Topic: Should I reduce my max. RPM after a overbore?

[Soos]

http://www.sohc4.us/forums/index.php?topic=26433.0;topicseen

I was reading this post, and am wondering if I should back off on my max rev limit after I complete my overbore from 59.8mm to 65mm pistons.(on a '79 cb650)

I have just recently finished machining to get correct deck height, grinding to balance piston weights, and polishing the domes to attempt to keep carbon from building up quite as fast.
The weights of the pistons varies from 70.61 grams to 70.63 grams.(minus rings, circlips, and gudgeon/wrist pin)
hrmm... that reminds me, I probable need to weigh the new piston rings along with the wrist pins and with the pistons so the final piston assemblies are very close to each other in weight when installed in the engine.


Should I even worry, since the stroke of the cb650 is only 55.8mm, and the redline is stock at 9500?
The speed at which my pistons travel at 9500rpm  would be around 3478.346 according to the piston speed calculator at http://www.csgnetwork.com/pistonspeedcalc.html.
And thats pushing the 3500 limit that has been a good rule of thumb for a long time, but not over.


Have yet to weigh stock cb650 pistons though, so i'm not too sure if the weight will make a marked difference between the 2 sets of pistons. (affecting load at TDC and BDC where a weight difference between the 2 sets of pistons weights could be causing an excessive amount of stress.)

Anyone offhand know the approx. weight of a stock cb650 piston? (?paulages?)
I plan on taking my stockers to work, and weighing them sunday night, will post weights to compare...



I'm

[MRieck]

 I would use the stock redline no problem. Are those cast pistons?? They aren't very heavy. Forget about weighing the rings. Weigh the pins and just match them to the pistons. You are super close on the weights anyway.

[Tower]

That 0.02 gram difference is applying a differential force (torque) of about 1.6 ft.lb./sec/sec on the crankcase at 9,500 rpm.  So, Soos, you're bragging are you? 

[Soos]

The pistons are ART 65mm (836 kit) installed in a 750 that was not (IMO) worth reviving due to a serious cam failure before I bought the bike.


They are cast, and before machining or balancing weights, they varied from 72.5'ish to 74.1 grams.

I mainly ground on the insides to round over machined surfaces(near pin bosses), remove uneven casting marks, and even out weights.

And having a accurate scale, lots of sandpaper, and a good rotary tool(besides a bit of patience) helps a lot.


l8r

[Soos]

not really bragging....

Ok, i guess so, I'm (as my wife says) overly anal about fine details at times.
And going this big of pistons, i figure it cannot hurt to even things out as much as possible.

If I thought I could do the machining to my own crank without jacking it up, I would lighten and balance it as well...
I know I could machine it lighter... but as to balancing it..... I dunno there.
The conn. rods however... I'm thinking of just balancing, and having them shot peened(and double checking the weights after peening as well)

I'm tring to do all this on a shoestring budget, so all I can do myself I'm up for... even if it means I wait a season or two to get everything done to a level that makes me happy.

After I finish fabbing up the valve spring compressor, I plan on doing a head-job as well(unless the missus allows me to spend a few hundred to have that done...  hi MReick...! )


l8r

[paulages]

Anyone offhand know the approx. weight of a stock cb650 piston? (?paulages?)
I plan on taking my stockers to work, and weighing them sunday night, will post weights to compare...

if i can find my stock pistons, i'll weigh them for you.

[Tower]

Sounds like a great plan.  Consider, the rods could only be improved by removing the casting flash and smoothing before you shot peen.  (As long as you don't go nuts with the grinder). And if no cracks show up after peening, there's always a few micro grams that can be taken off by polishing them and hardening in the oven.

Balancing the crank has got to be the biggest gain for revability right after piston weight matching (maybe even more important?).  If you can do all that, never mind 9,500 - you'll easily be into the 10K-11K range.

Here's some food for thought:
If the pistons, rings and pins weight 80 grams, then at 9,500 rpm they are applying a force of 6,500 ft.lbs on the connecting rods.  At 10,500 rpm the force is about 8,000 ft.lbs.  But, at 5,000 rpm its only about 2,000 ft.lbs.

[Soos]

Sounds like a great plan.  Consider, the rods could only be improved by removing the casting flash and smoothing before you shot peen.  (As long as you don't go nuts with the grinder). And if no cracks show up after peening, there's always a few micro grams that can be taken off by polishing them and hardening in the oven.

Balancing the crank has got to be the biggest gain for revability right after piston weight matching (maybe even more important?).  If you can do all that, never mind 9,500 - you'll easily be into the 10K-11K range.

Here's some food for thought:
If the pistons, rings and pins weight 80 grams, then at 9,500 rpm they are applying a force of 6,500 ft.lbs on the connecting rods.  At 10,500 rpm the force is about 8,000 ft.lbs.  But, at 5,000 rpm its only about 2,000 ft.lbs.

...... 8000 ft lbs?? @10,000 rpm???
wow.....
I think I'll possible forgoe something(cam?)... or save for a while and have my crank balanced IF i decide to machine it.
Anyone know of some buisness that does(or has done) lots of cb cranks?(lightening and balancing)

I'm hoping to make this build last at least 20,000 miles of me riding it...
lately i've hit 9 grand about 5-6 times a week on my stock motor(well besides the carbs and coils).
The sound of the motor at 9500 in fourth gear is nice.... 
Gotta pass those truckers somehow....

l8r

[Jerry Rxman Griffin aka MuthaF'er]

If you want your crank lightened and balanced talk to Big Jay (one of us) at APE.

[Tower]

Not to worry Soos, steel connecting rods have an ultimate tensile strength (stress tolerance) in the range of 80,000 - 180,000 lbs/sq.in. and aluminum around 70,000 lbs/sq.in.

Stress = Force / Cross Sectional Area

So, an 8,000 ft.lb force acting on a rod (say its about .4" thick by 1" wide, hence having a cross section area of .4 sq.in.)  gives that rod a stress of about 20,000 lbs/sq.in.  Meaning, even at 10,500 rpm those rods have plenty of room before failing.  Indeed its probably just barely reaching the yield stress point or at the worst its comfortably within the plasticity range of your rods.