Author Topic: Measuring cylinder head volume  (Read 152 times)

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Offline backofbeyond

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Measuring cylinder head volume
« on: June 10, 2021, 04:40:10 PM »
I've got to the stage with my DS6 rebuild that I want to check the compression ratio before closing it all up. I've got the engine propped up and a burette clamped above the plug hole waiting to go (well more or less anyway  :)) What do you guys use as the measuring fluid? Petrol? Oil? Something else? I presume a ring of grease around the edge of the piston will seal it well enough?
70 YDS6
70 H1
72 XS2
76 B120
76 400/4
79 CBX
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Offline Batman

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Re: Measuring cylinder head volume
« Reply #1 on: June 10, 2021, 06:26:21 PM »
You mean combustion chamber volume? as cylinder head volume check is done independently of barrels. For head I've used purple sprit from Homebase.
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Offline Barnett

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Re: Measuring cylinder head volume
« Reply #2 on: June 10, 2021, 06:31:29 PM »
Head needs to be upside down and plug screwed in for measuring cyl head vol. Idealy you will need small sheet of flat glass with a hole drilled in it to introduce fluid. Grease head face with Vaseline to bet glass plate to seal, +1 for purple spirit, or meths.

Offline 5port

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Re: Measuring cylinder head volume
« Reply #3 on: June 10, 2021, 07:04:54 PM »
I've got to the stage with my DS6 rebuild that I want to check the compression ratio before closing it all up. I've got the engine propped up and a burette clamped above the plug hole waiting to go (well more or less anyway  :)) What do you guys use as the measuring fluid? Petrol? Oil? Something else? I presume a ring of grease around the edge of the piston will seal it well enough?

You are going for the volume check using the engine assembled.  I have not done it this way but, it is reasonable to use light oil or paraffin.  I think the difficulty is to judge when the level actually reaches the bottom of the plug hole.  Other option is to fill to top of the plug hole and subtract the volume of the threaded bit but, add the volume of the plugs'  air space in either case.
Myself, I do it with the head separate and factor the piston crown vol and the squish gap vol.
Cheers

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Online Bluehaze

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Re: Measuring cylinder head volume
« Reply #4 on: June 11, 2021, 12:01:46 AM »
Measuring combustion chamber size of an assembled engine will be most accurate I think, taking into account the piston position at TDC, head dome shape and cylinder base gasket thickness.

Offline 5port

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Re: Measuring cylinder head volume
« Reply #5 on: June 11, 2021, 07:26:12 AM »
Measuring combustion chamber size of an assembled engine will be most accurate I think, taking into account the piston position at TDC, head dome shape and cylinder base gasket thickness.

Both methods can take account of all factors and be accurate or inaccurate, depends how well they are performed.   ;)
The real test is the actual compression achieved.  However, overall, the figures are good guides for comparison if building an engine or doing development.  I prefer the disassembled head measure method, but I only do this for a few engines, not standard ones that are just squish checked.  That said, I have done more quick checks of heads for volume where the head is unknown.  Easy to do that, you don't have to build the engine!  :)

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Offline backofbeyond

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Re: Measuring cylinder head volume
« Reply #6 on: June 11, 2021, 07:32:22 AM »
Measuring combustion chamber size of an assembled engine will be most accurate I think, taking into account the piston position at TDC, head dome shape and cylinder base gasket thickness.

That's the way I was thinking but I'm certainly open to greater experience with this stuff. Measuring the head volume separately would be easier and (for once!) I have all the bits lying around to do it but having to account for the piston dome shape etc seemed to make it more tricky to get right than just measuring directly. The downside of doing it directly though is the risk of flooding the bottom end of the engine with whatever the measuring fluid is if the seal round the piston isn't 100% + any leakage throwing the result off. So no water based fluid and petrol may well dissolve grease used to seal the piston. 
70 YDS6
70 H1
72 XS2
76 B120
76 400/4
79 CBX
87 XR600
01 CCM604
02 GoldWing

Offline 5port

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Re: Measuring cylinder head volume
« Reply #7 on: June 11, 2021, 08:26:24 AM »

That's the way I was thinking but I'm certainly open to greater experience with this stuff. Measuring the head volume separately would be easier and (for once!) I have all the bits lying around to do it but having to account for the piston dome shape etc seemed to make it more tricky to get right than just measuring directly. The downside of doing it directly though is the risk of flooding the bottom end of the engine with whatever the measuring fluid is if the seal round the piston isn't 100% + any leakage throwing the result off. So no water based fluid and petrol may well dissolve grease used to seal the piston.

I covered the measuring of volumes for CR on my RD350 Track Bike thread.  Overall, even with the complete compression volume directly measured, you need an accurate exhaust port height to factor the swept volume and, you should measure the squish to at least make sure it is not too tight, and you use that figure to correct for deck height and gasket thickness etc.
CR measurement does require a bit of understanding to get right.  But, a simple head volume check and a squish gap check can tell if you are in the ballpark for a standard engine.  You have the Yamaha figures for the head vol and corrected CR I think?
BTW, the use of parrafin is because it has a very low surface tension and can get into that small squish gap and all corners of the chamber at TDC.  On the same point, the cylinder/ engine should be vertical so that no air is trapped.  Cheers

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Offline backofbeyond

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Re: Measuring cylinder head volume
« Reply #8 on: June 11, 2021, 11:42:39 AM »
I need to go back over your track bike thread again. It's a long thread though and last time I was looking at gearbox shim issues. Port height I've got and squish I've got, as well as checking how far below the top of the bore the piston is at tdc. There's some slight differences between the two cylinders but how significant they are is one of the things I'm pondering.

Figures are

Squish : Left hand cylinder (LH) - 1.35mm. Right hand cylinder (RH) - 1.21mm. Measured by the squashed solder method.

Exhaust port height : LH - 29.1mm. RH - 29.6mm

piston height at tdc : LH - -0.72mm. RH - -0.60mm (minus number =  distance below the top of the bore at tdc)

It was the left hand cylinder that holed the piston, so that's the one I've really got under the microscope.

So far, unless any of those numbers tell a tale, I've not seen anything that really explains why that happened.   
70 YDS6
70 H1
72 XS2
76 B120
76 400/4
79 CBX
87 XR600
01 CCM604
02 GoldWing