Valve face diameter?

Can anyone give me a rough estimate what the face diameter of a valve for my 550 is? I have an an idea in the back of my head about calculating something, but I need the diameter that the piston would "see".

I'm thinking it's around 1"? I checked the manual and there's no spec there.

OK, the reason why I ask... I'm really bored...

So, I blew my #2 piston. Got a new one, rings and valve job. I remember after the valve job that I had to go about 1 mm smaller shim. Now that piston has lower compression than the others (150 compared to 130ish). I was worried my cylinder might have been warped a bit... But, in theory, I increased the volume of my chamber because the valves now are a bit more recessed.

Here's the math I used. If we go with a 1/2" valve face diameter and 1 mm recessed into the head, I added about 126.6 mm3 to my cylinder.[ (12.7mm/2)^2 * pi * 1mm ]

Now the displacement of my bike is 553 cc (or 5530 mm3). If I use the ideal gas law and assume the operating temp will be the same, I get P1 * V1 = P2 * V2 (p being pressure and V being volume). So, my P1 would be 150, V1 would be 5530/4=1382.5 and V2 would be 1382.5+the additional 126.6.

When I run this calc, I get 137.4 psi. This really amazed me that just that little bit could make such a difference...it also depressed me a little bit, because I'm thinking that the compression on this cylinder will never get much higher than what it already is.

whoever did your valve seat job needs to have his hands cut off his body immediately.

he must have used one of those heavy pneumatic tools for car or truck seats

motorcycle valve seats should be done slowly with a hand cutter

why on earth would anybody take 1mm of a valve seat?

gosh

in any case, havent opened yet my 550's head but if my CB500/4 has 27mm inlets, guess a 550 kawi's must be something like 28-29mm.

great...lol.

I thought he did it by hand... I'll double check my notes, but I think I went almost 1 mm lower. Then again, it is possible that maybe I was on the tight end of the spec before blowing up and I know I put it on the loose end after blowing up.

1.) The shims would have started somewhere around 2.3 to 2.6 mm.
The smallest shim you can get is 2.00 mm.
If he took off 1mm, where would you get the shims?
Is it possible he actually took off .1mm?

2.) 1 cubic centimeter equals 1000 cubic millimeters.
So your bike's displacement is 553cc = 553,000mm^3.
So cylinder displacement is 138,250 mm^3

3.)You should be using the combustion chamber volume at TDC, not the displacement.
Compression ratio is 9.5 to 1, so:
Assuming ideal volumetric efficiency, and adiabatic compression (both unrealistic).
The starting volume is 138,250 mm^3.
Finishing volume (combustion chamber volume at TDC) is 138,250 / 9.5 = 14,553 mm^3

4.) P1V1 = P2V2 does not really hold up for the most part, as we can see by double checking the compression test result. We could theoretically round up the atmospheric pressure as 15 psi, then multiply by 9.5 to get what the compression test should show. 15 x 9.5 = 142.5 psi.
So we can see the reading of 150 psi is already too high, showing us that we cannot use the above assumptions if we want any precision.

5.) But let's throw away that 150 psi number, and stick with the above assumptions anyway to eliminate as many variables as possible, and isolate it to just that variable of the valve-face volume. Let's idealize that compression ratio of 9.5 to be precise, and thus the stock, idealized compression test is 142.5 psi.

That leaves us with V1 = 14,552 mm^3 (stock volume at TDC).
Now let's change it for that 1mm valve job (as you previously calculated).
Then V2 = 14,552 + 127 = 14,679 mm^3 (volume after valve job at TDC).
P1 is 142.5 psi.
Four variables, one unknown, and some simple algebra and P2 = 141.27 psi.


Conclusion: So if we idealize everything (even beyond that of ideal gasses) and take 9.5000 as the compression ratio, the valve-job's volume-change would only affect the compression by 1.23 psi (Assuming one valve at .5" diameter, 1mm cut).

Going further, if we take the valves as 1-inch faces and include both valves, we get 133.3 psi. That is a significant change, but still only 9.2 psi, and the exhaust valve is nowhere near that big.

I think you need to run the bike a few more miles before everything gets seated in order to get a better compression test result.

BTW, the intake valve is about 27mm at the seat-face ring.
The exhaust is closer to 20mm.

loudhvx wrote: 1.) The shims would have started somewhere around 2.3 to 2.6 mm.
The smallest shim you can get is 2.00 mm.
If he took off 1mm, where would you get the shims?
Is it possible he actually took off .1mm?

2.) 1 cubic centimeter equals 1000 cubic millimeters.
So your bike's displacement is 553cc = 553,000mm^3.
So cylinder displacement is 138,250 mm^3

3.)You should be using the combustion chamber volume at TDC, not the displacement.
Compression ratio is 9.5 to 1, so:
Assuming ideal volumetric efficiency, and adiabatic (both unrealistic).
The starting volume is 138,250 mm^3.
Finishing volume (combustion chamber volume at TDC) is 138,250 / 9.5 = 14,553 mm^3

4.) P1V1 = P2V2 does not hold up for the most part as we can see by:
To double check that figure, we could theoretically round up the atmospheric pressure as 15 psi, then multiply by 9.5 to

get what the compression test should show. 15 x 9.5 = 142.5 psi
So we can see the reading of 150 psi is already too high, showing us that we cannot use the above assumptions if we want

any precision.

