As stated in title - I need to figure out the volume of the cyl head chamber so I can determine compression, never done this before so not sure how. I figure someone here has done this numerous times and knows some tricks!

running Arias forged pistons, not sure how that affects the calculation as its certainly not flat which I assume changes how one computes the actual space between piston and head at TDC....

head gasket compressed at .041 - felpro Printoseal - steel sandwiched gasket, have original compressed one and new one for rebuild.

Thanks for the help!

Another question - stem seals on exhaust valves, good insurance or unnecessary ??

thoughts....

valve stem seals are unnecessary. They are only used to reduce the amount of oil sucked into the cylinder

There is no fast way. If you don't want to remove the head, tilt motor so plug is highest point and vertical. Set at TDC so valves are closed. Remove plug, measure and fill chamber to bottom of spark thread with oil. Knowing what that is you'll be able to get a pretty accurate figure for the compression ratio. Don't worry about a few drops up or below the threads. It's too small an amount to affect the CR. (CR=swept volume+combustion chamber divided by combustion chamber.)

For cyl head chambers - piece of flat plastic with a small hole in it near one edge over a chamber, sealed against the head surface with a smear of oil or grease. valves should seal themselves if in good condition. spark plug in obviously. The hole is to pump in fluid with a large syringe with ml scale. The hole will need to be slightly larger than the syringe to allow air to escape as fluid enters. Having the hole near an edge allows you to tilt the head slightly to keep the hole higher to allow air to bleed out as you fill it - if it were in the middle you'd have trouble getting air bubbles out.

Pistons - same sort of process to get the capacity of a dished piston. For a domed piston, nick a container out of the kitchen with ml marked on the side. Fill it to say 100ml then dip the piston in just to the edge of the ringland, so just the dome is in the water. The scale will show you how much water was displaced above the 100ml starting point and that's obviously the volume of the piston dome, as long as there's no air pockets in valve recesses. If there is, you can fill them with playdough or something before dipping them, and then measure the valve recess by filling it with fluid. It's fiddly, but that's life.

Exhaust VSS are considered optional by some as there isn't any real vacuum in the exhaust to suck oil in, unlike the inlet side. But high speed air leaves areas of low pressure and can suck oil in, plus crankcases and valve covers get pressurised slightly. So for the sake of a few dollars and a minute's work I always use them on exhaust valves as well. There's zero benefit to leaving them out.

What is the fastest most accurate wa to check CC's in cyl head ?
LMAO when I read the title. There is a 3d hand held scanner you
can use these days (read $$$$ plus the fluid dynamics software
even more $$$$$) but you still have to pop the head.

Look at it this way, you are not pulling out a chain driven
twin cam head or any modern donk which are worse than Medusas head.

Fastest way is look up the compression for the Arias forged pistons in the Nissan Competition catalog. Oh wait, your pistons have been milled down

Nissan Competition Arias forged pistons



Your engine

Both advice seems sound. DVR's method sounds genius. Just when you are done, rotate the engine over with plugs out to get most of the oil out. Then crank it with plugs out to get the rest. It'll still smoke for a while after starting but no harm done


I bought one of these: less then $3 at the feed store. ask for a 35cc horse syringe, which will measure any A-series chamber (usually 28cc to 32 cc)



The plastic was from a CD case

the grease



hole drilled in the plastic -- I used water, less of a mess



This is also a good way to tell if the valves are sealing well. It should hold water for several hours with no visible leak. If the water leaks out, it's time to lap the valves!

Even easier, use your favorite CD like Gandalf did -- it already has a hole in it

calculation

Cylinder volume for stock 73mm A12 engine (be sure to measure as it may not be stock)
1171cc/4 = 292.7 cc

Gasket = 0.041 inch = 1.04 mm
bore of gasket (might be 76 mm)
area = pi * r2 = 3.14 * 76/2 * 76/2
cc of gasket = bore * area = 4.54 cc

Head CCs
Dome CCs

Ah forget it ... just use a calculator http://bing.com/search?q=compression+ratio+calculator

Id expect something around 12:1

the head is off so I did all the CC volume on both the head and piston volume I just haven't gotten to the final calc's to figure out the ratio.

I set the piston at .300 down in the hole then filled so I need to compensate for that to work backward to the ratio, no time for math today!

I dont think the pistons have been milled either.

No seals on exhaust = smoke out of tail pipe....right ?? going in the chamber, burning in next cycle or in the pipe I'd assume, seems like good insurance - right or wrong?