Disassembly of an O.S. Max .61 SF motor

[ticedoff8]

I have a new project: I found a really nice Cobra with lots and lot of mods. The rumor is that it used to belong to Peter Chao - the owner of Helicopter World.

It has been sitting for a long time, but it is complete - including an O.S. Max .61 SF with a black helicopter head and a 7H carb.

The motor was frozen with gunk, so I've been slowly disassembling it. Now that I have it torn down, it is clear the main bearing has rust and may need to be replaced. But I've reach the point where I need to take out the piston / rod so I can remove the crankshaft and bearings.

But, I can't get the lower rod bearing off of the crank pin. I've already removed the steel cylinder liner - but the rod only moves about 1/2 way off the crank pin - then no more.

On my O.S. Max . 61F SR-H, the front of the crankcase is bolted on and it is easy to remove. You then slide the crank forward and the rod bearing slips off the crank pin.

On the .61SF, it is a one piece crankcase. I can't slide the crank forward.

Does anyone remember how to disassemble this?

I suspect that the rod has some play at the top wrist pin that allows it to slide to the back - giving me a few more mm to get the bottom rod bearing off the crank pin. But, so far, I haven't gotten the wrist pin to move. It seems a little stiff, so I am soaking it with penetrating oil to see of that frees up the top wrist pin.

If someone can confirm there is play at the top wrist pin - I can try a little harder. But if there is some magic I'm not seeing - I'd appreciate some pointers.

Thanks.

[heli-cuzz]

I recently did an overhaul on the same exact engine with basically the same problem. The engine sat on the bench for a few years. I soaked the entire engine in denatured alcohol overnite and still had trouble removing the connecting rod from the crank pin.
I have a hook type tool and it took some pulling force to remove it from the crank pin.
I also slightly rotated the crank while pulling on the connecting rod to help free it up from the crank pin.
Make sure the crank pin is at its highest point{TDC} and pull the con rod while slightly turning the crank.
Once removed I noticed the wrist pin was limiting the con rod from sliding completely over.
It was the most difficult engine I have ever disassembled. Most likely from being gummed up.

Good luck!

[ticedoff8]

As I suspected - the upper wrist pin was gummed / corroded up so badly it wouldn't slide the extra 2mm I need to get the lower rod bearing off of the crank pin.

Once I "persuaded" the wrist over far enough, I could get piston / connecting rod out. If I had to do it again, I think some heat from a blow torch would have helped.

Boy, this engine was a mess. I had to heat the crankcase with my torch to get the bearings and wrist pin out. The rear main bearing and crankshaft have a major coat of rust, and everything is gummed up with old oil. I had to chip the ring out 1mm at a time - it was frozen into the ring groove of the piston.

Using a tooth brush, 600 grit emery paper and dish soap, it took me all day to clean the rust and old oil - but now it looks like it is new again - inside & out.

I have a new set of bearings and a ring on order, then I'll start it up and see if it is back to its spec power. If so, I'll have another nice 60-size heli ready to fly.

PEOPLE !! - Please use after run oil or a fuel that contains caster oil !!! If not for your own sake, for the sake of people like me - who buy your old stuff and try to get it flying again.
I'd say this engine is a classic example of the "rode hard and put away wet" engine management style.

[RCdude]

The 61SF was a decent engine back then. I've got both the Ringed and non-ringed (ABN) version and I've just brought the ringed version back into service. Anyway on the subject of heating the case. I use an old toaster oven to heat the case after soaking the engine in kerosene for a while. Just remember not to leave any of the o rings in or on the engine that you need or the kerosene will cause them to swell up effectively making them useless. Once the engine is loose, Remove any excess kerosene from the engine and pop it in the toaster oven for about 4 min.then using a glove take it out and remove the liner if its frozen in the case then the piston/rod assembly. Once that's all out, (you may have to reheat the engine) give the case a moderate whack back plate side down on your work bench and the bearing and crank or bearing if you were able to remove the crank should just drop out.

A note about the rod, (you all may already know this but other readers lookin' at this may not and its easy to overlook )Pay special attention on how the rod comes off the crank there is a radius (Tapered) side of the bushing and a flat side. the tapered side goes toward the crank it will go on the other way if you hold your mouth right. but if it goes on backwards it will place the top of the rod off center on the wrist pin causing wear.
Take care (RCdude)