Topic: Engine Balancing - Weight of Stock Pistons and Pins - CB750 K4 ??

[RRRToolSolutions]

Does anyone have the weights of the stock pistons, rings, and pins for the K4 series 750? Mine are long-gone by way of 836 ones. I'd also like to know the weight of the Wiseco 836 pistons, rings, and pins if someone just happens to have a set out and a grams scale.

I've just re-read and freshened my memory of the technical differences between centripetal force and centrifugal force. We're fortunate in this regard as the balancing of a CB750 is quite easy and can be done without counterweights or balancers, and without using the special equipment made for checking rotating assemblies. Since we have a "180 degree" crankshaft which has 2 pistons up and 2 down in 180 degree opposition, the engine can be perfectly balanced to within .5 grams (or better) by using a digital grams scale capable of measurements in the tenths. I've just bought a new grams scale today on-line with 500 x .1 grams capacity for $54 inclusive of shipping. I don't know how much difference there is between stock or forged pistons, rods, pins, and rings (yet), but I do know that 1 ounce of difference at the end of a crankpin having 2.4" of stroke equals 1.2 ounces at 2,000 rpm, but watch out -  it's 9.6 pounds at 4,000 rpm, and a whopping 153.6 pounds by 8,000 rpm! So while nothing is likely to be even 1 ounce away - there is serious benefit to be had from getting those rod ends, pins, pistons, and rings all to within a few gram/tenths of one another.

The plan -
1) Measure and record each rod "end". Not the total rod, but each end by supporting alternating ends so each is measured and compared. I wil post the results of the stockers and the new Carillos. Hopefully nothing has to be ground from the Carillos, but we'll see how they measure up.

2) Measure and record each piston pin.

3) Measure and record each piston.

4) The rod-ends will be compared (small to small and large to large) and the appropiate material removed until the heaviest is equal to the lighest since you can't add material (always taken by grinding/filing perpendicular to the crank). When they are all exactly the same - those are balanced.

5) On the pistons and pins, - I will match the heaviest piston to the lightest pin (keeping each piston with it's original cylinder since it has 800 miles on the new motor). Using the best combination of pin/pistons, remove material from the heaviest piston's boss area to match the lightest combination and so on until all are exactly the same. Again, slow and measuring often to get the perfect balance.

Before some ask why this is necessary and question the small amount of vibration force compared to the torque applied to the throw during combustion - this vibration is what causes premature failures in cranks and cases. The flexing can cause bearings to fail and induce stress cracking. I'm going for 11,000 rpm and this balancing is free (except scale purchase). Knowing that it's balanced will sure let me sleep better too.

I will keep a log and post the weight results. If someone can give me those stock piston weights - I'd really appreciate it.

Thanks,
Gordon

[bwaller]

I can only offer weight from a decent ?? mechanical scale, but weighed each component three times and this is the average of that.

standard piston (13101-300-030) 157 grams

standard rings   14.3 g

piston pin  39g

Keep us informed, as a matter of fact if someone has one, I would like to see the comparative piston weight of a Wiseco competitor, aren't Wiseco still considered "heavy"?

[Jerry Rxman Griffin aka MuthaF'er]

Gordon,

K8 piston, rings, pin, one clip is 218gm with very minimal carbon. F0 piston, rings, pin, 1 clip and having larger valve reliefs is 215gm. Incidentally both pistons are -392 but they have different domes.

I'm looking forward to your individual Carillo weights. I'd like to think they are dead on for $1000 a set. I'll be shipping my crank this week for lightening (before weight 21.3lb) and balancing and will be having a set of Carillos installed while there.

Jerry

[RRRToolSolutions]

Thanks for the inputs.

BWaller - as long as the 4 piston/rod assemblies are the same, they can be heavier or lighter than stock. It's the 2 going up and the 2 coming down that all need to be equal to eliminate the vibration. Jerry, I should see them in 2-3 days, but I only have a 200 x .1 grams scale. I ordered the new 500 x .1 gram scale today and it may not show for several days. I'm expecting the Wiseco 836 pistons to go over the 200 grams mark and I know the rods will exceed that. I also have to set-up a small jig so the rods are pivoted exactly the same way each time using a bearing/roller so there is no error. I'm hoping that $1,000 includes some balancing too! We'll see.

I'm really curious as to what the differences are in stock rods, stock pistons, pins, are. This data may be in print somewhere, but I've never heard it discussed here before. Balance a V engine is complicated and requires the spinning and measurement of the rotating assembly. 180 degree motors like in-line twins and fours don't because of the before mentioned off-set of up/down. Balancing these assemblies just makes sense and it's too easy not to check and do.

I will loan my scales out once I'm done to anyone wanting to do their own.

Regards,
Gordon 

[bwaller]

I should have mentioned those parts above are new, unused and the oil ring is three piece.

[Big Jay]

Couple of things. In order to have your assembly in balance by measuring and matching all of the components, the crank itself needs to be dynamically balanced or you are wasting time. No factory street bike cranks are balanced very close.  We get the crank itself to less that .25 (that's a quarter of a gram) end for end.

Second, if you have Carrillos, they will be perfectly matched, and the card that comes with them will show the rotating and reciprocating weight.

Jay

[Jerry Rxman Griffin aka MuthaF'er]

Jay,

Thanks for the info. My crank should be in UPS today headed your way.

Jerry

[cappy]

Jay,

When you balance the crank do you use bob weights to similate the recip and rotating mass?

Thanks,
Greg.

[Big Jay]

No. Inline four cylinder cranks are "self canceling" Meaning the  design ( two thows up, two down) cancels itself out. The crank is spun up and balanced alone then if the component weights, are the same, no matter what they are, and the entire assembly is still in balance. Inline sixes and flat fours are the same. All other crank configurations require bobweights to be attached to simulate the component weights.

This means, you can take the stock lightweight oem pistons out of your 750 and swap them for heavier, say, 1000cc pistons, without screwing up the balance.

Jay