Baker Monitor 1/3 scale Engine

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Bruce W-S
Project of the Month Winner
Jan 30, 2015
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Perth Western Australia
I have a 1/3 scale Baker Monitor engine built by someone else about 15 years ago. While the engine ran once for about 15 minutes, I haven't had much success since. One of the issues was the apparent lack of compression. I would have expected somewhere around 4 or 5 to 1 but it doesn't feel like it.
I decided to remove the head and check the rings and in the end, I replaced them. I was surprised at the amount of free space between the piston at TDC and the head and combustion chamber. The overall shape does not make it easy to calculate the volume with any accuracy so my question to the Forum is, does anyone know what compression I could expect from such a model?

Thanking everyone in advance

Bruce W-S
Baker used the name "Monitor" for many things, including their windmills.

The "Little Monitor" (1 1/4 hp) that is commonly seen in a pump jack configuration used a "GA" prefix in the repair parts catalog list of parts.
The engine had the vase-shaped water hopper.

The Monitor "GB", "GB-A", "GD" and "GF" were the vertical ball hopper engines, 2, 2, 4 and 7 horsepower respectively.

Baker had a line of horizontal engines, and I don't see a prefix assigned to them, but they were 3, 5 and 8 horsepower.

The term "VJ" was defined as "vertical jump spark ignition", and this term was used to describe all of the Monitor gasoline engine styles.

One of the old Baker catalogs shows photos/engravings of pump jacks powered by both a vertical ball hopper style engine, and a horizontal Monitor engine, so the pump jack engines were not just the vase-top type.

I am not sure if the vase-top engine was only built in the 1 1/4 horsepower size, but it would appear that this style was only 1 1/4 hp.

So assuming we are discussing the vertical 2 hp ball hopper style Monitor, the catalog indicates a 2.5" bore, and a 3.5" stroke.
With the piston at TDC, there is a 1.0" clearance between the top of the piston and the bottom of the head.

There is additional space in the passage leading to the valve chamber, and space in the valve chamber itself between the two valves.

Calculating the volume of the cylinder above the piston, with the piston at BDC, we have:

(pi x r-squared) x 4.5 = 22 cubic inches

with the piston at TDC, the remaining volume is

(pi x r-squared) x 1.0 = 4.9 cubic inches

Neglecting the area in the valve chamber and associated passage, the compression ratio would appear to be approximately 4.5 : 1.

If the valve chamber and associated passage are included, then perhaps 4 : 1 compression ratio?

Perhaps someone can check my math on this item.

Given the questionable quality of gasoline back in the day, I would assume the small engine manufacturers would err on the side of caution, and keep the compression ratio low.

A 1/3 scale ball hopper monitor should have about 0.333" clearance, assuming the folks that made the kit accurately scaled the full sized engine.

Most of the hit & miss engines are fairly low compression. I don't have the 1/3rd scale PMD one that I built but it was possible to bounce the flywheel back against compression if you did not pull it hard enough to overcome the compression.
Many model hit and miss engines have a compression ratio lower than the original which allows tuning to run at a nice slower "scale" speed.

I have a Baker Monitor and it is a 1/3 scale of the 4 hp original. It was built by Ellis Smith (a very good builder) and it does not have much compression and runs great. They are very sensitive to even slightly compromised spark plugs and like a nice strong spark. I use 12 volts with the buzz coil. While they can run on automotive fuel, I would never put it in any of my model engines. I use Coleman or Crown camping fuel (white gas) or a pure fuel like 92-94 octane yard tool fuel. I mix each at 20-25 to 1 with Marvel Mystery Oil.

I had videos of it but they seem to have vanished from YouTube. Go figure!

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