Topic: Quench Band / opening combustion chambers for 65mm bore question ?

[Garage_guy_chris]

Hey guys. Im at the point where i need to finish the head. One of the last things i have to do is to adjust the combustion chambers to match the new 65mm bore by adding a quench band. Hondamans book and other threads on the forum has this procedure pretty clearly illustrated my question is this:

The edge of the intake valve seats are more or less even with the heads surface so do i cut into them to get the quench band or work around the radius of them in the effective area?
The exhaust valve seats are recessed enough i can get the quench band in there without effecting the seats.

I used some dry erase markers to color things so different parts can be easily identified



Here is the layout for the quench band i just used sharpie as i ran out of blueing layout ink flipped the newly bored cylinders onto the head and scribed around the bore. Now i know where the taper needs to start into the combustion chamber

[hotdog]

Exactly what I'll be doing.

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[turboguzzi]

send some goods pics of the piston crown and we'll be able to help more.... whatever you do, dont touch the head surface, you are too close to the seats.

[Garage_guy_chris]

the pistons are the Cruzin_image flat-top 836cc kit'

[Old Scrambler]

At the very most............you could dress the edge of the chamber 'skull' to eliminate any sharpness. As long as you have 35-thou or more of clearance between the piston and the skull you should be good to go.  HONDA did the same when they put larger pistons in their 250cc twins to make the 305.  Your choice of head gaskets and proper torque-specs may be critical.

[PeWe]

My head got that back in the 83-84. The head must have matching pistons, if not lower compression.
My first pistons forged  Russ Collins 836. My tuner told me that they machined the crowns and matched head with opened chambers. I blew them and replaced them with Action Fours 836 pistons of std model. Engine got less CR  but worked well and nice despite lower compression.

I found  pistons 2012 that probably are earlier Russ Collins cast pistons, only one oil ring. I reused my old 3 piece oil rings from old AF pistons.
The NOS RC have wider crown and match my chambers better. I have read about others on this forum using same pistons that got ticking sound when pistons kissed the edge of the chambers when they had std size.

Here photo from my head after refurbish 1 year ago by same tuner that did the job for over 30 years ago. Chambers are about 65mm in diameter. Inlet valves 34mm, Ex 28mm.

4 chamber most pitted of the chambers since almost 20 years in barn without preserving oil sprayed inside. Only kicked the engine twice a year to avoid rings to rust and got stucked. I should have sprayed thin layer of oil in plug holes every 3 year or so.. Seats corroded too.

[turboguzzi]

ok, seen the pics.
you could get a pretty good squish band from the black area that you marked, you dont need to do anything to pistons. you'd need to do a dry assembly of block and head with the final gaskets you are going to use , torquing the bolts (No cam needed) but before dont forget to place 4 pieces of solder of at least 1.4-1.6 mm of solder in four corners of one piston. turn that piston over TDC so that the solder gets sqaushed (hopefully). for good effective squish you'd need the solder to be squashed to about 1mm/0.40". thats pretty much the minimum you need to get from the squish combustion enhancing turbulence. in my race motors i go as low as 0.75-0.8mm
if solder doesnt get squashed you are way out with squish, if solder measure say only 1.4mm, then you need to mill the block (not the head, this will not change squish) by 0.4mm. Lightly chamfering the chamber's edge is not a bad idea.
You happen to catch me in a day when i been doing just that in my Supermono racer, so here are  pics that explain. as i am using an MSL gasket, i expect the squish to go down to 1mm when fully torqing. for your check you can tight the bolts around that psiotn snug but not full torque to not risk wrapping

[Garage_guy_chris]

Pewe: thats a sweet looking head, lacking all shrouding, just a dome or Hemi'd head if i know what Im talking about. Your Intake valve seats look sunk deeper into the head aswell

TurboGuzzi: Thanks for the explanation and photos that clears a lot up for me, BTW nice yamaha! I bet a 250/450 supermoto would be so much fun.

Ok this is all gold and incredibly helpful I can't explain how much I appreciate all the help. I'm getting a better understanding of whats going on and what needs to be done. I started by just referencing hondaman's book, I hope he doesn't mind the photo.



Well it looks like the thing to do is to finish the bottom end. After that I will have to check and set clearance's before doing any modifying to the head. Hopefully the pistons will come in under the deck height @ TDC with just one base gasket and no spacers but i will visit that as i get there. 

Maybe some of this is overkill for what Im building but I want to do it as its good practice and the components are cheap enough i wont cry (to much) if it all catastrophically fails. Hopefully down the road i can jump in the deep end and get into 900cc Plus stuff. These old 750's are such an awesome platform with so many options and to boot in such a classic beautiful package.

[bwaller]

Even that narrow width will help squish the charge into the open chamber, as long as the gap isn't too large. If you massage away that material you lose most of the squish effect with a flat top piston.

