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Discussion Starter · #1 ·
The 720 stroker kit we installed this winter had a serious problem this weekend. A little history...The engine was completely torn down and cleaned several time before assembly, the crank/rod assembly was installed by a local Yamaha dealer so it could be pulled in properly, The cylinder was bored and honed by a friend of mine that does all the custom engine work for several local Harley Davidson shops and he also set the piston ring gap per manufactures specs. The final assembly was done by a mechanical engineer friend of mine that has 30+ years of engine experience.

This bike was broke in and had been ridden for maybe 10 total hours when it just stopped dead. After getting the jug off we found the bottom of the rod was seized to the crank. Luckily there was no other damage to the piston, valves, etc. I double checked all the oil passages leading into the crank and everything was clear. When I blow air into the end of the crank some of it blows back at me and I can hear some escaping inside the case but I cant feel it coming out around the rod bearing

So any thought on what may have happened?
 

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Discussion Starter · #3 ·
Yeah thats what I figured but I checked all the passages and everything is clear except the rod bearing. When I blow air into the end of the crank I cant feel any air coming from around the bearing. Oil pressure was not a problem in this bike. Im thinking the oil passages may not have been lined up properly at the factory but I guess I wont know until I send it back for inspection.
 

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check for damaged seal on crank cover clutch side, theres a seal that goes over the end of the crank that is often damaged when puttin on casecover
 

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What is the "willykiller oil mod"
 

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Discussion Starter · #7 ·
Yeah what is the "willykiller oil mod"?
 

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Discussion Starter · #8 ·
check for damaged seal on crank cover clutch side, theres a seal that goes over the end of the crank that is often damaged when puttin on casecover
I'll take a closer look at it but I think it was ok...I know that part didn't come with the stroker kit gasket set so it was the stock seal and it was not removed during the rebuild so I don't think it was damaged.
 

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It doesn't do anything for the lower rod. It is for the wrist pin.
 

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When the crank was assembled, the lower piston pin probably wasnt aligned properly to the hole in the crank web, this restricts the oil from getting to the lower rod bearing, this was a problem with the early YFZs and they had a ton of lower end problems................

As far as the oil mod, it helps lubricate the upper rod bearing, piston skirt and also helps cool the piston.............
 

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Discussion Starter · #12 ·
When the crank was assembled, the lower piston pin probably wasnt aligned properly to the hole in the crank web, this restricts the oil from getting to the lower rod bearing, this was a problem with the early YFZs and they had a ton of lower end problems................

Thats what I was thinking...It sucks we have to completely tear this thing down again to fix a problem that was caused by a bad part. At least nothing else was destroyed during the seize.
 

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When the crank was assembled, the lower piston pin probably wasnt aligned properly to the hole in the crank web, this restricts the oil from getting to the lower rod bearing, this was a problem with the early YFZs and they had a ton of lower end problems................

Thats what I was thinking...It sucks we have to completely tear this thing down again to fix a problem that was caused by a bad part. At least nothing else was destroyed during the seize.
It isnt really a bad part, it is poor workmanship, you should be able to verify if the pin is in alignment by checking the location of the timing mark on the pin in relationship to the crank centerline..................If it is aligned, you will have to dig deeper, if it isnt aligned, you found the problem.............
 

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Discussion Starter · #14 ·
They told us that the counter balance shaft wasn't tongued properly and it spun loose and slammed into the piston rod...Now since that counter shaft is keyed onto the gear and the nut was still tight with the lock tab in place how is that possible. The key was sheared on both the crank gear and the counter shaft gear so who is to say what failed first...I think the crank froze in place causing the to gears to shear free allowing the counter gear to then spin free out of time smacking the piston rod.

What do you guys think?
 

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They told us that the counter balance shaft wasn't tongued properly and it spun loose and slammed into the piston rod...Now since that counter shaft is keyed onto the gear and the nut was still tight with the lock tab in place how is that possible. The key was sheared on both the crank gear and the counter shaft gear so who is to say what failed first...I think the crank froze in place causing the to gears to shear free allowing the counter gear to then spin free out of time smacking the piston rod.

What do you guys think?
If you don't use a new keyway on the counter balancer then it could have sheared. I have had it happen 2 times when reuseing them. Most of the time because the tabs on the lockwasher gave out. I locktite and spot weld them now:D and don't have a proble. Way overkill but I don't want it to happen again. Was you crank getting oil though? Could have been a number of things though. Who is fixing it?
 

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Discussion Starter · #16 · (Edited)
I checked all the oil passages during the tear down and everything was clear and oil pressure was good when it was running. All the locking tabs were in place when I tore it back down. The keys were new and everything was torqued according to the service manual. Just to clarify the counter shaft key was sheared and the flywheel key was sheared. The large key on the crank gear that runs the counter gear is intact so I assume the weaker of the two sheared when everything locked up thus releasing the counter shaft to spin. We are waiting to see how much it will cost to replace the crank before we decide to rebuild. I guess we can use the JE 10.75:1 piston that came with the kit on the old stock crank lol....If it wasn't for bad luck I wouldn't have any :)

Would that Stage 3 cam work without a stroker crank?

This F'ING SUCKS!!!
 

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I have had two counterbalancer failures, one where the key way split in half length ways and the second where the gear failed as the pictures I posted will show.
In the first with the key failure, the nut was still tight with the locking tab in place, I suspect that the key just failed from fatigue, this was on an 01 that was about 6 months old and had never been apart, the counterbalancer struck the rod and stopped the engine dead, and I was laying on the front bumper.
The second was under race conditions, this terminated the engine, causing very extensive damage, teeth broke on either the counterbalancer drive gear of the crank drive gear causing the balancer to go out of time and struck the rod, the rod was a carrillo rod and it split the bearing housing in the rod, it was bent slightly, and the counterbalancer exited the engine smashing the starter......................







 

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WOW!!! I am so glad i was at idle when my locktab sheared and my counterbalancer smacked my crank. Looks like high rpm shearing pertty much ruins the whole engine. Seems like that whole counterbalencer keyway/locktab is a big downfall in the 660's.
 

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I now loctite it all together...............key and shaft to gear..............
 

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Discussion Starter · #20 ·
WOW...Thankfully this one happened at a very slow speed so the only thing that broke was the piston rod and the counter shaft is slightly bent. The company is working with us to get the crank assembly repaired. Hopefully they can help us out and it won't be a hugh expense. After the winter stroker/tranny rebuild the owner is working on a very tight Raptor budget.

Willy what type of loctite do you use for assembly and do you just coat the counter shaft and the key before putting the gear and nut back on?
 
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