Yamaha TRX850 Enthusiasts Forum

Has anyone cc'd a standard head? Just thought I'd ask before going on a treasure hunt for a bit of Perspex and a horse syringe.

QuickDraw wrote:Has anyone cc'd a standard head? Just thought I'd ask before going on a treasure hunt for a bit of Perspex and a horse syringe.

Easy to do, use Isotropic Alcohol as it will evaporate and leave no residual, won't attack/melt the perspex and is less likely to create havoc when you remove the perspex

Thanks Rod! I'll do the piston-top too while I'm at it.

Had pages and pages of measurements and calculations from modifying my head.
Have searched and found a few scraps of paper showing that I measured std combustion chamber volume (cylinder head removed from engine and inverted [using a greased 12mm Perspex disc to provide a boundary seal]) is 33.7cc.

Standard Yamaha piston crown dish is 4.6cc.
I allowed 0.75mm for head gasket thickness when compressed.
I measured deck clearance initially at 0.5mm. I reduced this to zero by lapping during the build to reduce my squish clearance and to help raise my finished compression ratio.

Sorry I can't provide more data, but hope that helps a little.

Mark.

sanddune51 wrote:Had pages and pages of measurements and calculations from modifying my head.
Have searched and found a few scraps of paper showing that I measured std combustion chamber volume (cylinder head removed from engine and inverted [using a greased 12mm Perspex disc to provide a boundary seal]) is 33.7cc.

Standard Yamaha piston crown dish is 4.6cc.
I allowed 0.75mm for head gasket thickness when compressed.
I measured deck clearance initially at 0.5mm. I reduced this to zero by lapping during the build to reduce my squish clearance and to help raise my finished compression ratio.

Sorry I can't provide more data, but hope that helps a little.

Mark.

The only scrap of paper I can find indicates 33cc for std chamber volume measured using the same method as Mark (perspex + oil + syringe).

DW

Awesome! thanks guys! That helps a lot

sanddune51 wrote:Had pages and pages of measurements and calculations from modifying my head.
Have searched and found a few scraps of paper showing that I measured std combustion chamber volume (cylinder head removed from engine and inverted [using a greased 12mm Perspex disc to provide a boundary seal]) is 33.7cc.

Standard Yamaha piston crown dish is 4.6cc.
I allowed 0.75mm for head gasket thickness when compressed.
I measured deck clearance initially at 0.5mm. I reduced this to zero by lapping during the build to reduce my squish clearance and to help raise my finished compression ratio.

Sorry I can't provide more data, but hope that helps a little.

Mark.

Reckon you've underestimated your gasket squashing Mark - 0.75 would give you a 10.86:1 compression ratio. 1mm gasket equates to 6.2cc and gives a tad over 10.5:1 which is what Yamaha state.

I actually made no claims as to anybody elses compression ratio only my own.

I'm very happy with my measurements and stand by them. If I'm wrong then I'm more than happy to be corrected by demonstration

QuickDraw asked if anyone had cc'd a standard head, I have, so gave him my measurement. This was subsequently verified to within 2.13% by Phil. I also had other associated measurements that I personally made at the same time. I thought these would make a helpful contribution and submitted them that others could also benefit.

I fail to understand why you think my measurement of a compressed head gasket would be in error by over 33%. If I recall, the standard head gasket is less than 1mm when new.

I used a Moore & Wright 0-25mm micrometer that zero's perfectly. I've been using these pieces of equipment for 34 years so I don't reckon I've misread.

Perhaps your own calculations might just be incomplete? Did you include deck clearance and piston ring land volume?

Mark.

I wasn't having a pop at you sandune - I was just saying that the volume of a 0.75mm gasket equates to 4.48cc but a 1mm gasket bumps it up to 6.29cc - gets important if you're pushing up the compression ratio and need to avoid detonation.

To follow up on this one, I checked piston to valve clearance today. So with a .2mm gasket and an extra 0.84mm valve lift (exhaust) in an otherwise factory spec motor, I measured about 2mm (or 0.080") clearance which is minimum recommended for most engines from what I read (0.100" being the general safe rule of thumb). I'm ok with that.

For my compression calculations I used the following numbers:

Std Engine Volumes;

capacity cc= 424.44
chamber cc= 33.7
piston cc= 4.6
deck clearance cc= 3.14

--------------
Using std gasket
std gasket clearance cc = 4.4


total combined vol = 470.28 cc
less capacity = 45.84 cc

calculated compression ratio = 10.3:1

--------------
Using 0.2mm gasket

0.2mm gasket= 1.26cc


total vol = 467.14cc
less capacity = 42.7cc

calculated compression ratio = 10.95:1

Did you end up having a copper gasket made ???

Not yet. I just de-laminated a standard stainless one to do a trial assembly. When the gasket paint is cleaned off, the thickness of one layer is exactly 0.2mm.