Continental o-300 Repair or Replace?

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minnesotaairboat
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Continental o-300 Repair or Replace?

Post by minnesotaairboat »

Hi There,
I'm looking for some advice on this beauty I found. My hopes are turning it into an ice fishing machine here in Minnesota.
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The owner had it sitting in a barn since he picked it up in Utah at least five years ago. He said the previous owner fired it up for him but it has not been run since. It's now mine for a pretty cheap gamble and I first want to figure out how to proceed with the engine, which is a Continental o-300b. Should I attempt to get it running or sell it and put a different (easier and cheaper) engine in its place?

It's not able to attempt a start without some work including on the gas line and pump wiring. In the meantime, I decided to do a differential compression test on the cylinders. They were horrible numbers. Cylinders on one side were about 10/80 and the other side of engine were 30/80. I could hear and feel the air coming out of the oil fill cap during the tests, but couldn't notice any from the exhaust or carb. Just to head off questions, I did learn how to set the cylinder positions for the test, took out the other cylinder plugs (top set), and opened the throttle.

I figured the rings might need some oil after sitting and put oil in top of all the cylinders and spun the prop for a while. The new compressions tests showed all three cylinders on one side at 71/80. The opposite side cylinders were still terrible from 10-30/80. Opening up the valve covers, I found the same pattern. The bad side was absolutely filled with crud. The better side was far from clean but did not have all the loose debris like the good side. With the blow by in the valve covers I was surprised I didn't also feel air coming from the exhaust or carb during the compression test.

I am a novice on engines, but at the least I'm guessing it needs rebuild of the valves etc along with piston rings? At that point would it be better to overhaul or replace the entire cylinder? I see complete replacement cylinders run about $1k each. Based on the poor shape of the valves I would guess there is a risk of issues in the crankshaft and accessory parts as well.

I'm wondering what else should I look for in the engine, suggestions for a repair plan, and whether I can take it to a small engine mechanic rather than a aircraft mechanic? With all the work, would it be better to find a different engine? What are my options for something easier or cheaper? Also, is there any value in the Continental engine as it sits?

Thanks for all the help!
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Slidin Gator
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Re: Continental o-300 Repair or Replace?

Post by Slidin Gator »

Minnesota,

Welcome to the addiction!

Dude, that is a cool looking boat. I think you are well on your way to a great ice fishing machine. Just remember, you don't cut the fishin hole in the bottom of the boat :violent1: :violent1:

I do recommend you put a full cage over that prop, the Great Salt Lake must rot brains :scratch:

I am not a Continental guy and am not familiar with the O-300, but I believe it is a bit rarer than other options, I imagine you can find parts, it is just gonna take a bit more searching. It looks like a lot of those engines were made in Britain. Loaded cylinders for most other models run $300 for good used running to $650 rebuilt new. The $1000 each you mention might be right for a rarer beast and aviation folks up north (or even inflation :dontknow:). As for the bottom end, I would not write it off based on the condition of the cylinders, aviation crankcases are work horses and pretty bullet proof. You want to know if it makes oil pressure.

If the engine is running as you say, I would light it up and get it good and hot (at least 180 oil temp, 200 or more even better). Once hot check oil pressure at idle and WOT. I don't know exactly what that engine calls for, but I would be looking to make at least 40 at idle and 60+ at WOT. The link below takes you to the stash of Continental manuals on this forum. No 300 manuals, I would use the 470 as a guide on oil pressure if nothing else.

You also want to figure out the oil circuit, is there a relief valve you can adjust?

https://www.southernairboat.com/pdf/air ... ntinental/

If she makes good oil pressure, I would slap on 3 cylinders, buy 3 more for spare and give her hell this winter. Otherwise, consider upgrading now before you pour money into an antique.

What are the dimensions of that hull?
I still think I-10 is the Mason Dixon line.
2013 Mike Stossel Boats, 13' x 7' deck over, IO-540, 74" NGQ less winglets.
minnesotaairboat
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Joined: Fri Jul 16, 2021 8:44 pm

Re: Continental o-300 Repair or Replace?

