backpressure: what are the facts?

Nedwreck, did you mill the head or the cylinder? How much clearance was left between the top of the piston and the head? After you milled everything, was it a really noticeable power difference? What did you torque the head bolts to? I pulled the head and cylinder from one of my motors for the first time, and one thing I noticed was the casting flash around the cylinder ports.....pretty much all of them. It wasnt extremely bad, but its bad enough that I want it gone!
 
Well it's not exact I made a reasonable guess of the cylinder dimensions,for a 48cc engine (bore 4.0cm stroke 4.3 cm) and guessed at the CR,it actually increased from 6.5 to 7.4,If it had been lower say 6.0 it would have gone up to 6.85.I don't know why higher ratios are not common as presently used in 4strokes.I surmise that it has something to do with the dome shaped combustion chamber used with the Schnuerle loop scavenging setup but that's just a guess.
 
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head shaving (planing)

Ratty I planed just the head using the flattest surface I could find a plate glass display shelf and adhesive backed 3M stickit. Proceed carefully if you try it avoid tipping to maintain a flat gasket surface. My head had lands (grooves) designed to grip the gasket these I used as my flatness guide. When they were gone that was my first .020 I FIrst used a w pattern turned the head 90º and used a circular pattern. Turn frequently one way then the other. 150 grit should leave a rough enough surface for gasket grip. Coloring the milled surface frequently with a marker can also show if you are removing material evenly across the surface. These are not hard fast rules as I am sure you have developed your own shop skills and methods. I measured the second .020 by laying feeler gauges on the unplaned surface using a straight edge across the planed surface. Piston to head clearance I checked by holding the head on the block include gasket sparkplug out sat on the bike and rolled back and forth with clutch out you will feel interferance. Index the spark plug if you like I'm not sure this makes a difference but it couldn't hoit as they say. Also bear in mind the combustion chamber is hemispherical (HEMI!) and as you remove material the dome edges move in. Power increase YES! Immediate noticeable torque hp increase. I could hardly wait for things to cool to do the second .020. I also bought a black powder coated 80 for my chopper build and took the motor rigHt out of the box and milled the head. I rebolt the head in an X pattern tightening them evenly (I do this when re-assembling anything) bearing in mind that a torque sPec is a measurement of bolt STRETCH not tightness. Our tech duivendyk hopefully can give us insight into probable power gains. Have fun with your lo-buck no-buck mods!
 
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head milling

It seems to me that removing material from the cylinder rather than the head might work better, as the relation of the domed piston and domed combustion chamber would remain the same. I think simply using a thinner head gasket might work well, mine is just a simple one made from aluminum. I would almost bet that one could be cut from a sheet of thin metal....maybe aluminum flashing. I have read about the guy here that removes the cylinder to block gasket to increase compression, but that seems like it would lower the ports adversely....I dont really know enough about port work to fool anybody .
I seem to remember indexing washers for spark plugs sold by Moroso or MSD.......maybe even Accel. One of those hot rod companies.
On a different note, here is a link to the company that supposedly manufactures this particular engine I am currently experimenting on......
http://www.zm-bicycleengine.com/product2.asp
This page list some specs that seem a bit like inflated claims, who knows. There are some torque specs somewhere in that site. I understand torquing is truly a measure of stretch, but on this engine I think a simple cris-cross, incremental torquing will do just fine. You should be able to feel if the studs start to stretch too much. I have never torn into any of my other motors, but I have seen pictures of them torn apart . The crank on this little dude seems to have some balancing from the factory; there are pictures of it on the web site. I have never had this one running yet, it came on a project bike that had only been run for about 15 minutes. After looking at the cylinder and piston, I can vouch for those claims. Oh well, enough rambling...
 
beyond 4cycling

I milled the head to avoid having to sort out a ring compressor when removing the cylinder. Thinner head gaskets are also an idea but I figured the stock one had the right strength and wouldn't blow with increased compression as soft shim stock might. Hard shim stock won't help the parts mate. Taking out the cylinder gasket as you said probably would work but as you said may change port timing and again may require a ring compressor. So a quick no money spent performance upgrade I feel is still head planing I did mine in the living room while watching a movie (The Departed) LOL I'm goin to that site you mentioned. You've started me thinking abou cleaning up the ports too. I gasket matched the muffler as it had some flash from welding
 
Look ma, no brains!

I pulled the cylinder off last night just for poops and giggles. The bottom of the cylinder is tapered on mine, so it is sort of a built in ring compressor. I have worked on motorcycle engines like this too, so I assumed this one would be the same and it is. I used a feeler gauge to separate the base gasket from the cylinder and bottom end, it came off nice enough to reuse it, no prob. I am really tempted to sand this cylinder down, or have it cut at a machine shop. I just wonder if the head and cylinder could be 'lapped' so they could fit together with no gasket. I will have to do some searching on this one, that gasket has got to be at least .030" thick....it would help just by removing that.
 
nedwrecks conTinuing education

Now I know its a good day! I've learned something new! Now to clean up those ports for another no buck performance mod. Where's my dremel? Ah well I'm off to the web to research port modification in 2cycle motors thanks ratty.
 
Inside the cylinder was planing to use cut off wheels wheel stones I've had. For 25 yrs and haven't used LOL I went to another site and it was suggested by a pro tuner to bevel the piston top at the take ports to. Increase flow decrease turbulence aNd that the bottom of the exhAust port Should be at the edge of the piston at BDC. A person should proceed carefully however the cost of replacement pArts is so low (entire motors 90$). Forge ahead!
 
I found the specs on those engines on that engine site,and recalculated.The comp.ratio listed was 8.0 .If we can trust them on this,it increases to 9.0 for 0.020 and to 10.2 for 0.040.That would seem dangerously high,if the 8.0 ratio can be relied upon.I'm inclined to be a bit leery if someone comes up with these nice 'round' numbers.
 
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