TOPIC:

I'm getting 6.24 and 7.02 respectively, assuming the other numbers are right. This is best done by direct measurement of cc 's of oil in the combustion chamber and then summing the piston crown hump displacement.

CR = swept volume + TDC volume divided by TDC volume.

Am I wrong on this ?

Take the power wire of the Dyna and stick a comp gauge on it...... if your getting 100-110-120 psi wouldn't that show it ?

That would be unreliable due to the new rings and only 200 miles on it. I'm checking my valve adjustments this morning. I will do the wire test. I have some heavy gauge wire I can use. I can measure at the side where the piston is flat and then measure the center of the dome if that would help.
Steve

I can measure at the side where the piston is flat and then measure the center of the dome if that would help.

Not sure it would.

A cc syringe and plate glass with hole in it over combustion chamber sealed with grease and filled with oil and no bubbles gives the combustion chamber volume, usually 1 cc within factory spec unless it has been skimmed or had a valve job.

Then, unless a perfectly flat top piston with no valve pockets, you calculate the piston dome displacment which comes from direct oil cc measurement.

You might be able to get a rough measure by tipping the bike on an angle, filling a bore through the plug hole with oil at TDC, noting the volume, then go to BDC and see how much you add, using the bottom of the spark plug thread as your fill mark.

Add the first two then divide by the first. Probably accurate within less than half a point of compression ratio I guess.

Usually the piston dome is above the deck height and subtracted from the combustion chamber volume and gasket width to give V(tdc) , in this case it will be much less, that's why we are making assumptions by back calculating from factory specs and some sort of direct measurement might be neccesary.

If nothing else it would give me a stroke measurement. If it turns out to be 66mm, OK, it's a 903 crank. If anything else, it's a 750.
8mm would be a lot to miss.
Steve

I get it, you are measuring the stroke length with a wire.

The more I think about it, I think Tyler's numbers might be out , as V(tdc) is based on the 900 crank throw, shorter stroke = higher V(tdc) number.

Try the oil fill test , you got me interested now. Otherwise I get easily distracted on oil seal threads

Hind sight is 20-20...LOL....
it sure would have been nice if you had measured with a steel rule how far the piston drops from TDC in 180° !! HA ha
I would not be surprised to find out you do not have a 750 crank from what everyone is saying.
I think you "might" be able to drop a straight wire in a plug hole and mark it off against a top fin and rotate the engine to see how far it goes up or down.... etc.
A negative 8mm would be fairly easy to see if you can keep the wire straight up and down ....and always compared to the same fin and the same spot the top of the piston.
Seems like a goofy backyard mechanic way to do it, but what the heck... lol
8mm, that's .315" or 5/16" of an inch from a 903 at 66mm / 2.6" stroke.
So if you measured only 2.3" I guess it is a 750 crank.

Tyrell Corp wrote: I'm getting 6.24 and 7.02 respectively, assuming the other numbers are right. This is best done by direct measurement of cc 's of oil in the combustion chamber and then summing the piston crown hump displacement.

CR = swept volume + TDC volume divided by TDC volume.

Am I wrong on this ?

Your correct.

CR= V(BDC) / V(TDC) where V(BDC) = V(TDC) + Cylinder displacement

If you take the stock displacement and CR you can calculate V(TDC) that includes chamber volume + Gasket+ deck height - piston dome

Then you can determine the additional piston dome volume for the 10.25 : 1 pistons

Again this is just estimation, the real measurements would be needed, but it should be in the ball park, and that doesn't look too good.
Someone familiar with these engines can comment about the normal head volume and so forth, I haven not had a 1015 apart.
In Swest's case its the reduction in cylinder displacement that really knocks the CR.

If this is the case with your engine Swest you may need to deck the block and plane the head if you want to run it that way.

Ok, I'll do my valves this evening. Here it is. My last reading is 54.50. I took multiple pics because the measurements changed slightly.
TDC

BDC


54.51mm


Old gasket




Another pic of depth.


I'm now sure it IS a 750 crank. 54.51 - 1.40= 53.11.
Next is the compression test.
Steve

Yup looks like you got it.
Now what?
Pull the engine crack it and replace the crank and rods?

Maybe I'll list it on eBay for $9000 OBO like that fool, buy a 903 crank for $200 and maybe buy a bigger kit.
Steve

There is a flaw in the oil test CR check, that could be worked around if doing valve clearances - I'll leave you to work it.

over and out .