davidej

I bought a 110a cheap from a fellow club member as he said it needed bearings replaced. I ran it on a test bed and it seemed fine but after using for a couple of flights,it was obvious he was right so I started to dismantle it.

So far;

Head off
Backplate off
Liner withdrawn

Now to get the gudgeon pin (piston pin as OS call it) out. Piston at bottom dead centre, small drift through hole in the front and first the PTF pad and then the pin drop out.

But the piston still seems to be connected to the con-rod (at the front of the piston, not the back). Doh! what is wrong?

I look at the pin and it is obviously much too short to have gone right through. Worse, there seems to be clear evidence of a fracture of some sort at one end

It seems to me that part of the pin is left in holding the front part of the piston to the con-rod.


Anyone got any suggestions how to proceed from here?


PS If necessary, I can try to post a photo of the bit of the pin that I have extracted.

speedracerntrixie

My Feedback: (29)

You should not be trying to separate the piston from the connecting rod. You should be separating the rod/piston assembly from the crank pin. I'm sure there are several videos available on youtube, look some up and spend some time educating yourself before ruining your engine.

757jonp

My Feedback: (48)

I've never heard of a piston pin failing in one of these, but that sure sounds like what happened. I don't see much option other than to drive it out, which is probably going to ruin (if isn't already) the rod and piston. Probably was hydrolocked at some point and forced? How's the cylinder look?

davidej

Trixieboy

Thank you for your unhelpful (and marginally rude) reponse.

The only youtube i can find is for an OS FS52 and it clearly shows that you should remove the gudgeon pin , not try to get the big-end off the crankshaft pin.

If you had looked at the motor in question before posting, it would be obvious to you that you advice is quite wrong.


I hope yet to have some helpful replies, but thank you again.

speedracerntrixie

My Feedback: (29)

Well the exploded view did not clearly illustrate the hole in the front of the case so it does seem that OS designed the engine so it could be taken apart by pushing the piston pin out. This however is not the norm and at this point you may want to do as Earl and I suggest and try to get the rod off the crank pin. You may end up damaging the rod in the process but it's most likely damaged anyways.

earlwb

Well although there are some engines in the past where you had to remove the gudgeon pin to get the rod off. The more modern glow engines had quit doing that. What you normally do is move the rod back inside the piston towards the rear of the engine as far as it will go. Then the rod will usually just pop off the crankpin then. Sometimes it is a bit tight but usually it will come off. Sometimes applying heat to it will soften any old congealed oil that might be keeping the rod from going back farther. Using a "L" shaped tool can help here too. As you can get it behind the rod to help pull it back more.

Now since part of the gudgeon pin broke off inside the piston. The broken edge could have belled out or swelled out at the break point and that may be jamming it up.
If you can't get the rod to move back that extra tiny bit. You may have to force the issue with something to help pry the rod off of the crankpin. Or worse cut the rod in two. Another thought is to use a small rod or shaft that fits inside the piston and rod to see if you can push the broken piece more forward and also get the rod to move back as well. Then it may move enough to get it off of the crankpin.

davidej

if you don't have to remove the gudgeon pin before getting the big-end off, why would OS put two holes of the right size and in exactly the right position to allow you to do so?

To put any end to this disagreement, I will ring up Ripmax in the morning and get their advice.

speedracerntrixie

My Feedback: (29)

It may have been done for ease of assembly on the production line. This is the only engine I have encountered this on in 40 years of running glow engines. Like I said, I have taken apart literally hundreds of engines by removing the sleeve and then popping the rod off the crank pin. I see no reason why this can't be done here.

cahusker

My Feedback: (50)

I have disassembled many OS four stroke (FS) engines but none of the newer alpha series. All older OS FS engines have to have the piston pin pulled out in order to get the rod off the crankpin. Sometimes it can be a real pain in the butt to get them out. The idea of having a hole in the front of the block to push out the pin is nice and sounds like it worked well for you except it didn't all come out. Two ideas to share would be to try to push the broken piece back into the rod just a bit to see if it releases the front of the piston then, if that fails then you will need to figure out how to continue pushing the piece out as you started. A little heat on the piston with a heat gun may help, just heat the top of the piston and let it heat soak for a few minutes and see if the pin isn't easier to move.

