Inliners International Forums General Discussion Engines "Light at the beginning of the tunnel"

Hi! I'm new around here. One of the reasons I jumped in was to learn. I've worked on my own cars since before I had one (worked on cars with my Dad beginning at age 10... I'm 60 now), and I'm a licensed mechanic, but this is a new one: I have a Chevy "250", unassembled, on the engine stand, waiting to be put together. All nicely balanced and blueprinted, line-honed, decked, bored .030 over, big valves, double springs, (lots of fun things), but recently, I decided to build it to run on propane. I'm still learning about this and have learned that you benefit more if you raise the compression ratio, and now is the perfect time to do it since the engine has not been put together yet. However, the head IS together. I'd like to mill the head and get the compression up to 10:1 or 10.5:1. Anybody have any suggestions as to how to calculate just how much to have milled off the nice new "ready to go" head?? I'm thinking that I'll have to put a sparkplug in and turn the thing upside-down (level it) and fill the combustion chamber with oil, carefully measuring how much it takes to fill it, using some kind of a syringe to do the measuring, and calculate the displacement of the stroke of the piston and add the two and the figure the ratio of the two added together, and the combustion chamber itself. (How'm I doing so far?) It should be pretty easy to figure the displacement of the cyl using simple geometry and taking into account the .030 oversize bore, (and a table to convert inches to CCs) and also, by figuring how many CCs the combustion chamber SHOULD HAVE, to give me the right ratio, and by filling the upside-down combustion chamber with so many CCs of oil and measuring the depth from the head surface to the oil level and taking that as the amount I need to have milled off the head. But what about the head gasket, and the amount of effective combustion chamber it adds? Will the "non-flat" piston top affect the calculation? (not a "domed" piston- just a "shaped top", I guess you'd say) Am I getting ready to get myself into trouble here? Ruin a good head? Any ideas? Patrick Cowdrey- White City, Oregon (I consulted with Schneider Cams, and they asked me to send my new cam back to have it re-ground for use in a propane-burning engine. Will do.)

PC...I used propane pistons in my 292 to up the compression. Maybe they are available for the 250??? S

Useing the 307 piston it has a 4 valve relef in the top of the piston. It will bring you much closer to the compressing your after. You could never mill enough off the head safely with the stock 250 piston it is dished to deeply. The stock 250 pistons compression is 8.5-1. The 307 piston In it's OWN bore is 8.5-1 also But the stroke, But the stroke difference useing it in the 250 block will bring it Just shy of about 10-1
For every .010 milled off gives you about 2points in compression is my understanding.
hope this helps.

Ps Welcome to the Inliners

I already have my pistons- some gorgeous "Silvo-lites"- they are not stock. I think there is plenty of meat there for milling. I'll check it before we do anything. But Twisted6 says you can't mill enough off to do any good, but he says that milling off .010 gives you one point of compression... .010 seems like hardly anything. I'm thinking I'll need to mill off at least .050- maybe more. But then- how do you check for valve interference?

to check for valve to piston clearance,the cam and Pistons will need to be in the motor.Use clay on the piston top ,Put the head on torqued to specks roll the motor over Remove the head and check the clay for thickness. This will be your valve clearance. some feel .100 is what your after
I have ran as little as .090 (with stock rods)
more clearance is needed if useing Alum rods is my understanding.