REAL Push Rod Tube STC

[7133]

I have a C90-12F and changed the push rod tubes over to the REAL gasket type a while back. I have been trying to eliminate oil coming through the air box, dripping onto the inside of the cowl and then running back and getting blown onto the belly. I assumed it was because of intake valve guides having too much clearance but have eliminated that cause. I am wondering if the REAL push rod tube being slightly smaller internal diameter than the original Continental tubes are holding up the flow of oil back to the sump and filling up the rocker hats and then going down the guides and through the induction tubes. Has anyone else had this oil through the airbox problem after changing to the REAL pushrod tubes?
Michael

[5099]

Wow! You must have an oil pump from a Chevy 350 to pump enough oil to fill a push rod tube and force oil pass a valve guide.
From what I was reading, you are getting oil from Inside the airbox and not from the outside????
If you've already “dealt” with the valve guides (interested just what you did) and are sure you don’t have an oil leak there. . What about oil getting pass the piston oil rings?

Next time you have the urge, take the upper spark plugs out, run the pistons down and using a bore scope, look at the area just above the piston and the lower part of the cylinder. Do you see a puddle of oil just above the piston? You may have an oil ring problem that won’t show up during a compression test. Of course, some oil in this area is normal but what you've described, it sounds like a lot.
JMHO, Steve.

[6643]

Easy enough to test your theory. Run the engine then pull one of the rocker box covers.

[7133]

Thanks for the replies
I have done that, run the engine and then quickly pulled a rocker cover, my theory was wrong, no excess oil in the rocker hat.
I had a couple of inlet valve guide replaced which were worn, but still seem to be getting oil coming down the induction tubes to those cylinders.
The oil rings on those pistons are fine.
I cannot see that it could be coming from anywhere but the valve guides when the engine is at idle and the carb butterfly is closed allowing the suck from the cylinder to pull oil through the guide. Unless there is somewhere else?
Thanks
Michael

[6643]

How much oil are you talking about?

[7133]

Not sure how much, but enough to drip out of the air box run down the inside of the cowl and spray onto the firewall and belly.
Michael

[6643]

Not to be a pain, but are you sure it's dripping out of the airbox? Could it (e.g.) be coming from the spine of the case?

[7133]

Any dialogue is good to help me solve or accept the oil leak.
No definitely the airbox. I can put my finger into it and feel the pool of oil. Also when I take the induction tubes off I can see oil lying in the spider above the carby.
Michael

[6298]

Do your rocker arms have holes in the upper end? Later engines went to undrilled intake rockers because it was determined that the intake didn’t need the extra oil. The exhaust side did because it runs much hotter than the intake. Since the intake valves suck air into the cylinder, the also suck oil in. The intake elbows are below the cylinders and some of the excess oil ends up traveling down to the carburetor and the airbox.

[7133]

Hello Randy
Thanks for your input.
Yes they do have holes at the upper end. I know that because when we have pulled a cylinder in the past we noted that the input and exhaust rockers were the same, both with a hole at the upper end. From what you are saying it would be OK to change those input rockers to later models with no hole at the top! I think you may have found my solution.
Cheers
Michael Sheppard
1948 Cessna 140 VH-TMO