I thought you mentioned that big chunk of aluminum was too big to chuck. From the picture, it looks as if you could easily chuck it up. If you reversed your jaws and grabbed from the outer diameter, you could get that extra 3 or 4 cc's.

No -- the problem here is that if I try to re-cut the combustion chamber, my cutter will start to chatter bad because I already cut the sparkplug hole. Also the sparkplug would then protrude into the combustion chamber. All I can really do is make a new one or try to run this. My current plan is to bolt on a stock head and measure the compression. Then remove it and bolt on the 30cc head and see what the compression is. If it is not to crazy I will run it.

My point was you wouldn't have to use the spark plug to chuck it up, and you wouldn't have to use the custom cutter.

Ok I got now. I didn't use the sparkplug to chuck this small piece up. I was able to grab this smaller piece with the reversible jaws. I don't cut the combustion chamber on the lathe. I have special cutter I made to do that and I do it on the drill press. Once a pilot hole is cut you can't use this cutter anymore because of the chatter and it tries to wander. I only cut the squish band in the lathe.

Edit: That picture you probably are talking about is misleading. I was just dialing in the cutter before I set the angle. I know I chucked it with the sparkplug but once I had the cutter set I used the reversible jaws.

Well adnoh gave me push.
Found an online calculator for measuring compression ratios.
Not sure how accurate it is but I think it is close.
With a stock oddy head (35cc) it came in at 8.4 to 1 and that seems reasonable.
With the 30cc head it came in at 10.16 to 1

Then I found more crap online about compression ratio and octane needed.
The 10 to 1 needs 100 octane ---------------- that's AVGAS -----

I'M GOLDEN

I estimate I will get 200 psi on the compression tester.
We'll find out soon enough.

Here are some pics of that online calculator results.
The first pic shows the 30cc head numbers with no head. I'm not running a gasket. I'm running an o-ring.
Second pic is the projected CR.
Third pic is the stock 35cc oddy head.
Fourth pic is the projected CR.

Interestingly when I did the calculations with a standard head gasket (.042") I got 8.25 to 1 with the 30cc head and 7.12 to 1 with the 35cc head.
So if I have to I can still run a full size standard head gasket to bring down the compression.
I am really interested right now to see how close these calculated numbers are gona be.

Some baseline info on both the 1st and 2nd edition 350 Oddys.

1st edition 350.
Bore x stroke. 80x68mm [3.15 x 2.68 in]
Displacement 342cc [20.9 cu in]
Compression Ratio. 6.7 : 1

2nd Edition 350
Bore x stroke. 78.5 x 68mm [3.09 x 2.68in]
Displacement 329.1cc [20.08 cu in]
Compression Ratio 6.0 : 1

I have both Gen Honda workshop manuals 1st and 2nd edition,so I know these to be true.
So this info should be helpful in the design process.

Where did you get your stroke # from? 37 It's wrong...68mm correct.
You have been Understroking!

No the stroke number is not wrong because I measured it from the time the exhaust closes.
If your exhaust is open your not stokin nothin.
Quote: "You have been Understroking" --------- I can assure you I have not been understroking.
Apparently your exhaust is open !!
That's what I heard anyway

Yes you are WRONG,because your NOT measuring it the correct way.
Stroke is measured by the crank lobe pin centers.
Picture the crank lobe at TDC (Top Dead Center),then at BDC,the distance measured from their centers is the stroke.
I have a spare ol Ody crank in the backshed somewhere,but can't find it atm,maybe sold it?.Was going to do the measurement,but I'm sure this is how Honda did it.
Anyone have nude crank?

Right, you don't have to use the custom cutter that chatters. Since you can chuck it up, you can use the lathe. Can't you just cut the angled section where the combustion chamber and the squish meet? I only suggested it because you mentioned you would need to start over if the compression ratio was too high. Considering how strongly you campaign for avgas in a stock Engine, I can't believe you are even considering going to 10/1.

And I understand your thinking on the stroke issue. You're saying it doesn't start to compress till the piston passes the exhaust port. For measuring Engine size [cubic inches or cc's] the total stroke of the Engine would be measured by the total distance the piston travels. This makes me wonder if your program is designed to calculate 2 strokes? Or maybe it's just for calculating 4 strokes? Either way, I think your correction for the stroke works.

I wonder if the programing has a built in exhaust valve/port figure? Meaning it adds an average valve timing to the calculations? I wonder if it's accurate for both 2 and 4 stroke engines? If not, your correction on the stroke input might not be right?

No I can't just chuck it up in the lathe and cut the angled section between combustion chamber and squish. That's why I made that custom cutter.
The other radius cutter that bolts to the lathe didn't work out for me on that radius cut and that first large piece (6") that wouldn't fit in the lathe is now junk. So I started over with a 4" piece that I could chuck up. I will post a pic of that larger piece later so you can see what happened.
Now I understand what you are talking about LOL. I'm a slow learner but I do learn.

You guys might be right on that program but we will find out soon enough who is right.
If the compression comes in like I said at around 200psi then I was right. If not you're right.

