View Full Version : Low compression picture

12-08-2005, 09:05 PM
To follow my yesterday thread picture explain better than words This pictureshow how far the piston is on desd center
this one is general view of the hit miss engine plus the head
What is the best solution a spacer on top of piston, new longer piston, longer connecting rod,
How close from the head I can go

12-08-2005, 09:18 PM
I'd go with a much longer connecting rod. Nice work.

12-08-2005, 09:37 PM
Yes longer rod,infact one that brings the piston to within .030" of the head.

Nice looking engine also!

12-08-2005, 10:42 PM
That is a very nice looking engine and nice work too.

It is a gas, not gasoline, engine? I ask because of the plumbing on the intake side of the head.

Also, is that the ignitor on the side of the engine, and does there have to be clearance for the ignitor. Will increased piston stroke block the spark from the combustion chamber?

J. R. Williams
12-08-2005, 10:54 PM

It appears the engine is designed for an internal ignitor and the space is for that part. Your engine has a magneto installed and I do not see a spark plug.


12-08-2005, 11:19 PM
Nice work Luke. Is that a functional mag on I
see on the engine?

If you make a longer rod, a heads up on a problem you may encounter. Do the math on the
new rod design before you make one. It may hit
the back of the cylinder as the crank rotates.


J Tiers
12-08-2005, 11:45 PM
AFAIK, most of those engines are pretty low compression, maybe 5 or 6 to 1.

Your picture does seem to show VERY low compression, roughly what your other post said. 1.5" stroke and 0.910 headspace is about 2.6 to 1. That would give really low power, power increases with compression non-linearly, I believe.

Obviously, if the spark plug is next to the magneto, the piston needs to clear it. So that establishes your minimum clearance.

You have a couple options.

If the geometry allows it, you can adjust the rod until your 1.5" stroke length is 4 or 5 times the remaining clearance, i.e. clearance is around 0.3" to 0.375". That gets you 5 or 6 times compression, and ought to go OK.

If the geometry does not allow that, you may have to re-make the head with an extension into the bore so as to bring the effective headspace down and yet leave the spark plug where it is. That "looks like work", since it is already finished.

I suspect that there is a really bad dimension somewhere on your plans.....

Was there ANY part that you might suspect you had to take too much off when machining? Or a part where you had barely enough material to finish all the features correctly?

The first might suggest a wrong dim that is small, the second a wrong dim that is too large.

And, where IS the spark plug?

12-09-2005, 12:00 AM
Hmm. Trying to imagine what errors are on the plans that threw things out. Is the crankshaft too far from the cylinder?
I kind of think the longer rod would be the best answer. I wouln't want to increase the weight of the piston too much, and to shorten the cylinder looks to be tough as well.
What are the chances that this engine is SUPPOSED to run with that low compression?
Seems odd to me. Something else I would be looking at is the travel of the piston in the cylinder- does the entire piston stay within the cylinder at BDC, or does some of it stick out- or does it not come to the bottom of the cylinder at BDC?

12-09-2005, 01:03 AM
Old John Deere "D" tractors were designed to run on distillate fuel(they started and warmed up on gasoline)and had a compression ratio of 3.92 to 1. They had petcock compression releases on the cylinders to allow hand starting the 501 cubic inch two cylinder engine by hand turning the large flywheel. The point is, engines will start and run with VERY little compression. Petcock on cylinder and the large flywheel can be seen in the photo below:


[This message has been edited by Carl (edited 12-09-2005).]

J Tiers
12-09-2005, 01:04 AM
Hey...... wait just a moment here......

I'm looking at the head in the picture..... and the cylinder pic.

There looks like there is a "spigot" that goes into the cylinder a bit. Maybe anything from 0.2 to 0.3 or so, its pretty hard to estimate.

The cylinder pic looks like maybe you haven't allowed for the spigot in your 0.910 dimension.

That distance comes off of your 0.910 from piston to end of cylinder, less any gasket space.

If your spigot is 0.3 effective, the clearance is only about 0.6, and compression is really around 3.5 to 1, which is getting closer to reasonable for a low compression engine.

What is the basic problem?
Is it that the thing won't run?
Have you tried to start it yet?

You might try to start it, and see if you have a problem with it running.

Most of those don't "pop over" compression like more modern commercial engines.

12-09-2005, 08:16 AM
a lot of those old engines had a very high lift on the cam and a lot of adjustment for valve clearence and if its a hit/miss eng the intake actually is held open by the gov to allow the eng to coast---, or run down,till it needs to fire again.

12-09-2005, 08:42 AM
You could also make an insert that attached to the cylinder head above and below the valves, and extending into the cylinder almost to the top of the piston. It would look like this:

Actually, I hope it looks better than that. http://bbs.homeshopmachinist.net//redface.gif

That would fill the void and raise the compression without restricting the flow around the valves, and it wouldn't interfere with the ignitor.

You might be able to have a little lip on the top to fit between the cylinder and the head, and cut the gasket a little to clear the it. That would eliminate the need for drilling and tapping the head for the fasteners.


12-09-2005, 09:09 AM
Thanks to all
I like the wedge idea.
The kit was made for spark plugg ignition. The igniter is of my own design. Doing that I add cubage
Short story. In the late years,here in Canada engine dealers can sell a barrel of nail as an engine if it had a Webster on it. The spark was hotter on cold morning with the Webster

12-09-2005, 10:22 AM
<font face="Verdana, Arial" size="2">....if its a hit/miss eng the intake actually is held open by the gov to allow the eng to coast---, or run down,till it needs to fire again.[/B]</font>

Generally Hit/miss engines had only one push rod and it was the exhaust held open by the cam. Intake operates atmospherically. Air pressure pushes air/fuel in to replace the partial vacuum as the piston draws back to bdc.

I know there is some space at tdc on these types of engines but that picture looks excessive. But as someone else said, the big ones ran with very low compression unless ignition or fuel delivery problems. Though models can be much more temperamental. I have a Briesch Lil' Brother (3/4" bore x 3/4" stroke that I've not been able to make run, compression is low but I think the problem is ignition.)


12-09-2005, 10:39 AM
<font face="Verdana, Arial" size="2">Originally posted by pkastagehand:
I have a Briesch Lil' Brother (3/4" bore x 3/4" stroke that I've not been able to make run, compression is low but I think the problem is ignition.)

. </font>

I built the little Brother also. I have gotten it to run sporadically, but not steadily. I plan to redo the point set up at some time, either with a small automotive set or a solid state set up like Jerry Howell sells.

[This message has been edited by JCHannum (edited 12-09-2005).]

12-09-2005, 10:45 AM
I'd ask the same questions as J.Tiers: does it run, not start, or what? These little engines can be much worse with high compression than low, in some cases. One well-known model builder reported that one of his older engines, built 10+ years ago, was always tempermental. Hard starting, kick back on you while cranking, jump all over the place while running, etc. He lowered the C.R. (don't remember what he did exactly) and it's a different engine. I'd surely try running it, getting the ring(s) seated, etc. before modifying the C.R.

We learned a hard lesson on our first engine project: there must be NO excessive friction in any part of it, or it simply will not run. Don't think that "it'll wear in", it won't.

12-09-2005, 03:19 PM
I built my Bob Shores Eagle hit and miss with 5:1 compression. I wish I would have gone with a little less. The idea with the low compression is to get the engine to "idle" very slowly to sound more like a full sized hit and miss. When mine fires it still does so with a lot of power even at 5:1