RZ/RD 350 & Misc. 2-Stroke Tech BBS

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 Post subject: crankshaft rebuilding
PostPosted: Wed Nov 28, 2007 10:56 pm 
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what is the go with rebuilding oem cranks?

can someone explain this process to me, i read alot on the net that they are not rebuildable, then some places claim to be able to do it...

also where do they "weld" the cranks for increased revs?

is it worth rebuilding and welding/truing an oem crank?

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PostPosted: Thu Nov 29, 2007 8:55 pm 
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The cranks can be rebuilt, to a certain extent. On these cranks, the rod pin is cast into the center crank web. So if the pin diameter is out of spec, then you need to buy a whole center section from YAMAHA. But a lot of the time, the wear is on the rod bearing and/or rod. You just have to measure the crank pin to find out.

The general process of rebuilding the RZ crank is as follows:

1) Disassemble. This requires a pretty decent hydraulic press. And you need some special press plates to support the crank. You basically press the pins towards the center while capturing the outer crank web. Once you have separated the outer crank web from the center section, you can remove the old rod, spacers, and bearing.

2) Inspect. Measure the crank pins to make sure they are in tolerance. Buy a new center section if they are not.

3) Install new rod bearings, spacers and the rods. Do this one side at a time.

4) Press the outer crank web on. This is the nitty gritty. The more precisely you can do this, the less time you will spend truing the crank later. Most people build a jig to keep the outer web aligned to the center. This is easier done one a single cylinder crank. Twin cranks provide the obstacle of the center bearings, so the jig is not as simple. Google "crank jigs" and see if you can come up with a picture to see what one looks like. While pressing, you will want to use feeler gauges to set the gap ro the rod side clearances.

5) Repeat #4 for the other rod/side.

6) True the crank. This is the even nitty grittier part. The end of the crank can be "out" on the axial eccentric from the crank or on the radial from the crank pin. Or both! You have to put the crank in a jig and measure the run-out with a dial indicator. A high or low perpendicular with the crank pin means it's out on the axial, meaning you have to twist the crank web on the pin to line it up. If the high or low is parallel with the crank pin, you have to either pinch or spread the crank webs on the side away from the crank pin. This is a tedious process which requires removing the crank from the measuring jig, make the adjustments (usually with a brass hammer and a wedge), and re-measure. When done, you should have no more than .007 (I think this is the manual's tolerance) run-out on either crank end. I'm a mechanical neard so I shoot for .001" or less. :)

7) Weld. This is simply welding the pin to the outer crank web. This keep the press fit on the pin from moving any at all. It really doesn't have to be a big weld and you don't want a big honking bead that might rub the cases either. Also, once you weld the crank, you will have a devil of a time ever re-building the crank ever again.

Rebuilding a stock crank can get expensive if you have to buy the center section. Now that there are so many new (and cheap) Banshee cranks on the market, I question whether or not it's even worth it. Some say the Yamaha cranks are better. Other's think that a after market crank is better. But either way, if you have to buy a center section, you might as well buy a whole brand new after market crank. The price is nearly the same these days.

Steve

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PostPosted: Thu Nov 29, 2007 10:47 pm 
I had Tony Doukas racing rebuild my RZ's crankshaft a few years ago.new center(rebuildable),rods,welded and bearings fro $280USD to my door in two weeks.Over 10K on it and last year the rod side clearance had changed little from new.I'm happy.


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PostPosted: Thu Nov 29, 2007 11:40 pm 
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T-Rex, That's a great price....especially because you're happy after 10k. Michael


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PostPosted: Fri Nov 30, 2007 1:20 am 
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I have just brought a wiseco Hot rod crank. Is it generally a good idea to get these trued and welded, or are they pretty good out of the box?

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PostPosted: Fri Nov 30, 2007 2:16 am 
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T,

$280 is a darn good price! Holy cow... I've been working my ass off building my own and spending more than that!!

Saving4Parts,

At least check the crank for run-out before you put it in. I would bet that they are pretty good out of the box. But one never knows what happens to the box after it leaves production! The shipping company could have dropped it several times. Before I started building my own cranks, I sent one out and got it back out of true. It wasn't packaged well and you could tell it had been dropped. Guess when I learned to true cranks? :)

As for welding, it depends on what you are going to do with it. If it's a stock bike that revs to 9.5K, then you might not worry about it. If you think you are going to spend much time over 10K, then weld it. I think T-Rex told a sad story about messing up one of his cranks at a drag strip. All it takes is one missed gear and a trip to the moon with the revs and it's all over. :(

On a rebuilt crank, If the crank has been taken apart before, I would definitely weld it because the press fit will loosen up.

