.
Put in the crank, place the plastic gauge, put the other half on and torque down, measure the clearance.
I put some red loctite on my mains to take up any clearance and not allow them to move around.

[Sci85]

I can't see the main bearings moving in this engine as it uses dowels in the journals and bearings themselves. These are roller bearings btw.

[GregT]

It looks like a couple of the big ends have "spread"...ie the web has moved on the crankpin. Happens with 2 stroke cranks when they're over revved.
It is I suppose always possible that the crank has been rebuilt and those webs not pressed home far enough - unlikely I'd have thought.

Didn't know the 550's were prone to this....

At my end of the world, I'd probably press up the spread joints and weld after checking the crank was true. Nothing I can see looks overheated or very bad - and down here 550 cranks aren't common.....