ST G21/35 Combat Special Compression Ratio

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ST G21/35 Combat Special Compression Ratio

Postby Ken McClenahan » Fri Jan 12, 2007 11:14 am

I have a ST G21/35 Combat Special that I bought back in the early 70s and used for fast combat back before the Fox Mark III became available. A few months ago I took the ST out of the can and flew it. Predictably, it banged my finger hard on the first flip. I did get in three flights with Tom Chambers doing the starting using a chicken stick.

The engine was very quiet so it is apparent that it is vastly over compressed, which goes along with the finger banging. I would like to reduce the compression ratio but don't have the foggiest idea how much to shim the head up. I can make aluminium head gaskets of any thickness over about 10 thousandths out of bar stock using my Unimat but it takes several hours for each so I would like to have some idea of what works if anyone out there has ever done this.

I have noticed that some people believe that this engine was designed for use with FAI fuel but I don't klnow why World Engines would not have informed the Italians that we don't use FAI fuel for fast combat and, in any case, I have never noticed that the amount of nitromethane in the fule had any appreciable effect on ignition timming.
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Postby gossie » Fri Jan 12, 2007 12:28 pm

With high comp. it will run better on FAI fuel.
If you shim the head how ever you wish to, to lower the comp., then it will run well on nitro.
The lower the comp. the more nitro.
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ST G21/35 COMBAT SPECIAL COMPRESSION RATIO

Postby John Kupinski » Tue Jan 15, 2008 4:40 pm

Set your deck clearance (piston to squish band gap) to .015 and the motor will run beautifully on 25%. I sometimes would run 45% in Fast and would have to increase the clearance to .018.

I used to be heavy in combat and was competitive with the ST until the Nelsons came out.

To measure the clearance, I used pieces of automotive feeler gauge inserted carefully thru the exhaust port and with the head on tight and plug out. If the engine could be turned over with an .015 shim in it and not turn with an .017, I was good.
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ST G21/35 Combat Special Compression Ratio

Postby Ken McClenahan » Tue Jan 15, 2008 9:40 pm

Many thanks for the information.

I can measure head clearance by using a screw through a metal bar (a home made end micrometer) and an ordinary outside micrometer. One does have to carefully count a lot of turns on the screw though.
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Postby loucrane » Sat Jan 19, 2008 1:12 pm

Ken,

Don't know if it is still available, but in the 1970's I used a 'squashable material' - Plastigage - to check deck clearance. It came in at least two ranges, or at least I still only have two...:0.002-0.006" and 0.004-0.009". It was a Perfect Circle product, wih an address as Hagerstown, MD (no further details.)

As I said, I don't know if it is still around, or if so, is available in a suitable range for your G21/35. Simple enough to use: lay a short length of it on the piston where you want to know the deck clearance, turn it through TDC and remove it. Compare its squashed width with bars printed on the sleeve it ships in.

The thin round-section strips are color coded to reflect their range.

By the way, it may be quicker to cut head shims out of light aluminum sheet, like that used on pull-tab coffee can lids, disposable oven roasting pans, and such. These are a soft, easily cut aluminum.

A quick and dirty example: mark the bore diameter on the stock and use a 1/4" paper punch to remove most of the material inside the line. A Dremel red point can get right down to the line; debur the ground edge. Center the hole on the sleeve and mark your OD to fit. Tinsnips can get you close enough to the line.

Make sure the 'shim' clears the other pieces for assembly. Stacking as many as needed to make up your desired clearance is as good as turning a very thin piece, and both quicker and less messy.

You can do about the same with hardware store 0.020" roofers' 'flashing' aluminum, but might wear out a few paper punches... and that's several times the thicknesss of the can lids/roasting pan material. You might need a thickness that doesn't measure in units of 0.020"...

Luck!
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ST G21/35 Combat Special Compression Ratio

Postby Ken McClenahan » Tue Jan 22, 2008 9:49 pm

I used my homemade end micrometer by putting the bar across the bottom of the head and adjusting the screw down until it hit the flange. With the screw in that position, I then put the bar across the top of the sleeve and adjusted the screw down unitl it hit the top of the pistion at top dead center. The difference was a quarter turn on the 4-40 screw.

I used an outside micrometer to measure the thickness of the aluminum head gasket, which was 0.0104 inches.

Therefore the head clearance is 0.0104 + 1/(4 X40) = 0.01665 inches.

