Crank not spinning smoothly

[Fingerscrossed]

Hello, I’m currently doing an engine rebuild. I picked up a refurbished crank from Geoff’sXS in Sydney, anyway put the crank in and aligned the dowel pins with the holes, cases went together easy. As I’m increasing torque on the cases the crank is not rotating smoothly, there is a ‘stuck spot’ at the same point on every rotation, when the con rods are closest to the sump. It doesn’t happen when it spins in the other direction. I’ve attached a video, does anyone have any ideas? Is it just that the crank hasn’t been put together 100% right?

Video

 

[JOHNDADDY]

??? no help, sorry.
Curious myself though.
Best of luck with it!

 

[Signal]

Possibly the cam chain catching and bunching up between the sprocket and the case. After that I would be double checking that all the bearing dowels are aligned.

 

[jay760]

If the cranks been welded its possible the weld is too thick and catching the case, spin it a few times take yhe cases apart and see where and what is catching as there will be catch marks on the casing

 

[Fingerscrossed]

Possibly the cam chain catching and bunching up between the sprocket and the case. After that I would be double checking that all the bearing dowels are aligned.

Cam chain is free, and all the dowels are definitely aligned-I've taken it back apart twice and checked.

If the cranks been welded its possible the weld is too thick and catching the case, spin it a few times take yhe cases apart and see where and what is catching as there will be catch marks on the casing

Yeah I'll have a look. I left it yesterday but I'm hoping it's just torqued down harder on one part of the cases or something-although I was doing it evenly and the crank was getting tighter with each round of torquing. I didn't get to 14lbf but I probably should have tried that

 

[Signal]

What I have seen when truing a crank is the centre bearings lifting out of the engine case. I was using the engine case as a truing stand and the crank was rephased and welded. It did come right but it took a lot of time and patience.

 

[Tone]

I'd say the crank is not true. Easy to check, support the two inner bearings spin the crank and measure the runout either side with a dial indicator. I could be mistaken hard to tell with the video but that inner web looks like it wobbles a little.

 

[Fingerscrossed]

Thanks guys! Im just slowly doing up the bolts now to check what’s happening at the correct torque.

Ive noticed that it spins fine anticlockwise, and when I spin it anticlockwise a few times it will then spin fine clockwise a few times until it starts getting stuck at the same point, when the rods are closest to the sump, then it’s free for the rest of the rotation…

It doesn’t matter how many times I spin it anticlockwise, when I spin it clockwise again it will start getting stuck after 3 rotations. Does anyone have any ideas why this could be? I really have no idea

 

[Fingerscrossed]

Really embarrassing, but for anyone else who runs into this issue- make sure you use the crank oil seal and not the camshaft oil seal, or you may experience these symptoms

 

[Its been a long time]

Really embarrassing, but for anyone else who runs into this issue- make sure you use the crank oil seal and not the camshaft oil seal, or you may experience these symptoms

That's not embarrassing, just a mistake. I've made plenty. It's good to hear it's sorted out for you. Good luck with the rest of the build.

 

[arcticXS]

Really embarrassing, but for anyone else who runs into this issue- make sure you use the crank oil seal and not the camshaft oil seal, or you may experience these symptoms

Would you mind explaining how the incorrect seal would cause the crank to bind? I am really curious

 

[Jim]

Would you mind explaining how the incorrect seal would cause the crank to bind?

Yeah, I'd like to understand that also.

 

[Fingerscrossed]

Would you mind explaining how the incorrect seal would cause the crank to bind? I am really curious

I can't say I understand either I'm afraid! I just know it was binding before and it isn't now

 

[arcticXS]

Hmm. We once had a 125 with a Yamaha/ Minarelli twostroke engine, and it seized a piston badly. Some pieces of the piston ended up in the crankcase, so i flushed it out several times with diesel. Even when the crankcase seemed perfectly clean, the crankshaft would still bind slightly in one spot. I never investigated further, as I gave the bike away. But it must have been a tiny particle in one of the bearings. The XS crankshaft is very similar to a twostroke crankshaft, so maybe it could be a similar kind of issue?

 

[jaybar6]

it is a two stroke crank. Probably from the rd350/400. It has full flywheels to fill the crank chamber, I have seen pictures, with plastic inserts filling in the round cylindrical holes in the flywheels. They removed the crank seals and added the sprocket wheel to control the camshaft, in changing to a four stroke.

I have thought of hack-sawing the flywheels to a lighter "Thor's hammer" shape.