McCoy 35 engine problem

LAST EDITED ON Mar-10-04 AT 07:21 AM (CDT)
 
I learned something new today about my McCoy 35 engine. This is the engine that has lots of compression but refuses to run. I can hold the propeller and let gravity pull the engine around the crank shaft and it takes 19 seconds for the pressure to leak past the piston. Piston diameter is .750. Cylinder wall is .7505 above the ports. Cylinder wall is .755 below the ports. Thats .005" clearance between the piston and cylinder wall on the bottom of the stroke. The engine has NO crankcase pressure thats why it won't run. When the piston goes down it is suppose to pressurize the air and fuel mixture in the crank case and then the piston passes the port opening the pressure shoots around the piston and goes into the cylinder. With no crank case pressure that means the engine is not getting fuel. I can squirt fuel in the cylinder and it will start and run about 3 second then die each time I do that. The crank shaft bushing was wore out it had .006" clearance. I made a new bushing .001" clearance. Now the engine will start and run slow with the new crankshaft bushing but it still has no power it still has too much leakage around the cylinder wall with .005" clearance. I will make a new cylinder for the engine maybe tomorrow if I get time. It should run like new again.

>Piston diameter is .750. Cylinder wall is .7495 above the ports. Cylinder wall is .745 below the ports. Thats .005" clearance between the piston and cylinder wall on the bottom of the stroke.>
>Gary Weaver [email protected]

Gary,
If the piston is .750 and the cylinder is .745 isn't the cylinder smaller by 5 thou? Must be a typo?

LAST EDITED ON Mar-10-04 AT 07:30 AM (CDT)
 
Your problem may be the fit of the cylinder in the case. Squirt some oil into the exhaust stack where the cylinder meets the case, not into the cylinder. Roll the shaft around and watch for bubbling as the piston descends.

The looseness at the bottom of the cylinder should be there, I taper my cylinders up to .001 on engines as small as .09. In a well fitted lapped engine in good condition you would not be able to measure the difference between the piston and the top of the bore without the very finest instruments and the skill to use them.

LAST EDITED ON Mar-10-04 AT 07:39 AM (CDT)
 
I had it backwards. Thats what happens late at night when I am tired and my brain isn't working 100%. I changed it. I should have said, .005 clearance in the cylinder wall at the bottom and .0005 clearance in the cylinder wall at the top and .006 clearance in the crankshaft bushing. We use to fill the inside of the crankcase on the outboard motors and go cart racing engines with anything we could, epoxy, silicone, metal spacer, anything to increase crankcase pressure and the engine power would go way up. We could change the steel rod to an aluminum rod and get more crankcase pressure because the aluminum rod was physically larger than the steel rod and took up more space in the crankcase. More oil in the fuel would increase crank case pressure too but we had a problem with too much oil would foul out the spark plug sometimes a new plug would foul out before the end of a 10 lap race.

Ron B.
F4Fguy

Gary:

Listen to Randy,that engine should run with those fits! I doubt they were much different new.The crankcase packing was perfectly adequate as designed it doesn't need any more. I suspect Randy's right,the case isn't sealed.Check the cylinder base gasket and also the backplate gasket.These are both composition gaskets and fail regularly.While you're replacing gaskets,also replace the head gasket,these are even more notorious.Be very careful torquing the cylinder/head bolts.These engines' cylinders are very thin-walled and easily distorted.Another point to check;when you rebushed the shaft,did you duplicate the intake opening in the bushing exactly in both size and position?
Even worn out,I've never seen one of these that wouldn't run if it was sealed up.Even with .005" clearance at the bottom,at 9000RPM there's not time to leak down enough to stop transfer.

I made a new gasket for the crankcase cover and the gasket that goes between the crankcase and the cylinder. The engine still does not run. I removed the needle valve and spray bar and made sure it was clean and lined up correctly and the engine still will not run. I held the engine upside down in a pan of oil and turned the prop and FINALLY found the problem. The head gasket is leaking but it is not leaking around the head. The leak is around the 6 bolts that hold the head on the cylinder. Bubbles are coming out between the cooling fins next to the 6 screws. I made a new head gasket using .020" roofing aluminum. The engine seems to run Okay now. I am trying to get together 4 good McCoy 35 CL engines. I have enlarged a set of Sterling Ringmaster plans to 5 foot wing span. I also enlarged a set of B-17 plans to 5 foot wing span. I am going to use the Ringmaster wing and the B-17 fuselage to build a B-17 with 4 McCoy 35 engines. I know the 35 size engine is probably too much engine but I don't see many of the smaller McCoy engines on ebay. I also want to build a twin engine airplane first to experement with before I build a B-17. I got a refrigator cardboard box and I cut out a wing shape and fuselage shape with cardboard then placed the 4 engines on the wing and took some measurements. It is starting to look like 5 ft. wing span it too small for 4 engines. I don't mind to built a larger airplane as long as I can get it in my truck.

With all the good engines available in smaller sizes then the McCoy 35 I don't see the point in using them. OS 20 FP's can be had for very reasonable prices and are so much more reliable.

Bill