Topic: balancing question to Big Jay and others

[turboguzzi]

Been a bit silent lately for the simple reason that I'm working nights to get the 500 ready for next racing season. First race is coming up on March 9. Will post on the updates to the bike as soon as I'm finished.

Among the improvements for next season, I finally managed to get hold of some very nice lightweight pistons for my CB500. They have thinner rings for less friction, semi slipper design and most important, weight a good 20 grams less than the modified stock ones I've benn using. weight is down from 160 grams to 140, 12.5% less.

Question is now if its worth the while going into crank rebalancing. In theory a four is inherently balanced (have matched the piston's weight too), but still, is it worth touching the crank webs? and if yes by how much?

Your feedback is welcome

Buona notte
TG 

[paulages]

mike rieck and jay will have the answers you're looking for (i'm betting they'll argue in favor for sure), but if it's any consolation, my 650 crank was almost in perfect balance from the factory. it only required minor balancing. the stator wanted quite a bit.

[Sam Green Racing]

Turbo, imagine geting off a Harley and then on to a Honda, That's the diference of having your crank lightened and balanced.

Also, as there is less mass to move, you will find the pick up much quicker.

You might also find a very small increase in power as the motor don't have to work as hard spinning a heavy crank.

Sam.

[Big Jay]

You are correct that you can change piston weights on an inline four crank without messing up the balance ( as long as all pistons weigh the same).

You would really enjoy what a ligthter crank does for your bike.

Jay

[sparty]

You would really enjoy what a ligthter crank does for your bike.

That couldn't be more true.  I will never regret my APE crank.

Turbo,  You should get a lightened and balanced crank.

Sparty

[FunJimmy]

Turbo

What pistons did you finally find?
What is the bore of these pistons too?

I found some interesting 59mm forged pistons for Honda's GY6 engine, but can't find any other specs like wrist pin dia and distance to top.

Did you check those out?

Thanks

[turboguzzi]

Thanks for the feedback.

Will push now to get that cranklightened in the month I've got before closing the engine....

In any case, got quite alot of rotating mass off as I am not running alternator or starter so took the rotor off the crank and sprag clutch off the primary shaft. The lighter pistons should help here too I guess.

Fun Jimmy,

i wish i could run 59's but I am limited to 57mm max as I run in the 500 class, that's 500cc +3.5% displacement allowance for overbores. I've got 56.5's.

Wasnt aware of that GY6, what is the original bore in those?

Got a link to those 59's?

Cheers
TG

[FunJimmy]

TG

I was just searching feebay for 59mm pistons and came across the GY6 pistons that I mentioned

Here is an ebay link for some 57.4mm cast GY6 pistons.

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&ih=023&sspagename=STRK%3AMEWA%3AIT&viewitem=&item=360016714617&rd=1

And the 59mm forged pistons.

http://cgi.ebay.com/ebaymotors/Forged-Piston-kit-for-GY6-59mm-cylinder_W0QQcmdZViewItemQQcategoryZ111107QQihZ014QQitemZ330206101989QQrdZ1QQsspagenameZWD1VQQtrksidZp1638Q2em118Q2el1247

It would be nice to find some more specs on these.

[dusterdude]

Turbo, imagine geting off a Harley and then on to a Honda, That's the diference of having your crank lightened and balanced.

Also, as there is less mass to move, you will find the pick up much quicker.

You might also find a very small increase in power as the motor don't have to work as hard spinning a heavy crank.

Sam.

sam,did you get off on a harley?you perv.

[turboguzzi]

Hi Jimmy

Thanks for the link. The 57.4 mm piston might just cut it, even if for the time being .

What makes you think that it's cast rather than forged?

Doing a short search I cam upon this

http://ginospr.com/page3.html

Irrelevant for me but you might want to give it a look.

TG

[FunJimmy]

What makes you think that it's cast rather than forged?

Assumptions really.

Thanks for the link. I’ll email them and get some specs.
Those lightened pistons look a little short term.

FJ

[turboguzzi]

Jimmy

Is yours a 550? What's the stock bore there and how big you want to go?

Might have a look at the catalogs I've gathered and see if there's something.

TG

[FunJimmy]

Yes, mine is a '77 CB550F and the stock bore is 58.5mm.
I would be interested in pistons up to 61mm maybe 62mm.
I know the 605 big bore kits use 61mm pistons.
What I'm unsure of is the max I can safely go on stock cylinders.
A little higher compression would also be nice.
Do you know the stock wright pin diameter?

Thanks for all the help.

Cheers
Jimmy

[turboguzzi]

15mm

Bingo! my piston catalog says the Honda XL-185 is 63mm, has 15 mm pin and the compression height should give you a nice CR boost. it's 25.3 against 23.00 stock.

You could machine the piston edge to have good squish against the head and you are there.

Enjoy

TG

[Sam Green Racing]

I think the XL185 piston is very flat topped, I ordered six for a mate who used them in a turboed Z1300 to keep the compresion down.

Just my thoughts.

Sam.

[turboguzzi]

hi sam

little 500-550 CB's combustion chamber is quite different from 750's much more tight and shallow, so do not need much bump to get the CR up. After machining the block, that is.

With the very flat topped piston in the left of pic I managed to get 12.5:1 after taking of 1,6 off the block.

TG 

[FunJimmy]

Turbo

Thanks for looking that up. I'll check them out too.

Jimmy

[bwaller]

Jimmy, I'm using 61mm pistons and the 550 sleeves are, if I remember correctly, about 0.085" thick at that bore.

[FunJimmy]

bwaller

Thanks for checking into that.
I guess 61mm is about as big as I would go.

FJ