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Old 12-20-2008, 08:36 PM   #1
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Default Nitro Engine rebuilding

Howdy folks, I have a couple questions about engine rebuilding.

I've gotten a couple of engines on used buggies/truggies that have qustionable compression. Can I bring these engines back to life by simply changing the piston/sleeve and bearings? The cranks on both engines look smooth with zero scoring, and the head buttons look like they are in perfect shape. Neither will pass the gravity test though, so I was considering putting a new sleeve, piston and bearings on both engines. One is an Ofna Picco 27 wich I gather is a pretty cheap engine. The other is a GS B01 .21, that I understand from talking to a few people was a pretty good engine in it's day.

Are there any other things that I need to look at before deciding to rebuild these engines? And is there anything else that should be replaced at the same time?

I don't have any nitro engine rebuilding experience, but I have built/rebuilt everything from from a lawnmower engine to the custom 502ci engine in my speedboat, so I should be able to handle the technical aspects.


Any input from you engine guru's would be greatly appreciated.

Thanks,
Kris
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Old 12-20-2008, 08:41 PM   #2
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Yes all you would heve to do is change piston/sleeve/rod and bearings to get them back to spec. Either that or get them pinched. Depending on the price of the motors and the price of parts for them it might not be worth it to replace those parts. A pinch can be gotten for fairly cheap and if everything else in the engines is in working order you could get them to last 3-5 more gallons.
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Old 12-20-2008, 08:43 PM   #3
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the gs bo1 is nice not the fastest but run so good i have 6 gallons of racing on mine and never opened it up then i sold it and they beat the hell out of it and is still running. i bet a good port job wood make that motor mean.
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Old 12-20-2008, 08:48 PM   #4
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well u can send them in and have them pinched. u want to replace the rods. they tend to wear out after the engine loose compression. i'll look up where to send the piston and sleeve into. if u want me to. thanks nate
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Old 12-20-2008, 08:55 PM   #5
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os rocket
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and a dude on here 22racer
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Old 12-20-2008, 09:24 PM   #6
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Kris,

whether you get a pinch or fresh P&S, definately replace the bearings; they're cheep. Inspect the piston face and look for pitting and cracking (this is more likely to be seen on the outer edge). Do the same with the head and check the squish band. If the piston face or squish band looks rough, or bad in any way, the rod, wrist-pin & bearings have had a hard life and should be replaced. Other wise keep the old rod & pin (they're broken in.. not broken). I would also suggest measuring the roundness of the crank pin. if the differance between high and low is greater than 10%, replace the crank. (this is also a good way to judge the number of hours on the engine).

If you really want to get into the nitty-gritty, take some measurements while the engine is in pieces. This information will help you set your pipe length accurately and know what kind of performance the engine is tuned for.

Oh yeah, how did you check the compression?


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Old 12-20-2008, 09:39 PM   #7
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Here's a link to my thread. I don't agree with changing bearings, only if the need to be changed.
http://www.rctech.net/forum/nitro-of...ebuilding.html

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Old 12-20-2008, 10:15 PM   #8
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Thanks for the info guys.

Both of the engine's squish bands look great. There is zero noticable pitting, cracks or scratches on the pistons, cylinder walls or the combustion chamber in the heads. The top of the pistons look clean with almost no discoloration, and the cylinder walls look smooth with only a slight change in shade showing the top of the piston travel.

Chunk - I'll go ahead and mic the crank pin (Hoping the crank is still good). But I'm totally confused as to what you mean by taking measurements helping me set the pipe length accurately and knowing the performence that the engine is tuned for. What exactly do you mean by that? Thinking of automotive engines, I understand how knowing the exact displacement and the volume of the combustion chamber will help determine the overall compression of an engine, but how does this relate to a nitro engine?

At this point I'm thinking that the Ofna engine might only warrent a pinch and a new set of bearings, while the GS B01 will get a new sleeve and piston as well as bearings.

I just want to do this right and end up with functional engines and not a huge headache instead.

Once I either pinch or replace the sleeve/piston, will I need to do a complete break-in as if they were new?

Thanks for the help,
Kris
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Old 12-21-2008, 09:28 AM   #9
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yes u will have to break them in again.
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Old 12-21-2008, 01:37 PM   #10
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Kris,

These little nitro engines are two stroke pumps with ports in the sleeve and crank. As the piston goes up & down it changes the pressure in the crank case and in the combustion chamber. The action of the piston sliding up and down past the ports regulates the flow of fresh fuel and burnt fuel. The TIMING (duration a port is open) is one component of the larger equasion of how the engine was intended to produce it's peak power.

The exhaust system we use is actually a combination of a header and a tuned resonance chamber (Pipe as we like to call it). The header aids in removing burnt gasses and pullining in extra fresh charge; this is called scavanging. The PIPE reflects the impuse after it has travelled through the header, back into the header & ultimately the exhaust port; this is called packing. these two actions allow the engine to burn more fuel than its actual displacement. Think of it as super charging.

An engine that effectively uses the tuned exhaust produces more power, gets better fuel efficiency and runs cooler.

Just PM me if you want to know more specifics.

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Old 12-21-2008, 04:47 PM   #11
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Quote:
Originally Posted by CaptCrunch View Post
Neither will pass the gravity test though, so I was considering putting a new sleeve, piston and bearings on both engines.
Can someone please explain the gravity test?
Thanks
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Old 12-21-2008, 05:06 PM   #12
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Chunk, you have a PM.

Herb, the way it was explained to me, if you hold the engine by the flywheel at a 90 degree angle right as it is heading into the compression cycle, the compression should be able to hold it sideways for at least 10 seconds before the weight overcomes the compression and the engine falls downward.

Folks please correct me if I'm wrong.

Kris
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Old 12-21-2008, 06:59 PM   #13
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Quote:
Originally Posted by mungen View Post
yes u will have to break them in again.
With a new piston and sleeve you will need to break it in. Some people that pinch motors do them pretty tight and you will need to rebreak in. When I pinch a sleeve I try to get it about like a motor after a gallon on it. There is no need to get it as tight as it was new as it has already worn in as a set. I only recommend a couple tanks 2 hours richer.

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Old 12-21-2008, 07:25 PM   #14
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How exactly do you pinch the motor?
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Old 12-21-2008, 10:53 PM   #15
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how come all the sudden there is a bunch of guys trying to advertise that they will work on your engines , (not just you rex) but holy smokes there are about 8 guys that are trying to get you to send them your engine .
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