Considering 10/1 --- well sort of because I didn't want to start over again.
I don't care if this thing blows up . It has a welded piston anyway .

End result irrelative. It's about having fun in the shop.

liduno --- here is a closer pic of both of those heads.
If you look close you can see the chatter marks on the 6" head.
What you can't see unless you were here and holding it in your hand is that the cutter "ran off" to one side by nearly and 1/8" and the sparkplug hole is no longer in the center of the combustion chamber. It's junk. I suppose I could buff out those chatter marks and use it anyways but I don't want to. An oddy cylinder head has the sparkplug angled so maybe it is "good enough" but I'll pass.
I suppose I could start over and make another head but I suddenly got a woody when I ran the numbers and figure I could build a grenade with 200psi compression and a welded piston . If this thing holds together it might be a Polaris/Honda slayer .

Another issue was that I could not get the proper radius profile with that bolt on cutter I was using on the lathe. I could not get close enough.

Since we're not scarred to do some redkneck engineering, could you lower the compression ratio by chucking up the head and using sand paper on the combustion chamber? Get her spinning fast and go at it with some 80 grit..lol I bet you could get those few cc's pretty quick. That is, if the psi test comes back too high.

Using that calculator --- if I used a stock head gasket I can lower it to 8 to 1.

Maybe I missed it, but why not spin the work piece and put the cutter in the tailstock? I doubt the homemade cutter is perfectly balanced and when it spins, I'm sure it is adding to the chatter. Wouldn't this also keep the plug hole centered and make it much easier? Just tossing out ideas. I do enjoy following you posts CO.

Yes that's the first thing I thought about but unfortunately we run into one of my problems here ---- aka the mickey mouse little home hobby stuff.
The homemade cutter won't fit into my tail stock --- FFFFFF.
I can't turn it down because of the design. I would have to make a new cutter with a smaller shank.

could you use a dead center made for your tailstock and mill a slot in it to accept the cutting tool and braze it in place?

Perhaps anneal the dead center before you mill it. ?????

It actually works well in the drill press as you can see by the earlier pics.
Don't really want to spend a lot of time making a new cutter because this one seems to work.
I just can't use it in the lathe.

A little more thought on the compression calculator. For calculating a 4 stroke, there wouldn't be a need for them to add in any type of calculation for valve timing. During the compression stroke of a 4 stroke, both sets of valves would be fully closed while the piston was traveling up. It would use the full stroke of the piston to make compression. A 2 stroke on the other hand, and like you mentioned, doesn't start to make compression till the rings are past the exhaust post.

So in calculating compression on a 2 stroke, you wouldn't enter the total stroke of the piston. The only time you would use the total stroke would be to calculate the size of the Engine. If my memory serves me, it's pi times the diameter times the stroke times the number of cylinders. That formula may be wrong so don't quote me..lol

Correct.
That's how I saw it.
Besides if you enter 68mm stroke you will get a ridiculous compression ratio. It's obviously wrong.
Yes I tried it -- after bug posted that.

Theres a better online calculator somewhere for 2-strokes.

Measuring CR using the distance from the top of the exhaust port for calculation will give you STATIC CR. Then, using the calculation for full Engine stroke from BDC to TDC (Top Dead Center) -> on a 2-stroke that is called DYNAMIC CR.

The actual CR of a 2-stroke will end up being somewhere between STATIC and DYNAMIC based on tune of the pipe, Engine rpm, efficiency, port configuration, etc. This is what makes determining the actual CR of a 2-stroke much more difficult to calculate.

What I would recommend is to use a fuel who's octane will cover 75~85% of the dynamic CR. This would be for safety of the Engine. Set the squish to .40 thou, then adjust your chamber volume to get your Engine compression up to about 180psi. Ending Engine compression in PSI will pivot around your STATIC CR btw.


I run VP110 or C12 on all my machines, and my jetski with a DASA Engine is running the highest CR and compression. Dynamically, I'm at 14.7:1 CR. Squish is at 50thou. Chambers are very small and the Engine is making 220psi on a gauge. Keep in mind this Engine is used for low rpm torque/brap which only sees full throttle for less than 5 seconds at any time (Because the whole jetski is usually inverted by the time the power comes on full).

Also how are you figuring your deck height to be 2.03mm? That seems to not be right at all and will affect your final CRs calculated. I think my Pilot and Ody have a deck height at about 0.75mm. Arnt you using an ody piston?

And another thing. When you are done making a mess with that custom cut dome, I suggest you buy a single Yamaha 701 ADA Racing dome. They support up to 83 mm diameter and have a range of CC sizes. They are CNC'd for accuracy and use an O-Ring for sealing. Plus when you roach an Engine, you can just replace the dome rather than fix the whole head.

Also they are cheap. Like $50 each. Often times you can find them for like $25 used, because often when someone blows their twin 701 motor (( Internal Combustion Engine ? )); they end up selling the good surviving dome (used) in favor of getting new matching pairs. I used to have one/some I'd send you one, but I don't have any anymore. The only other person I know on this board which may have one is Adnoh.

https://adaracing.com/dome-yamaha-701/

Then all you'd need to do is make a 'shell' for the dome.