Propworn

My Feedback: (3)

If it’s like other engines past or present that have these access holes then yes you must remove the pin and separate the piston and rod for removal. Be cautious of advice from experts who may suggest forcing the rod from the crank pin. You may do enough damage to make the motor unsalvageable. Never a need to force anything in my opinion.

Look at the part of the pin that came out. It should be hollow. Is the hole in the front of the motor big enough to allow the pin to slip inside? If so the pin should be removable from either side. The pin will be hard but if you can find something small enough that you can get it to bind inside the pin you may be able to turn it while gently pushing or pulling it out through either access hole. If you feel the need to heat it try putting it on a piece of steel plate in the BBQ no more than 250 degrees. A welding glove makes handling it afterword comfortable.

Hint it is important to keep the holes in the crankcase lined up with the piston when assembling and disassembling the engine. If you use the liner (just slip it over the piston far enough to keep the piston centered in the crankcase leaving the pin bore clear) it will hold the holes in alignment making the removal and replacement of the wrist pin easier.

Keep your eye out for two teflon/nylon caps that go in the ends of the wrist pin. If you have not removed this from the part of the pin left in the motor it may be hindering you from pushing the pin out. Sometimes a heavy straight pin forced into it can aid in removal or push it out from the far side through the hole in the pin. As a last resort I have used a small drill and tap to get it out.

If you try and force the rod off the crank pin and damage/bend it you will be replacing it anyhow so as has been suggested better to cut the rod and try and save the piston.

To me it sounds like you will be replacing bearings, possibly piston/pin/rod assembly so you might want to investigate the cost of replacing these parts compared to a new engine.

And no you would not be the first to repair an engine where the parts almost cost the price of new. I think many of us have done the same at one time or another.

Good luck with it which ever way you decide to go

Dennis

drac1

My Feedback: (4)

Every engine I have dismantled, you remove the piston pin first. You will not get the con rod off the crank pin while it is still connected to the piston with out damaging something.

speedracerntrixie

My Feedback: (29)

While I will admit that in this case I should have done more research before jumping into the thread, I have personally not ever torn down an engine that required the piston pin to be removed during assembly. Granted most of my engine work has been with 2 strokes. I have rebuilt a YS 91 AC and recently a Surpass .91. All engines that I have worked on have one thing in common. The lower end of the connecting rod has always had a chamfer on the bushing the side that faces the counterbalance. This is done to facilitate assembly and disassembly.

drac1

My Feedback: (4)

This is totally incorrect.

The large chamfer is to give clearance for the radius on the crank pin. If the con rod is put on with the large chamfer outwards, it will be tight on the radius, which will result in the engine seizing and or breaking the con rod. If the con rod can be taken off the crank pin while still connected to the piston, then there is an awful lot of wear or excessive clearance. It would be very noisy.

speedracerntrixie

My Feedback: (29)

Photo 1 crankshaft for OS 55. Note no radius at crank pin to counterbalance junction. Photo 2 and 3 shows chamfer on one side of connecting rod. Photo 4 shows crank in case with Piston/rod assembly lined up. Photo 5 shows rod seated on crank pin after small amount of pressure with index finger. Photo 6 sleeve installed.

I have built hundreds of engines this exact same way starting with Q500 engines in '80. Not once have I had a con rod failure. Nuff said!

drac1

My Feedback: (4)

The only way you can do that, is if it has excessive clearance which is equivalent to a worn out engine. I've never found them with enough clearance to do that.

There would be a radius there, because a sharp corner would create a stress point for possible failure. The 55 is a small engine, so the radius wouldn't be as noticeable as on say a 1.70.

And I can guarantee that is what the large chamfer is for.

davidej

"I beseech you, in the bowels of Christ, think it possible you may be mistaken".....Oliver Cromwell writing to the Church of Scotland.


To continue the saga

With a bit if fiddling I managed to push the part of the broken pin far enough into the conrod to separate the piston and complete the dismantling.

Needless to say, Ripmax don't have any spares in the UK or know when they will get any.

I have miked it up (6mm dia) and the very helpful people at Just Engines are looking through their spares to see if they have another one that will fit -if not, then I will have to order from the US (Tower Hobbies?).

I still don't know how it happened - Just Engines said they had never heard of one fracturing before even with hydraulicing, which they said normally bends the conrod.

Thanks to (most of) you who contributed.

drac1

My Feedback: (4)

Yes, most strange. Even when the piston siezes, it usually only bends or breaks the con rod.

