Urgent engine question:

[John Stiles]

On an OS Max 10 FP, the piston has an extra hole in it....should that hole be seen through the exhaust port, or would that be wrong? Thanks for any help, this is a used engine we acquired but won't start.

[david beazley]

I am not familiar with that particular engin but my guess is that the extra hole would go towards the boost port away from the exhaust port

[Tim Wescott]

What little I know of engine design tells me that the hole is there to ease the passage of gasses through a transfer port.  The only reason to have a notch or hole in the piston that uncovers the exhaust port would be for sub-piston induction, and that has (a) seriously gone out of style (it doesn't work too well with mufflers and pipes), and (b) it would be way down low on the piston.

So you probably got a deal on the engine because someone stuck the piston in backwards, and you should probably turn it around.

[John Stiles]

What little I know of engine design tells me that the hole is there to ease the passage of gasses through a transfer port.  The only reason to have a notch or hole in the piston that uncovers the exhaust port would be for sub-piston induction, and that has (a) seriously gone out of style (it doesn't work too well with mufflers and pipes), and (b) it would be way down low on the piston.

So you probably got a deal on the engine because someone stuck the piston in backwards, and you should probably turn it around.

Thanks fellers, that's exactly what I did and it fired right up. I also noticed there was no head gasket, nor was there a thrust washer......whomever tried to work on this engine rubbed[gaulded sp.] the crankcase cover and the aluminum on the front of the crankcase as well. Must'a really did some serious power assisted spinning on the little motor. I guess the good thing is that it's still able to run on cool fuel.

[Dick Byron]

As one that knows about passing gasses. There right. Hole on bypass side.

[Brian Hampton]

The "hole in the piston" thing has been used in many of the older engines. OS have used it way back with the old Max-I's and the same with Enya just to name two. Usually there's 2 holes but all will have matching holes in the liner. This allowed a flow path for mixture normally trapped inside the piston which helped cool the piston and made it easier to get lubrication up to the wrist pin. It's an excellent design feature and it's always baffled me why it's not used anymore. Doolings took it to the extreme .

[Tim Wescott]

I have two OS MAX 25, neither of which has a gasket.  Apparently what you usually take as a "head gasket" is a shim designed to let you fine-tune the compression, not really a gasket.  Certainly the engines I have with no "gasket" seal just fine.

Ditto thrust washers; I don't think any of my engines (lots of Cox 049s and a few bigger engines) have them.

I think the previous owner just got the piston in backwards then munged up the thing trying to start it.  Unless you're going to run it as a pusher, and unless there's no bits of metal on the backplate ready to fall off into the engine, you can probably ignore the galling and run the engine.

But if you're really worried about it you can send it to me, and I'll wear it out for you .

[Richard Grogan]

As one that knows about passing gasses...

  Ahhhhhhhhhhhhhhhhhhhhhhhhhhh. I can relate to that, Dick.









 

[John Stiles]

I have two OS MAX 25, neither of which has a gasket.  Apparently what you usually take as a "head gasket" is a shim designed to let you fine-tune the compression, not really a gasket.  Certainly the engines I have with no "gasket" seal just fine.

Ditto thrust washers; I don't think any of my engines (lots of Cox 049s and a few bigger engines) have them.

I think the previous owner just got the piston in backwards then munged up the thing trying to start it.  Unless you're going to run it as a pusher, and unless there's no bits of metal on the backplate ready to fall off into the engine, you can probably ignore the galling and run the engine.

But if you're really worried about it you can send it to me, and I'll wear it out for you .

I cleaned up the parts that were rough with 2,000 grit wet or dry......just to get any possible loose flakes........actually, the little engine runs very nicely with respectable compression.

[Dick Byron]

Gaskets sets are available on e bay for the old os 25. I just purchased 4 sets and there are many left.

[John Stiles]

Gaskets sets are available on e bay for the old os 25. I just purchased 4 sets and there are many left.

I'll be right on that.......maybe I'll get lucky, thanks for the heads up.

[don Burke]

A lot of the newer, OS included, plain bearing engines come with a hardened steel washer between the prop drive washer and the crankcase to stop the "wearing of the front of the crankcase" effect.  If it's not there, it probably got lost along with the note to assemble it with the piston hole on the transfer port side.  Good idea to put one in there, you might be able to find something usable at Tower hobbies.  I looked up a parts list at OSENGINES.com, thrust washer for the current 10LA is:
   #21620006  O.S. Thrust Washer 10-15 .

[John Stiles]

A lot of the newer, OS included, plain bearing engines come with a hardened steel washer between the prop drive washer and the crankcase to stop the "wearing of the front of the crankcase" effect.  If it's not there, it probably got lost along with the note to assemble it with the piston hole on the transfer port side.  Good idea to put one in there, you might be able to find something usable at Tower hobbies.  I looked up a parts list at OSENGINES.com, thrust washer for the current 10LA is:
   #21620006  O.S. Thrust Washer 10-15 .

Thanx

[George]

On an OS Max 10 FP, the piston has an extra hole in it....should that hole be seen through the exhaust port, or would that be wrong? Thanks for any help, this is a used engine we acquired but won't start.

As an additional affirmation, since my OS FPS .10 is in a plane I removed the backplate on my FPS .15 and took a peek. It does have holes on the back side to match the slots in the cylinder. There are no holes on the exhaust side. Additionally, it has the steel washer listed above.

Looks like ya gott'er fixed. Add the washer and you're all set. 

George

[John Stiles]

As an additional affirmation, since my OS FPS .10 is in a plane I removed the backplate on my FPS .15 and took a peek. It does have holes on the back side to match the slots in the cylinder. There are no holes on the exhaust side. Additionally, it has the steel washer listed above.

Looks like ya gott'er fixed. Add the washer and you're all set. 

George

Thanks