flywheel - left and right sides???

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Apr 15, 2004
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I got my crank back from the machine shop. Journals are all nicely reground.

I've been told there's a right side and a left side to the flywheel and that it's important to reassemble the crank correctly. Last summer when we took the crank apart we were going to mark it but the flywheel was already marked, with punch marks forming an arrow. But we never verified that the arrow was drawn in the correct direction, we just assumed it was. I've since come to believe that it's a bad idea to make any assumptions about anything on these bikes. So I'd like to be certain that I put the thing together correctly!

But. I was looking at the flywheel and I don't see any differences. It looks absolutely totally 100 percent symmetric to me. I can't see that it would make any difference at all which way I assemble it. Am I missing something? Is there some minor but crucial difference between the two sides? I don't want to put it together backwards and have the motor go BANG.

Debby
71/72 750 bitsa, coming back together!
 
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Jun 14, 2003
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Debby,

I don't think it makes a hoot which way the flywheel is assembled, so long as you balance the complete assembly afterwards.

Jason
 
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Jan 10, 2005
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Its because it was balanced that way at the factory and although the balance drillings are usually bang in the middle of the flywheel from side to side they are not in the middle of the hunk or iron when viewed front to back. If you assemble 180 out from where it was the balance will change. I'd put it together with old studs and get it checked by an experienced balancer first. If it's way out you can reverse it, recheck and then put it together properly.......just an idea.

I just had mine done to 63% dry and the side to side balance was so far out the guy took 1oz off the timing side cheek. Funnily enough it was super smooth at 4k + before I blew the motor but since the rebuild it's been reliable but not as nice to ride at higher rpm's. All I had done was a regrind with new pistons and rods which were 3/4oz lighter in total than the old parts, mostly the pistons. Then if you count the iron removed at -010" off both journals that's nearly another 1/2oz and that's a lot of force at speed...someone will work out how much.

While I'm on does anyone have or can direct me to a good clear side picture of a roadster tank with the 75 blue and red stripe paint job (74 would be OK) . I'm bored with black and plan to redo the bike in this scheme....just can't get the proportions right from the pics I have Thanks.
 
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That makes sense. And I see the balance drillings are quite asymmetrical. Perhaps having it rebalanced would be a good idea.

thanks! :)

Debby
 
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Jan 15, 2006
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Debby, have you closely examined the mating areas of the three parts? I don't recall if the factory ever did, but an experienced mech would match mark the parts. Typically, this would be done before disassembly of mating parts. Look for punches, chisel lines, or cross-joint scratches. Sometimes, you can match up by looking for shaddow or outline marks made by the two parts on each other. The crankends are probably not identical surfaces. Remember, these parts were made on old machinery by hand. Norton was being bled dry by the parent corp managment. BTW, the shop did put the required radius on the sides of the journals, didn't they? If not, can you say Boom? :(
 
Joined
Oct 9, 2004
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Jason's right on the money.

The orientation of the flywheel only matters if you're reinstalling it without rebalancing it.

Since we're on the topic and your crank's all apart - here's a couple quick things I've looked in to:

- At a minimum, have the flywheel magnafluxed to check for cracks.

- Deburring and chamfering the sharp edges on the flywheel and crank cheeks with a sand roll (before its balanced, of course) removes stress risers and is cheap insurance.

- Having the assembly shotpeened and/or cryo-treated is even better from a durability standpoint. The bill for mine was $75 for both treatments. Half the price of just one superblend.

- Coating the works with an oil-shedding dry-film lubricant. Performace Coatings does that work in WA, and I'm sure you've got jobbers in CO. I think it was like $45. Cheap as chips compared to a lot of NOS NVT stuff.

Anyway - have fun.
 
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