[turboguzzi]

It's a 700c.c. supermono, not a punny 450 .... been dynoed at 75 rwhp
Sorry if i dont agree with hondaman ideas, it will not be the first time. what i am trying to tell you here is essentially  DONT to do the angled cut in the head, i.e. NOT to do the "potential quench band cut" in the drawing. with flat pistons youll get wonderful squish if you bring the crown close enough to the head surface, so leave the head alone, get the right squish by milling the block.
the opening of the chamfers is a bit old school approach, current approach to squish is to maximize the flat areas of piston and head. check out latest heads of air cooled 2 valve Ducs.
the same idea is applied in all 4 valve heads today, nobody "opens the chambers" in them (though you could), instead, they leave large flat areas to do exactly the same as i am suggesting here.
The only evidence i have to the effectiveness of my idea (well, not entirely mine ) is the burn pattern i see that leave the piston crown clean in the squish zones due to the high gas velocity and the super low ign. advance i run to get max power, in the 25-28 degrees area.

[PeWe]

I guess that open the chambers and grind it into hemispherical shape is old school. My tuner got old memories when he saw his old job he did for over 30 years ago, mentioned the squish.
I found this site earlier when searching on the subject. "Hemi is no good...."
https://www.rbracing-rsr.com/squishcalc1.html

I guess the head jobs are different today.
This encourage me to use my old CB900 DOHC rods that are slightly longer in next CB750 SOHC project.

[Garage_guy_chris]

Well im finally back at it and building the engine. The pistons ended up pretty much even or slightly over the deck height.  I did the solder test as suggested and ended up with 0.048" - 0.050" with it torqued to 8.0 ft/lbs so it will shrink a few more thou with full torque. ~ 0.043"-0.045" clearance in the quench band. Based on reading this appears to be in a pretty good range.

 I think im just gonna clean up the edge radius and deshroud the chambers, check and match CC's and be done with it. 

[Old Scrambler]

That amount of clearance sounds GOOD

Be sure to begin on the rich-side when you start the motor and tune the carbs. It only takes a few moments for a detonation-event to eat a piston.......or more

[MRieck]

My current engine runs flat top pistons and required -.045 off the head to get the compression I wanted. A nice squish band was formed with the milling. Deck height is .040. Current head I am doing requires -.055 milling secondary to the chambers being enlarged for the old 71mm RC pistons. And it does get into the seats.

[PeWe]

My current engine runs flat top pistons and required -.045 off the head to get the compression I wanted. A nice squish band was formed with the milling. Deck height is .040. Current head I am doing requires -.055 milling secondary to the chambers being enlarged for the old 71mm RC pistons. And it does get into the seats.

Any photos of the technical art, Mike?

[MRieck]

My current engine runs flat top pistons and required -.045 off the head to get the compression I wanted. A nice squish band was formed with the milling. Deck height is .040. Current head I am doing requires -.055 milling secondary to the chambers being enlarged for the old 71mm RC pistons. And it does get into the seats.

Any photos of the technical art, Mike?

No but I should have the one with -.055 back soon. I'll take a pic.

[Garage_guy_chris]

Ok so i got a huge amount done today.

started by highlighting areas that needed attention


Second thing I did was use a small file to round over the sharp edge of the combustion chamber leaving as much quench area as possible.


While working i found more area's that needed attention


After cleaning things up with the dremel and a 1/4" sanding drum


After doing a preliminary on all 4 chambers i CC'd them and found 22.0cc, 22.1cc, 22.5cc, 22.0cc so i did some additional work and got them to 22.3cc - 22.5cc 



after making the adjustments i spent some time doing final cleanups and finishing




Oh and heres the ports i did about a month ago. I didn't go too extreme i just worked till i blended the molding lines out and cleaned up the area's suggested in hondaman's book and on the site.

[dragracer]

Okay, this is an old thread but are there any final pictures or notes??

[bear]

Sorry Frank no pics, but a different approach.
Good heads are at a premium.
Pistons are relatively cheap.
We modify our piston's configuration to suit the head.

Cheers,
Brian
 

[kmb69]

Excellent approach, Brian.

[dragracer]

Sorry Frank no pics, but a different approach.
Good heads are at a premium.
Pistons are relatively cheap.
We modify our piston's configuration to suit the head.

Cheers,
Brian
 

Brian,

That's the way we handle our KZ set ups. It just seems to make more sense than cutting a perfectly good head. However, we do massage the head somewhat to insure the pistons fit the combustion chamber without interference.

[dragracer]

Bumping it up for future reference

[PeWe]

 Garage_guy_chris, Is the bike a runner?
Did you use the cruzinimage 836 or other that can use a higher lifting cam?
Which cam, carbs and have you checked the rwhp on the dyno?

[Garage_guy_chris]

Garage_guy_chris, Is the bike a runner?
Did you use the cruzinimage 836 or other that can use a higher lifting cam?
Which cam, carbs and have you checked the rwhp on the dyno?

- The bike is definately a runner, i have about 4200 miles on it.
- It is a Cruzin image 836cc ki
- I wanted to use a Kenny Harman D grind cam i have but there wasn't enough clearance to use it so i settled on a Late F2 cam (popular choice in the $110 big bore thread)
- Im using regular 657B 72-76 CB750K link top carbs jetting is 3/4 turn on idle mix, D17-R needles 4th clip (these are a little richer than standard needles), 130 main jets
- I would really love to get it on a dyno and see some numbers the bike is awesome!

[seanbarney41]

Hi Chris, I am curious where you found info on which needles are richer/leaner.