Steve

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PostPosted: Sat Dec 01, 2007 7:43 pm 
Yeah I get parts at cost and it was cheaper to send it away.Even came with a 90 day warranty.BTW it has Hot Rods so I assume it's got a Wiseco center web.I wonder what he's charging now?

EDIT:I just checked.It looks like $480 trued and welded for a new crank.Still not bad if ya ask me.I e-mailed them about rebuilding cranks and I'll let ya know what I hear.


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PostPosted: Sat Dec 01, 2007 9:53 pm 
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Location: Alberta, Canada
check ebay, I've seen new cranks for like $260 or so.

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PostPosted: Sun Dec 02, 2007 1:01 pm 
I'll have to call them as the e-mail bounced back as "undeliverable".Ebay?Not me.


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PostPosted: Sun Dec 02, 2007 7:54 pm 
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i have a local guy in australia who can rebuild my crank for $200 if i supply the parts. im pretty sure all it will need is rods bearings etc... the crank hasnt failed i just want to do it for peace of mind as i am going to be racing my banshee in the new year.

i have spoken to 3 different crank builders in aus now and they have all said not to weld the crank. the last guy i spoke with (who i am going to get to do it) comes very highly reccomended and apparently builds alot of cranks for racers. he says he has a pressure guage on his press and he knows if the fit is no good when pressing the pin into the flywheel...

even graham bell's book says not to weld cranks as it makes the parts brittle. i am confused!

if i buy a wiseco hot rods crank assembly from the states that is already welded and trued, how am i ever going to be able to rebuild it?

also on these cranks the pins are fully removeable arent they? does this mean they are welded on both sides?

thanks for all the replies guys, really appreciate it!

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PostPosted: Sun Dec 02, 2007 8:19 pm 
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on the oem crank, does the centre come apart?

it must i assume, otherwise how do they get those bearings and the labyrinth seal on....

does this just press apart and together again? is it splined or anything? this must be a tricky part to put back together i assume as the pins would have to be aligned exactly 180 degrees apart? can you weld this part together too?

thanks, mark

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PostPosted: Sun Dec 02, 2007 11:53 pm 
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smurph wrote:
At least check the crank for run-out before you put it in. I would bet that they are pretty good out of the box. But one never knows what happens to the box after it leaves production! The shipping company could have dropped it several times. Before I started building my own cranks, I sent one out and got it back out of true. It wasn't packaged well and you could tell it had been dropped. Guess when I learned to true cranks? :)

As for welding, it depends on what you are going to do with it. If it's a stock bike that revs to 9.5K, then you might not worry about it. If you think you are going to spend much time over 10K, then weld it. I think T-Rex told a sad story about messing up one of his cranks at a drag strip. All it takes is one missed gear and a trip to the moon with the revs and it's all over. :(

On a rebuilt crank, If the crank has been taken apart before, I would definitely weld it because the press fit will loosen up.

Steve


Steve, thanks for the advice. I will check the run-out before fitting it. The crank is a 4mm stroker which i am using in my 421 kit. The engine is going in Aprilia RS250 chassis for track days. I imagine it might get up to 10K briefly, but don't expect to be spending alot of time over 10K. Using a digital ignition so i can limit the revs in case of missed gears.

On the weekend i stripped down my donor egine. I noticed there is a small amount of movement where the clutch basket is rivited to the primary gear. The basket would rotate 1-2 degrees on the gear if you applied enough force. Is this normal? I don't have any experience with RZ350 motors.

Cheers

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PostPosted: Mon Dec 03, 2007 1:19 am 
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yes that is normal they have rubber cushions in there to dampen the clutch

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PostPosted: Mon Dec 03, 2007 1:50 am 
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Makes sense, i have never owned an RZ350 bike so i didn't know if they ran a cush drive in the rear wheel. So they have rubber dampers in the clutch and no cush drive then?

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PostPosted: Mon Dec 03, 2007 3:51 am 
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no idea, my rz motor is in a banshee!

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