Topic: Modify the pistons, or the head?

[Soos]

Hey all,


Been lurking more than actively posting for a while, crazy hours at work.
(at least I have a job now though)



Anyways.....
I am preparing for my next overbore on a 650 motor that has been waiting in the wings for over a year now..

The block has been bored and new sleeves(CB900) installed for the 66mm kz750 pistons I will be using.



My most recent question I keep asking myself is this:


Should I try to get the proper clearances needed by opening up the head?
Or trim the pistons to acheive the proper clearances needed?



There is a ease of mod factor between the piston modification and  the head opening(and doing it right).

Modifying the pistons is WAY easier, and easily repeatable of the end results.
I CAN mod the head a bit but am worried about not getting each one exactly even.
I have access to do flow testing, but as to exact volume testing i'm limited on how accurate I am able to be with that.

If it's better to open the head( I am suspecting this is the case ) how close is close enough for volume of the compression chamber from cylinder to cylinder?
What would the noticeable efects of un-even combustion chamber cc's be?
 





thnx all





l8r

[turboguzzi]

I say touch just pistons.

The only work worth doing on the head if you are really maniacal is cc'ing, i.e. matching the volume between the pots,

pretty common in hotrodding

http://www.circletrack.com/techarticles/ctrp_0611_cylinder_head_volumes/index.html

[kos]

DEpends...


How much do you need to "open up"?
 ie: how close is the top of piston to head at closest place?

Mark

[Soos]

NICE LINK!

And the clearances, not sure yet.
Judging from height measurements of these pistons to the stocker cb650 pistons, there is a GENEROUS gain in deck height, as well as overall height(CL of wrist pin to top of dome) than the cb650 pistons do.

The chance of 3mm gain being runnable,without mods to either the head or pistons, is unlikely.
I want to up my compression but thats a bit much to leave alone.

IMO Both the head and the pistons could take the amount I want to remove without ill effects (I HOPE!)
I'll be modifying the pistons nontheless, if for nothing more than re-location of and depth of the valve releifs.
But the thought of keeping the KZ750's dome intact and milling the head is intriguing.




To tell the truth I was SERIOUSLY doubting that these KZ750 DOHC pistons actually had only 1 intake and 1 exhaust valve releif on the piston.... They do!
Found out that only 2 or 3 years of kz750 pistons are useable in a BC motorcycle as an overbore.
Ask me why I have 2 sets of KZ750 pistons... and one is for show, one is for this overbore.
One is 6(?)mm taller.
The useable set is 3 or 3.5mm taller overall.(center of wristpin to top of dome, deck height is not an issue, I'll re-set that as needed.)



Similar to the '77/'78 CB750 pistons, they have more of a dome to them then the '69-'76 cb750 pistons do. Because of that, they are fairly easily modified for a 61mm cb650 overbore.(1.2mm bigger, or 652cc's.)
And conversly the 550 would blow up using those same pistons, or you would machine a hole in the top of the piston trying to get the height down to a useable height.
But a 550 can use a '69-'76 set of pistons and get away with minimal machining effort needed.












l8r

[turboguzzi]

not sure that i can follow all the details, sometimes a picture is worth a thousand words...

in any case, there sure is a limit to how much material you can remove form a piston crown, go too far and you will weaken the piston's dome, there was a modified piston with a hole in the forums some months ago.

another easy option to reduce deck is to add more cyl blocks gaskets or even make a aluminum gasket, say 1/16" thick.

Assuming the camchain lets you get away with it

TG

[Soos]

I've measured the piston and 3mm is do-able and leave enough meat to hold up.
I have come across a few web sites on this matter, and know there is enough material to machine it as needed.

The head as well.
I plan on using a .5mm thicker base gasket, as well as a .5mm thicker head gasket.




l8r

[Pinhead]

Do both. Run as much compression as you can mechanically get. And do THIS.

[Sam Green Racing]

I think you'll find he knows about that as he was involved in the thread.

Soos
Just a butcher with a carbide hatchet, definitely not a
Master

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   Re: CB650 Surface Turbulence
« Reply #2 on: October 07, 2008, 11:56:57 am » Quote 

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Interesting...

Have come across this post elsewhere.
Are you doing a before and after dyno runs or anything to compare against?

Interesting to say the least.
I don't see how it would hurt, or hinder performance, not sure whether it gives a gain. Never tried it(yet?   )
Post the results either way good or bad.


I would watch for carbon buildup on those ridges.
Mabey pull a spark plug once every oil change and inspect the piston near TDC.
Carbon buildup can turn into pre-detonation over time with a lean engine.


l8r
 
 
« Last Edit: October 07, 2008, 12:02:08 pm by Soos »  Report to moderator    Logged 

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-=≡ Soos ≡=-
"Daisy" '79 cb650 - 652cc's modified to the hilt with leftovers and my custom made parts
 
Sam.

[Pinhead]

I know that, but it being a year ago I thought I'd bring it back up. Also, we've PM'd about the subject before. That's why I didn't make a lengthy post about it.

[Sam Green Racing]

That's fine PH, keep it coming.

Sam.