GT550 center pot dead bearing

[Cragdog]

So I guess this means the honeymoon phase is over between me and my triple...


For now, at least.
The last 7 months or so of riding this bike have been an absolute joy, I knew I would have to do a new top end eventually, but problems with the crank are my worst nightmare

[GT750Battleship]

Hi,sorry to see these pictures..at least now when you've fixed things,you'll get many years of enjoyment out of her,before another major rebuild.My 750 suffered a similar fate 6 years ago,two mains,rods & a cracked piston the bike has now traveled 38,000km after the rebuild & giving me lots of
Cheers,
Roger
GT750A

[Cragdog]

Does anyone know of someone who can rebuild this crankshaft on/near the West Coast?
I'm going to have to find the elusive bearings as well unfortunately.

[Cragdog]

Wonder if I am better off trying to find just a replacement crank with rods?

[Craig380]

Might be worth looking for a good used crank assembly, the drawback is you won't really know how good it is until it's installed and running.

You can use substitute rods - Mike (yeadon_m on this forum) from the UK has had his 550 crank rebuilt using KTM rods and suitable bearings. He would have knowledge of this. The only problem he encountered is the rod centres are 1mm further apart than stock 550 rods, so you need a 1mm spacer under the jugs to keep the piston / head clearance right.

[Cragdog]

Does anyone know if there were any significant changes to the crank assembly through the years? I am wondering if I can put a used crank in there from another year. eBay seems to have a couple 72 cranks for sale, my bike is a 74 however.

[Suzukidave]

I also see a piece of the center piston missing and what looks like a seizure on the end piston , lower end of the rod looks real dry maybe you lost oil supply and it took everything with it ?

[Cragdog]

I also see a piece of the center piston missing and what looks like a seizure on the end piston , lower end of the rod looks real dry maybe you lost oil supply and it took everything with it ?

You're exactly right Dave, I think what happened was the lower bearing started to go dry first, then went bad causing unwanted piston movement, the lower skirt of the center piston eventually made contact with the crank case or a port and sent pieces of itself all over. (I was on the freeway ~70mph when I heard/felt the piston explode and pulled over)
What I've left out so far (because it shames me to think I may have caused this ), is that the day before this happened I noticed for the first time EVER, not only was my oil tank dry, but so were my feeder tubes except for a small amount near the valves . This baffled my mind because no less than the week prior i had just topped off the tank with fresh oil, I've always been anal about making sure I have more than enough oil before I go ride, BUT what I also noticed when I realized I had no oil, was that the oil tank cap was missing from the tank and was actually next to the battery... This means I either forgot to put the cap back on after the fill up or it rattled loose. So my next thought was that perhaps a fair bit spilled out the top while riding, but yet I had no signs of oil leaking anywhere (checked back of rear fender, oil tank itself, swingarm pivot, the ground, no oil anywhere...) This led me to think what if the cap provided some kind of a minor vacuum for regulating the pump? If so and the cap was missing then the motor could have been sucking down oil faster than normal since I topped off...??? To add to the chaos, my truck refused to start that morning. So I primed the lines with fresh oil on the GT and filled the tank to the brim again and rode to work that morning taking special care to baby the motor and pay close attention. Everything seemed solid (I've been commuting to work on the 550 2-3 days a week 45 miles each way for months), didn't hear any funky sounds or changes in power so I thought I got real lucky and it hadn't actually starved anything from oil. Also rode around during my lunch break and same thing no issues same power, reliable bike. Day ended, I rode back home about 43 miles, was about 2 miles from my house just about to exit the freeway when the piston blew

So the mystery question for me is if it's possible that the cap being loose or off the tank could have led to an increase in oil consumption?

Be gentle, I'm still grieving a serious loss...

[Suzukidave]

My first guess would be you opened the oil tank thinking about filling and then didnt . That could explain the cap still off under the seat . With the cap off the pump shouldnt supply the engine any extra oil and then without any showing on the bike due to being blown out .. i am sticking with my first guess but still sorry for the expense this will cause you .

[Cliff]

"So the mystery question for me is if it's possible that the cap being loose or off the tank could have led to an increase in oil consumption?"
No. I broke my cap in 1979, rode until 2004 with a rag over the oil tank filler, held on with an elastic band and my oil consumption never changed.

[Spamy]

I just had my bottom end rebuilt by Crankworks in AZ. They did a great job for about $500. I got a crank off of ebay to replace mine, but it turned out the cranks bearings werent that great.

I ended up sending my crank and the one i bought off of ebay to them and using the best of all the parts to make one good crank. I sent them all new seals for the task and they supplied the bearings where needed.

I tried to use Bill Bune, but they never responded to my calls and emails.