Combustion Chamber CC measuring

[David Nicholls]

Dear Fraternity

When we "cc" the heads, we get the following measurements
cylinder #3 58.4 cc cylinder #1 57.7 cc
cylinder #4 60.1 cc cylinder #2 58.0 cc

Realising that this affects compression ratio, our concerns are the variation of cylinder #4
Our question is,
"Is this variation outside of limits and unacceptable, or should we install the heads as they are ? "

Many Thanks
David

[DonCichocki]

My manual says "no more than +1cc variation." I shoot for as close as I can get +/- .1cc. Take your time and do it right!

[Ron LaDow]

My manual says "no more than +1cc variation." I shoot for as close as I can get +/- .1cc. Take your time and do it right!

CR is an important parameter, so it's worth doing it right. And you want to do it in a way which doesn't hurt the combustion process:

Head and Cylinder - Copy.jpg (87.23 KiB) Viewed 989 times

The area outside the red circle is where the piston dome most closely matches the chamber; it's called the "quench" area. It should be very close to the piston at TDC, something like .04". Don't remove material there. (the circle size is the flat-top diameter of the pistons you're using)
You can safely round-off the edges (the red hash marks) and remove some under the intake valve.

[David Nicholls]

Dear Don and Ron
Many Thanks for your replies gentlemen, and your very valuable advice.
The Heads were reconditioned with all new parts.
We then measured the cc of each chamber about six times each just to confirm a consistency, #4 always measured more than the other three.
It was thought that even though there were no apparent differences, maybe #4 Valve Seat was recessed a little bit more than the other three.
The machine shop that reconditioned the heads will now check, and replace if necessary, the Valve Seat Inserts in an attempt to balance the "cc's". Maybe #4 needs to be raised (?) a fraction.
What ever happens, they will all be within 1cc, or less, before instillation.
Again
Many Thanks

[David Nicholls]

Dear Ron
Many Thanks for your continuing technical advice.
The effort that you go to in your contributions is fully appreciated.
We always read and digest your Posts.
Keep it up
David

[Vic Skirmants]

Most likely a difference in the basic casting of the chamber. Quite often one can see that the material at the vertical "wall" next to the intake valve is thicker on one side than the other. That's also a good location to remove metal without hurting "squish".

[C J Murray]

Ron, what chamber is shown in your picture? What engine?

[Ron LaDow]

Ron, what chamber is shown in your picture? What engine?

Really can't remember.
Several years back, someone posted it here asking where to remove material. I did a 'copy image' and used 'paint' to mark it up

[Ron LaDow]

...Many Thanks for your continuing technical advice...

You are more than welcome and this may have prompted Cliff's question:

Head and Cylinder Sealing Surface - Copy.jpg (88.78 KiB) Viewed 842 times

The green arrows indicate a cylinder/head sealing surface from a good OEM part or one which was properly machined; there is no reason for that surface to have an ID larger than the bore of the cylinders you are using. And very good reason not to:

Peripheral chamber.jpg (1.09 MiB) Viewed 842 times

The overhang of the chamber (non) sealing surface results in a peripheral combustion chamber (in red) which sees a compression ratio far above the number you have calculated, and is liable to preignition, leading to REAL damage as the ring lands collapse.
Chamber geometry deserves a lot of thought and effort.

[Harlan Halsey]

I seems odd that #3 and #4 have so much variation. Guessing that the crown volume is 12cc, the CR of a 58cc cylinder head is about 8.8:1 while that of the 60cc cylinder head is 8.5:1. That would a bit too much for me. And 8.8 is already on the low side. If you raise a 40mm valve by 1mm (040") you will decrease the head volume by 1.2cc, putting the 60cc cylinder to 58.8cc, which would be OK. So I would first look at the valve seats in that chamber, and see if they are low. If so, put in new seats. I am assuming that the valves are all new. If not new valves in that cylinder would help with the seat depth.

The alternative of hogging out the three chambers to 60cc is not attractive. I assumed a piston to deck distance of .040", and for a street engine, you might cheat on that a bit, say .035"