1QwkSport2.5r

If I were to hazard an uneducated guess, it likely could have been a manufacturing defect. The wristpin or gudgeon pin is hardened steel. It may not have been annealed properly perhaps? Nonetheless it doesn't matter as you got the broken pieces out. Good luck finding the replacement.

speedracerntrixie

My Feedback: (29)

Take a closer look at the picture. There is no radius, in fact its undercut where the pin meets the counterbalance. Please explain to me how to dis-assemble an engine when there is no access to the piston pin.

davidej

Surely we can all face the fact that for some engines you can pull the conrod off the crankshaft pin (without withdrawing the gudgeon) and for some you can't. Most 2 strokes I have dismantled fall into the first category but Surpass engines fall into the second.

The piston is constructed so there is almost no lateral movement of the little end on the gudgeon pin, so the only movement you have at the big end is the thickness of the liner you have withdrawn ie 1-2mm , far too little to take the big end off.

speedracerntrixie

My Feedback: (29)

Absolutely! Again in my first post I was mistaken and I have no issue admitting to that. Looking back I should have done things differently. I think we can all agree that the engine design will dictate what method is used. I think we all got a bit defensive here. With that said, everyone have a nice weekend and hopefully get some flying in.

davidej

Not here!

We have had a couple of lovely days but the wind is now 30+ mph

Another of those depressions that you North Americans ( Canadians actually) throw across the Atlantic to us this time of year

Bax

My Feedback: (11)

With O.S. engines, some of them will have you remove the cylinder liner first, then move the piston aft, against the crankcase, and then slide the connecting rod aft to remove it from the crankpin. Sometimes you need to pry it off. You can use the head of a large carriage bolt to help get the rod off of the crankpin.

Other of the O.S. engines will have a large hole in the wall of the crankcase. It is usually on the back side of the case. That is where you must remove the piston pin before you can remove the piston and connecting rod from the engine. If it's difficult to remove, just use a screw extractor to help it out. Sometimes a wood screw works, too.

The lower end of the connecting rod usually has one side where the bronze bushing is machined flat along with the aluminum of the rod. The other side will have a small chamfer machined on the inner edge of the bushing. That chamfer is there to allow clearance for the small radius where the crankpin meets the crankshaft. It is vitally important that the chamfer go against the crankshaft because the rod will tend to seize to the crank if it's installed incorrectly.

speedracerntrixie

My Feedback: (29)

I will have to fly enough for the two of us then. We are predicted to be in the low 80s and 5 mph of wind all weekend. Flying tomorrow morning and then back into the shop to lay up fiberglass fuselages for next seasons pylon airplanes.

Propworn

My Feedback: (3)

Most modern 2 stroke engines have enough clearance when the liner is removed to allow the piston and rod assembly to slide off the crank pin. Most modern engines use cir-clips to keep the wrist pin centered in the piston/rod assembly. These engines you cannot remove the cir-clips with the piston/rod assembly still in the engine, unless the cylinder unbolts from the crankcase exposing these clips.

Engines that have access holes to remove the wrist pin do not use cir-clips to keep the wrist pin from making contact with the cylinder liner. Instead they use some softer material usually in the form of a plug on both ends of the wrist pin to keep it centered and from making contact with the liner.

Engine configuration:

If the crankcase and cylinder casting are one piece be it two or four stroke two conditions arise.

In the case where there are no access holes the liner must be removed to allow the piston and rod assembly to be slid off the crank pin. These engines usually use a retaining device like cir-clips to keep the wrist pin in place and will have to be removed to disassemble the piston/rod assembly.

In the case where there are access holes be it one or two, chances are the wrist pin and piston will have to be removed from the rod to facilitate removal of the rod from the crank pin. With the engine that has access holes do not completely remove the liner just slide it up far enough to expose the wrist pin to the access hole/holes. This will keep everything in alignment and make removal/installation much easier. This combination of piston/rod assembly will not use cir-clips but end caps on the wrist pin to locate the wrist pin in the assembly. These may have to be removed prior to attempting to remove the wrist pin.

A third condition arises if the cylinder barrel unbolts from the crankcase most likely the piston and rod assembly will be held together with cir-clips at the wrist pin. The piston and rod assembly on these engines are easily slid off the crankpin once the cylinder barrel is unbolted and slid off the piston.

Hope this helps

Dennis