Since the engine is obviously overcompressed. it needs a lot more head clearance.

This ST G21/35 Combat Special from 1974 has a flat squish band and a single bubble. The flat squish band may have something to do with the problem although I had the same problem with a later version that had a double bubble head. I don't have the later version anymore so I can't look at the head.
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ST G21/35 Combat Special Compression Ratio

Postby Ken McClenahan » Sun Jan 27, 2008 3:36 pm

It finally occurred to me to calculate the compression ratio. Using some crude measurements, I come up with about 16:1, which is indeed excessive (no wonder it is a finger banger). Using a 0.020 inch thick head gasket would drop that to a much more reasonable 12:1.
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ST G21/35 Combat Special Compression Ratio

Postby Ken McClenahan » Sun Apr 27, 2008 8:18 pm

I cut a special head gasket that turned out to be 0.023" thick instead of the 0.020" I intended. I installed it and made four flights today. The finger banging is gone. I got one flip starts twice, flooded the engine once (my fault), and had a plugged needle valve jet on the forth flight.

Using 25% nitro, the speed on a combat model is about 90 mph at 5000 feet altitude, which is equal to about 101 mph at sea level. I am using a Rev Up 8-1/2 X 6-1/2 prop cut down to 8" diameter but it is too large so I will get some increase in speed whenever I figure out what the right prop is. Thirty years ago flying at sea level with the original head gasket I was getting speeds around 105-108 mph.
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Postby rick32559 » Tue Sep 30, 2008 9:35 pm

i have several g21 35 combat motors. ya pop em clockwise against the compression. use a piece of hose on your finger and slap the prop clockwise. it will bounce back and start in 1 or two flips
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Postby beaslbob » Fri Feb 25, 2011 2:03 pm

Yea we used to flip them backwards.

This has brought back memories of the 70 (71) nats where juniors from our area (des moines iowa then) pulsed all the combat guys at the nats on how the set up the tigres.

What they found out was to drop fit the bearings which the juniors (horror!) did with a dremel tool. Then lap the crank into the now binding case. :shock: And remove the head gasket. The g21.35 also had the intake on the left side of the crankcase (front view looking back) which had a "web" and the bottom turning into the case. They removed the "web". And obviously changed the crank timing. Then they changed from a 9x7 prop to a 8x8.

This all resulted in faster running engines that were harder to start and also ran backwards at reduced power. :shock:

And starting the engines with the back flip.

Awwwww memories. I wonder if I have the old g21/35 in shipping carton 185 or 123. Need to find it some day. :lol:
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Postby beaslbob » Fri Feb 25, 2011 2:06 pm

Also:

the increased compression started blowing out the seals of fireball plugs.

So we started reinforcing the plug seals with epoxi.

One junior told us he even tried soldering em. Said it didn't work too well with the battery connected. :oops: :lol:
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ST G21/35 Combat Special Compression Ratio

Postby Ken McClenahan » Sat Feb 26, 2011 9:39 am

I eventually got the speed up to 95 mph using a old Top Flite 9x6 prop. This is at 5,000 feet altitude using 25% nitro fuel and a very large airplane (over 500 sqare inches with a wingspan over 50 inches). A slightly smaller prop would work even better but I gave up on the project because I had proved that reducing the compression ratio to something reasonable does not cause a reduction in power output.

Changing to 40% nitro fuel would increase the speed to 99 mph. Using 40% nitro fuel at sea level would give a speed of 108 mph which is as good as I ever got at sea level 35 years ago.

Using an overcompressed engine and a chicken stick (or the equivalent) is a bad idea when there is no bennefit.

I tried using epoxy glue to fix the seal problem on Fireball glow plugs back in the sixties and it did not work for me. The 49 cent Fireball plugs drove all of the other brands off of the market for a while so I just gave up on flying combat for several years until decent glow plugs became generally available again in the mid 70s.
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Re: ST G21/35 Combat Special Compression Ratio

Postby kersplat » Thu Jun 11, 2015 7:38 am

Another easy method for checking Squish clearance. Take a piece of lead solder (no flux core). Bend it in the shape of an "L", put it through the plug hole and turn the engine over by hand. the solder will squish after doing this. take a Mic or a Vernier caliper and measure the squished end of the solder. you will have the squish clearance. Shim accordingly.
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