Post by minnesotaairboat »

Thanks for the help Slidin Gator, those manuals will be useful. The hull is 6' x 16'. It is full width up to 8' and then tapers to 2.5' at the nose. It seems like most people are running much bigger engines on smaller hulls. Am I going to be vastly underpowered with this engine?
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I did attempt to fire up the motor but there appeared to be a short somewhere in the system when I hooked up the battery (key off). I quickly turned the key and the starter did engage but I pulled off the cables before they began to fry. That will have to be resolved before I do anything else.
The prices you mentioned for cylinders sound much more reasonable. I was looking at the new Superior cylinder setups. Any ideas where exactly I could get some of those loaded rebuilds?
I could not find a pressure relief valve for the oil. Would it be associated with the oil temp fitting at the bottom of the accessory cover? Just wondering if anything else appears to be missing on the engine? There isn't an oil sump or filter like I see on other boats and it looks like the carb is missing the cover/filter.
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Does anyone know of any airboat places up my way (Minnesota) or does anyone in Utah recognize this hull design?

Thanks for advice
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Slidin Gator
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Re: Continental o-300 Repair or Replace?

Post by Slidin Gator »

minnesotaairboat wrote:
Sun Jul 18, 2021 11:24 am
Thanks for the help Slidin Gator, those manuals will be useful. The hull is 6' x 16'. It is full width up to 8' and then tapers to 2.5' at the nose. It seems like most people are running much bigger engines on smaller hulls. Am I going to be vastly underpowered with this engine?
I see manuals for that engine listed on ebay for $20, certainly worth the investment. It looks like it shares cylinders with the 4 cylinder O200. From Wikipedia I see the O300 is rated for 145 Hp and runs low compression 7:1 pistons. On a 6 foot wide hull that should be plenty of power, it looks like the boat/engine have had a good run at it and ice is pretty slippery (note that I read folks on ice carry a piece of wood to park on so the hull does not freeze to the ice). At that hull width you can only swing so much prop before it becomes scary, I imagine 180-200 Hp would be a reasonable limit.

What prop is on it? There should be a label showing diameter and pitch. I will caution here that the wooden prop should be suspect, you need to inspect it well before putting some RPM's to it, definitely figure out how to run it up from a safe location. Ideally you should send it to the manufacturer for inspection and refurb, a splintered prop will make it's own fishing hole in the hull.

With low compression pistons you can run that engine on pump gas, ethanol free rec fuel ideally. If I were considering an upgrade, I would be looking for a larger displacement 4 cylinder. From Lycoming the low cost entry would be an O320 which can be had in 150-160 Hp versions. Good used units might be around $4K. Otherwise O360 setups will net you 180-200 Hp and can run $6-$12K.

Some quick googling and I find a O300 for $8,200. It says it had a prop strike. FAA requires a complete tear down and inspection after a prop strike so a lot of times the owner sells and upgrades. This is where a lot of airboat engine start out.
https://www.trade-a-plane.com/search?ca ... ype=engine
minnesotaairboat wrote:
Sun Jul 18, 2021 11:24 am
The prices you mentioned for cylinders sound much more reasonable. I was looking at the new Superior cylinder setups. Any ideas where exactly I could get some of those loaded rebuilds?
I have no idea what/where you might find them locally. There are plenty of good sources for airboat aviation engines here in Florida, we can give you a list of phone numbers to call if you want to get parts shipped to you.
minnesotaairboat wrote:
Sun Jul 18, 2021 11:24 am
I could not find a pressure relief valve for the oil. Would it be associated with the oil temp fitting at the bottom of the accessory cover? Just wondering if anything else appears to be missing on the engine? There isn't an oil sump or filter like I see on other boats and it looks like the carb is missing the cover/filter.
I have no idea about the specifics of the oil circuit, hopefully you will get some Continental folks to fill in details. The air intake box on the bottom is also the oil pan, the intake runners run through the hot oil to keep the intake from freezing up, just what you need for winter. There are probably 2 filter screens, one on the intake and one on the pressure side. You can add a spin on filter kit, but I doubt it makes much difference on a well worn engine like this. Yes, you need a screen on the carb minimum (Flame arrestor), a K&N is even better. If you decide oil pressure is good and end up pulling the cylinders, make sure to check the condition of the cam lobes and lifters, if the cam is bad you are into a full rebuild.
I still think I-10 is the Mason Dixon line.
2013 Mike Stossel Boats, 13' x 7' deck over, IO-540, 74" NGQ less winglets.
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