Nytro rod bearing replacement

[jaydaniels]

Well, I'm in the middle of rebuilding a Nytro motor. My question is on rod bearing replacement. I have determined the correct rod bearings needed based on the numbers stampe on the crank and connecting rod. Are we now good to assemble or should we still check clearances to verify everything is correct? The manual recommends checking bearing to journal clearances with plastigage.

 

[raginyamaha]

Always check clearances.... you just never know. itll take you a few extra minutes for peace of mind

 

[sgilbert]

what was the situation with the motor? hard to believe that any of these sled engines are worn. the only ones I have ever read about were blown apart. Is it a new crank? either way, not checking would be a huge mistake. on the other hand, I wouldn't change the bearings for the hell of it. were they worn?

 

[jaydaniels]

The old bearings don't appear to be damaged, just changing them since we are into it this far. I wonder if we are jsut better off staying with the old ones. We made sure to index everything.

 

[1975FA]

The old bearings don't appear to be damaged, just changing them since we are into it this far. I wonder if we are jsut better off staying with the old ones. We made sure to index everything.

I would use platigauge to check the bearing clearance. If they are not worn then why replace them . On the other hand how much do new bearings cost? Has it ever been low on oil? I always like to mic. a crank for out of round and taper.

 

[jaydaniels]

Well, it is a motor I'm rebuilding for a friend. He had a leaky oil seal and took out the rod on his old one. The motor we are rebuilding is all new to us, but was not assembled. We are assembling. I'm always skeptical on used equipment and would hate to see another bearing blow out on this guy. I can see some wear on the bearing surface but its smooth, no scratches or scoring, just shinny compared to the rest of the bearing. New bearings are only $30.00 for all 3 cylinders so cheap. My only concern is that I put in new bearings and they end up further out of tolerance than the old ones. I think we will plastigage the old ones to see how they show. If not worn we'll re-use. All the Yamaha cranks have numbers etched no the them along with numbers on the rods. There is a formual to calulate which Rod bearing to purchase. From what I can figure, the numbers are related to the difference in size of the crank journal and the rod end diameter. You need the correct bearing so you end up with the correct clearance.

 

[cannondale27]

Only issue I see with checking the bearings is that you need to torque the rod bolts.They have been torqued once already from factory,once to check the clearance and once more to final assemble.Thats 3 times minumum.I would use new rod bolts for final since I think the originals would be stressed.Am I wrong in thinking this?

 

[jaydaniels]

Only issue I see with checking the bearings is that you need to torque the rod bolts.They have been torqued once already from factory,once to check the clearance and once more to final assemble.Thats 3 times minumum.I would use new rod bolts for final since I think the originals would be stressed.Am I wrong in thinking this?

NO your right on. Manual recommends changing them so I have some on order. I've rebuilt plenty of 2 strokes but this is a whole new animal. The torque sequence is a 3 step process.

 

[cannondale27]

Good manual then!I learned that one the hard way.I would bet that if you read the numbers and chart correctly and get new bearings they will Plastigage out perfectly but you never know so you have to check them.Is amazing the fits these motors are put together with from factory!

 

[Mtnviper]

Yeah, I would plasti-gauge them for sure. If I remember correctly the clearance spec for the rods is fairly tight, seemed like it was around .0012"

For assembly lube, I use my "secret sauce" which is a 50/50 mix of General Motors "EOS" (engine assembly lube) and Mobil 1.

Bill

 

[jaydaniels]

Thanks for the tip on the 'secret sauce'. Manual shows clearance spec of .0012-.0020 so pretty darn tight. This may sound like a stupid question but is that clearance the whole way around or just the difference in diameters making the actual space only half that. Where the plastigage shows only clearance where you place it do I need to take that number as being half of the clearance?

 

[sgilbert]

total clearance. again, if they are not worn, I wouldn't change them. more risk with new ones. and if they are worn, would guess the crank is too. I would be very skeptical on why the motor you are doing is apart

 

[jaydaniels]

So, I guess I'll check the original ones with platigauge and if they are in tolerance we'll keep them. We bought the motor from a performance shop. They mod them and have them apart quite frequently I guess. This is a stock one that was taken apart but never had anything done to it. Saved about 1300.00 buying this way vs assembled plus we have the opportunity to check everything out. You just never know what you are getting with a used motor.

 

[1975FA]

So, I guess I'll check the original ones with platigauge and if they are in tolerance we'll keep them. We bought the motor from a performance shop. They mod them and have them apart quite frequently I guess. This is a stock one that was taken apart but never had anything done to it. Saved about 1300.00 buying this way vs assembled plus we have the opportunity to check everything out. You just never know what you are getting with a used motor.

I was looking at my Nytro service manuel last night and even is you reuse the bearings you will have to change the rod bolts. They use the torque turn method of securing the rod caps. The bolts are stretched in this process and can only be used once. I would measure with platigauge using the old hardware and on final assembly install new rod bolts.

 

[Roadrider]

Pre-stretching has always been the way bolts are
tightened. We used to mic ours back in the 70's instead of using a torque wrench. The idea of the angle method is it gets you away from any thread or
flat surface friction differences so it's much more
consistant in how much it stretches the bolt.
(just a little additional info to 1975A'a post)
I wouldn't re-use the bearings or the bolts.