OS XM outboard piston fit

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Chris Trump

Member
Joined
Nov 7, 2012
Messages
10
Hi all,

My engine is new with maybe three tanks through it. The pinch to me is right where it needs to be for a new engine. First start is always good and it runs great. Second start the piston will give me a hard time locking up at the top....I mean real tight. Removing the head will free up the piston.

In the past I've had engines(various ABC engines) that tightened up with head on and some that lost their fit with the head . Never new why or how this was happening. I would love to Know why.

Should I just run it on the bench wide open very rich till it loosens up?

Thanks, Chris
 
Just run it. It will get better the more you run the motor. Pre run if you want, It will not hurt anything. I run two tanks and then hit the water. Run it rich and sneak up on the mix setting.(7-8 tanks) By the time you get your needle setting. it will be broke in. If you go lean, it will let you now. Keep a eye on your plug It will tell you a lot. Good luck
 
Thank you.

Now that I think of it, most engines seem to loose the fit with the head torqued. I was wondering why this one is the opposite.
 
I would not worry about that. I'm sure that tightened the head down will be different then when its loose. Also get rid of the stock prop and get a m442 from props4you or the new 1515 he has been working on. I have not tried mine yet.
 
Thank you.

Now that I think of it, most engines seem to loose the fit with the head torqued. I was wondering why this one is the opposite.
Over or under torqued head bolts that are placed outside the OD of the liners lip will cause upper cylinder distortion. There are engines designed with the head bolts passing through the liners lip. In these engines the liners lip will be as large as the OD of the case & the engine's head. Since all the pieces involved, the head, the liner's lip & the top of the crankcase come into full contact with each other, there are no distortion problems. This type of design also give the maximum amount of heat transfer from the cylinder to the case & head where the engine needs the most cooling.

Jim Allen
 
That makes sense to me. My BVM DF engines are through the flange types and they don't exhibit this behavior. I may try relaxing the torque on the bolts. Maybe I can get the fit a little less ornery.
 
That makes sense to me. My BVM DF engines are through the flange types and they don't exhibit this behavior. I may try relaxing the torque on the bolts. Maybe I can get the fit a little less ornery.
Good observation Chris. The same person manufactured the .45 long stroke already shown & the .96 Special Edition DF engine you mentioned. He & I discovered the value of building engines this way in 1987. The thickness of the liners lip was also increased to .200" as well. The number & spacing of the head bolts also effects any distortion that can take place. Miniature torque wrenches can be helpful.

JA
 
Thanks Jerry, Jim,

I did try less torque on the bolts and it's amazing how little it took to create distortion, as you put it Jim. Something a little over finger tight using the short end of the allen wrench.

Chris
 
Jim,

The .96 was and still is an great engine. One of my most reliable with hundreds of flights. In fact I have converted a BVM 91 to marine use which proved to be very successful. The rod was always the weakest link however after throwing a prop. I have since been using a steel octura flywheel which seems to make it a happier running engine.

Chris
 
Jim,

The .96 was and still is an great engine. One of my most reliable with hundreds of flights. In fact I have converted a BVM 91 to marine use which proved to be very successful. The rod was always the weakest link however after throwing a prop. I have since been using a steel octura flywheel which seems to make it a happier running engine.

Chris
We discovered that an aluminum connecting rod in larger size engines (.65 to 1.00 cu in) could not survive at RPM's approaching 30,000. Connecting rods made of 6AL4V Titanium were not any better because they needed separate bearing assemblies in the bottom end. Various types of drawn cup needle assemblies were tested with some success when engine RPM's were kept below 24,000. The right metallurgy (heat treatable steels) & a good bottom end roller design definitely solved the problem.

JA
 
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