Piston Wright - Add on

stiglarsenDK

I have sourced some HC pistals pistons for ST4s engine upgrade / drop in i was planning to do, yet they are 25 gram lighter than Stock... this would obviously require ballancing, but i was thinking if one could add those 25 grams i.e inside the rod- pin and there by avoid taking it all apart ?

brad black

i wouldn't bother unless the bottom end was apart.

Mr.R

I agree with Brad,

Usually I get everything balanced with a race motor, thinking that it's the best tuning mod' that you can do but...
Unless the motor is being tuned to the nth degree? Often it doesn't make any noticeable difference.
So I'd run the pistons as they are and if the motor vibrates so much that it's uncomfortable to ride?
Then I'd get the pistons/crank/rods/flywheel etc balanced together.

Steve R

Weed

brad black wrotei wouldn't bother unless the bottom end was apart.

A side question Brad.
What balance factor do you use for the Ducati motors?

brad black

whatever the balancer's do. don't know.

i put some hi comps in my 750 2v that were 45 grams lighter than the originals, and i couldn't tell any difference in vibration at all.

eric at clubhouse motorsports made a 620 into a 750 with std 750 pistons that are 460g, and he said it was vibey - probably too vibey to tolerate. not sure how much the 620 pistons weigh, at a guess i'd say a bit under 400g.

Weed

It's a question that has a few answers & I don't know the correct factor. Many years ago a guy in Melbourne that was a renown engine builder in his day balanced my 750 GT (after I converted to to 860) to the same factor as a Harley & it vibrated like shit.

griff851

I believe it's 50-52% depending on who's doing it.

Mr.R

griff851 wroteI believe it's 50-52% depending on who's doing it.

That's about as "Spot On" as you'll need.
I usually go for 50% but I also talk with the balancer to get his views on it after telling him where/at what revs the motor will spend most of it's time.
The smoothest motor that I've ever built/rode was balanced to 50% and...
That was a 950cc torque monster of a Bevel full race motor twin.
I get all of the Desmoquatro motors that I build balanced to 50%.

Steve R

stiglarsenDK

Piston Wright-ballance ratio-status quo

Seem we are walking into the black magic of balancing here.. My simple logic was ; make same piston/reciprocating weight and keep stock balance ratio as is..(unknown at this point). I'll do some ratio checking on other assembly's I have sitting from bottom end case tear downs..
Anyone with ratio trail and error experience ?

griff851

Bevan was searching for a new balancing guy to lighten cranks for racing. He chatted with the guy and gave him an SP crank to pork chop. It cam back with the bottoms of the fly weights cut off flat. It's now a door stop. 😁
I've seen other jobs where meat has been taken off the around the wrist pin area rather than under the crown. It's a jungle out there!

The reality is as Brad mentioned at 25g your probably not going to be able to tell the difference. The inherent vibration may move 1000-500rpm. If your serious give it to a guy that does dynamic balancing. I had mine statically balanced, but Noel had been doing balancing for years. Noel explained it all depends on whether you believe the oil in the pin cavitates or not.

Weed

I was told years ago that the factor was 100% of 1 piston (not sure if that was Bob Brown or not) so the 50% number rings true

Mr.R

griff851 wroteBevan was searching for a new balancing guy to lighten cranks for racing. He chatted with the guy and gave him an SP crank to pork chop. It cam back with the bottoms of the fly weights cut off flat. It's now a door stop. 😁 I've seen other jobs where meat has been taken off the around the wrist pin area rather than under the crown. It's a jungle out there! The reality is as Brad mentioned at 25g your probably not going to be able to tell the difference. The inherent vibration may move 1000-500rpm. If your serious give it to a guy that does dynamic balancing. I had mine statically balanced, but Noel had been doing balancing for years. Noel explained it all depends on whether you believe the oil in the pin cavitates or not.

All the dynamic balancers that I've used over the years have all allowed 25g for the weight of the oil,
for my Ducati twin cranks.

Steve R

griff851

Ah, so your guys are in the non cavitation camp. Here is my 888, flat tops and corello rods, crank figures statically balanced by old mate before he retired from active work to sell Sunnen crank balancing machines. He gave oil 2gm, he was in the cavitation camp.

The second is my Bevel crank done dynamically by another bloke Ron Young. I’m pretty proud of the Bevel one. I put Omega’s in with the corello’s hooked into the fly weights reshaping the leading edges. Came damn close just by dead reckoning. Not sure what factor he’s using or whether he’s a cavitation guy. Pete Smith is i’m pretty sure. Pete was on the Ducati importers 888 lets build a race bike team.
But it’s all a bit pie in the sky really, be buggered if I could tell the difference 20gm would make. Far more concerned with not ploughing into wallabies at 180kmph. And at 260 kmph way more interested in not dying period.😁

Weed

I'm pretty sure Ron Young uses the same guy that balanced my 750 crank

griff851

Ron is in NSW and always has been. He is the balancing guy.

Weed

I sent my crank WPG in NSW

marvin

balancing

It's a 90 degree engine so balance it like any V8. 50% I believe.