5.) So let's throw away that 150 psi number, and stick with the above assumptions to eliminate as many variables as

possible, and isolate it to just that variable of the valve-face volume. Let's idealize that compression ratio of 9.5 to be

precise, and thus the stock, idealized compression test is 142.5 psi.
That leaves us with V1 = 14,552 mm^3 (stock volume at TDC).
Now let's change it for that 1mm valve job (as you previously calculated).
Then V2 = 14,552 + 127 = 14,679 mm^3 (volume after valve job at TDC).
P1 is 142.5 psi.
Four variables, one unknown, and some simple algebra and P2 = 141.27 psi.


Conclusion: So if we idealize everything (even beyond that of ideal gasses) and take 9.5000 as the compression ratio, the

valve-job's volume-change would only affect the compression by 1.23 psi (Assuming one valve at .5" diameter, 1mm cut). If we take the valves as 1-inch faces and include both valves, we get 133.3 psi. That is a significant change, but still only 9.2 psi.

I think you need to run the bike a few more miles before everything gets seated in order to get a better compression test result.





P1V1 = P2V2, but the volume you are dealing with is not the displacement of the cylinder. V1 should be the volume of the

chamber when the piston is at TDC. You would have to measure that to be accurate, or you can estimate it from knowing the

compression ratio is 9.5 to 1:
Assume atmospheric pressure is 15 psi.
Assume atmoshperic pressure in cylinder at BDC.
Assume 150 psi at TDC.
Assume compression tester leaves no extra volume in chamber (they usually do leave extra since the plug is out).
Assume volumetric efficiency. (Not realistic since it's a dynamic process to build up compression).
P2 = 150
V1
P1 = 15

Riiiigghht! Headach! I hate math, but Agree, still need to seat the rings to determine if any loss is there. wouldn't sweat it too much, it could take quite awhile to get it "broke-in".

D'oh!

Elf, you quoted me before I had a chance to edit out the extraneous text towards the bottom.

LOL

I seriously doubt the valve could be that far back in the head. If it was sunk 1 mm, you would have to tip it 1 mm to get it back in the shim range. That would put thwe valve tip clear down on the keeper.

When the piston blew, did it damage a seat so bad that it had to be cut that much?

Hand cutters can be an issue as the different angels have to be done separately. This can result in recutting to get the seat width correct. Modern seat machines like the Serdi eliminate those issues.

APE Jay - yes, I had aluminum fragments everywhere... I never actually say the valve and the seat. I could have sworn it went from 3.5 mm to 2.5 mm shim. I'll check my notes tonight to verify.

Lou- What the heck? I thought you were an EE? Yes, I bombed the cc to mm3 conversion... I figured adiabatic compression because I figured any temperature increase from the compression would be neglegible compared to the heat generated by the combustion.

As for calculating volume of the cylinder, I just used the bore and stroke, but if I did that it would neglect the domed head space, correct?

Now, correct me if I'm misunderstanding Lou, but you telling me since the compression ratio is 9.5 to 1, that means that the best my bike can do at a given atmospheric pressure is 9.5*that pressure? I could have sworn in my clymer manual it showed compressions as high as 170 psi.

apbling wrote: APE Jay - yes, I had aluminum fragments everywhere... I never actually say the valve and the seat. I could have sworn it went from 3.5 mm to 2.5 mm shim. I'll check my notes tonight to verify.

Lou- What the heck? I thought you were an EE? Yes, I bombed the cc to mm3 conversion... I figured adiabatic compression because I figured any temperature increase from the compression would be neglegible compared to the heat generated by the combustion.

As for calculating volume of the cylinder, I just used the bore and stroke, but if I did that it would neglect the domed head space, correct?

Now, correct me if I'm misunderstanding Lou, but you telling me since the compression ratio is 9.5 to 1, that means that the best my bike can do at a given atmospheric pressure is 9.5*that pressure? I could have sworn in my clymer manual it showed compressions as high as 170 psi.

yep, carbon build up etc can bring measured pressure numbers to be more than the claimed compression ratio X 14.8 (1 atmosphere equals 14.8 pounds per square inch) though never got a pressure reading higher than the nominal numbers in al my life.

BTW, have no idea why you are digging so deep into thermodynamics, the deal is quite simple: on a 29mm valve, a 1mm dip equals 0.6c.c. over a combustion volume that is about 15-16c.c. like in your bike, this equals a loss of half a point in the compression ration. not nice but i would be more "depressed" over the bad inlet airflow into that cylinder rather than the lower CR

There is no 350 shim size that I've ever seen. While not impossible to have a valve with a 300+ shim size in a 550 motor, it's very unlikely. If it were so, it would mean someone before you was in there diddling around with the valve job.

Serdi, Sunnen, etc. etc. etc.... don't mean the seat is straight just because the machine cost a pile of money. It only means you can cut seats faster.

Rather than assume anything, put a leak down tester on the cylinder see what it tells you.

Other option is the good old oil in the cylinder trick. If the compression comes up, it's rings. If it doesn't come up, it's a valve issue.