lightened crank?

So this motor I bought has a lightened crank. The flywheel has been machined down a bit. That's a popular mod for Ducatis but I've never heard of anyone doing it on a Norton. So I'm wondering if it's a good thing or a bad thing. I don't know how much it was lightened, only way to tell is to weigh it and compare to a stock one.

I have to pull this motor apart to replace the cam so I could just put my standard crank in. And either way I'm going to have the crank rebalanced.

So, I'm looking for opinions. Go stock or go light? Has anyone ever done this before? :?

Debby

Deb,

I prefer the heavier stock crankshaft, as it makes the engine less prone to stalling at low speeds, compared to the lighter crank.

Jason

Lightened crank? You rarely see this mod on a Norton without a flowed head, larger valves, cam, etc. Biggest advantage would be quicker revs.
I agree with Jason, for the street, the extra flywheel weight makes a smoother motor at idle.

"Everything affects everything."

I have to hand it to you Debby, you seem to run into more problems or issues on your one Commando than some people do in a life time on several.

I want to say there are people on this forum who have forgot more about Norton's than I'll probably ever know but I do have strong opinions on crank weight.

I have owned 750's, 850's and my current project a Kenny Dreer VR880. I am sure you are aware the Dreer is a hot rodded 850 with a lightened crank. The Dreer screams but I am not sure I like the lightened crank. With it's high first gear in the CR gear set and high compression, it stalls easily and needs a high idle. You have to blast away on take offs. It makes for more of a hyper ride. Racing, it would be great, street use a little more difficult.

Between the 750's and 850's, I know I am going to take some heat here, but I think the 750 is a little sweeter. It just feels right.
I wasn't at the design revue with Kenny when he laid out the VR but I think the stock 750 crank weight feels right and think a hot rodded 850 with a 750 crank weight would be preferrable for street use.

I don't know how much your crank is lightened but I would lean to the stock crank.
The more I learn about Commandos, the more I appreciate how much went into the original design. We do make improvements and poke fun at things but overall the basic Commando is a nice, well rounded motorcycle.

Debby,

The stocker weighs in at 22-24-ish lbs. What's yours weigh?

Something avoided in this thread is that all that iron being slung around in the cases is one of the factors responsible for case half failure, shortened main bearing life, etc. As for the twin's need for a huge flywheel for relaxed "roll on" performance - Nortons make gobs of torque at low RPM's, so I'm having trouble accepting that we need all that stored inertia to move off the mark.

Conservatively and correctly lightening and balancing reciprocating / rotating engine parts can produce a much smoother running and more responsive engine. I brought my crank down to the bottom end of the stock weight scale (a little under 22lbs) without any apparent negative consequenses. Shakes a bit more than I would've thought (though less than most H*rleys I see). In its newish state, it idles at 900rpm w/ 32mm carbs, flowed heads, oversize valves, 10.25:1 CR and a Johnson j360 cam. Doesn't need a lot of rev's to get moving either (22t sprocket).

Just stirring the pot,
David

Dr_Hiller said:

Debby,
The stocker weighs in at 22-24-ish lbs. What's yours weigh?
David

Click to expand...

That's the $64,000 question. I don't know, and the guy who had it done "doesn't remember" :roll:

Only way to tell is to take it apart and put it on the scale. Probably safer to go with the stock crank, although if they only carved off a pound or so maybe it's ok.

Debby

What does lightened mean? If stock is 22 - 24 lbs and one preps a crank to 22 lbs, this is more consistent with blueprinting, not crank lightening.

IIRC the Dreer has been lightened 7 lbs. Which is consistent with what Steve Maney does for his full race motors. This makes for a different motor.


Maybe this crank was lightened within factory spec. Only a weight measurement will tell.

A Commando crank weighs 22 - 24 lbs.
Is the difference 750 vs 850, or are the tollerances that far apart?
I always thought the 850 crank was heavier?
Anyone out there weighed the two cranks?

Debby,
Speak w/ Bob Martin about this. If I remember correctly, Bob installed a 750 crank in his son Bill's 850.

Weighed my 850 stock crank on the bathroom scale 23.5 lbs. Journals were turned 10 thou but I doubt that would account for half a pound.

Scooter

This one supposedly was lightened about 2 lbs. Now I'm getting curious about what it, and my stock one, actually weigh!

Debby

Well, you're gonna want to clean out the sludge trap and install new bolts/studs before sending it out to be magnafluxed, micro-polished and dynamically balanced... so throw it on a calibrated scale (your health club, doctor's office, local butcher-shop, etc.) after disassembly, cleaning and test-assembly.

(You'll want to tear it down again and thoroughly clean it again after it returns from the balancer, so don't bend the lock tabs or peen-lock the nuts.)

If its 21lbs or greater - and passes the machinists inspection, I'd assume its fine.

I have to second dave's advice here. As has been pointed out by many people that do it don't be afraid to pull the motor down and find out what's happening. A couple days work will put your mind a rest for many years of hammer down riding. I can't tell you how many times I have been told that running 1 5/8 exhaust would make my bike weak at the low end by folks who have never had them. All I can say is lets run em ,they never want to. Same thing with points bikes they just don't live well at 7000 RPM, were the fun is . Two pounds off that cast center piece is just a clean up cut for a rider of your wt. Debby. norbsa

Yes, it occured to me that I'd better take it all apart and check everything. What if the guy who put it together didn't clean out the sludge trap and it's all full of metal shavings like my old crank was? Better safe than sorry!

Besides the crank needs to be rebalanced and the cam needs to be replaced anyway. So not that much extra work.

Debby

crank balance

Hi Debbie,

I had my '73 750 engine balanced in '83 at Penneman and Richards in San Jose. When you do this all the moving parts - pistons, rings, gudgeon pins, connecting rods and crankshaft need to go to the machine shop. Everything is weighed and metal removed so that both sides are equal. Then the balance factor is adjusted by drilling holes in the flywheel to remove mass if needed. It's kind of like wheel balancing. The nice thing from P&R was they stamped a number on the flywheel and kept records of the job so that when I needed go oversize on the pistons, I took the new pistons to them to be checked/rebalanced. I'm not that fussy anymore and P&R have retired.

Yep - I'd recommend pulling the motor apart fully at a good opportunity (a very good idea if it's never been done in your ownership). I was looking at a top end rebuild only last year, but decided it was worth pulling the bottom end apart as it had not been done in the 18 years I'd had the bike - glad I did as all sorts of carnage was evident when it came apart. At some point in the bike's past a cam follower (most likely) had disintegrated and taken chunks out of the bottom of the rods and left shrapnel embedded in the cases. Now I've got thousands of happy miles ahead after having the crank balanced, new polished rods, new pistons etc. inside the motor - money well spent!
I'd go for a balanced standard crank, and spend some cash on polishing the rods to ensure they are as good as you can get them.
Cheers
Nick

In balancing the crank is a 52% balance factor the best?