Wednesday, June 22, 2011

Oil Leaks -- It's not the gasket it's the surface

The only reason we use gaskets is because we can't machine a truly flat surface. Bugatti engine blocks were hand scraped to ensure that the surfaces were so flat that gaskets were not required for sealing! Almost true - they were hand scraped but Bugatti engines did leak oil but the point is that surface flatness is #1 when it comes to sealing surfaces.

Check your surfaces for flatness. A customer brought this cover plate into my store to purchase a gasket. I have this habit of placing plates on the counter upside down and pressing on the edges to see if they rock - a quick check for flatness - this one rocked! A new gasket wouldn't work any better on this plate than the old one. Pretty amazing to me that the customer was unaware of this. He is going to have to flatten this plate.

Another check for flatness is to lightly lap the surfaces and inspect the lap contact area. When I had the overhaul shop we used to do a lot of lapping. Had a lapping plate in the engine shop and a flat piece of thick glass as a lapping plate in the accessory shop. Saturate some 800 wet-or-dry sandpaper with light oil and then swirl the cover plate across the surface a few times - then you look at the cover plate and see if it is distorted. If it is then continue lapping. 

Another problem with gaskets is too much torque and you crush the gasket. The motivation is to tighten harder if you have a leak. But look at the picture below. More torque on the bolts and you just distort the surfaces more. The correct amount of bolt torque on a gasketed joint is set by the stress needed in the gasket material to effect a seal and sufficient torque to provide equal pressure across the gasket. Tightening should be done in stages to compress the gasket equally.

Lapping Technique:
The cover plate is swirled against the glass and paper. The wet sandpaper will stick to the glass so just pick up the plate and place it onto the sandpaper and lightly swirl the plate. You can lift the plate off and look at the surface and see the contact area. This will tell you if the plate is flat or not. At this stage you are just checking for flatness. If the paper is not removing metal from the entire surface then continue -- hold the plate with light but even pressure - do not force the plate onto the paper as you will press harder on one side than the other.

Here is a video - a couple of comments - outside of a flat-plate (lapping plate) the next best surface to lap against is glass. Do not glue the sandpaper down to the surface - what a mess. Light oil will hold it down. wet both sides of the sandpaper. I have never lapped with water - always light oil but I suppose water would also work.

Two essential tools missing from almost every aircraft repair shop - an arbor press and a lapping plate!


  1. Anonymous9:21 AM

    The picture of the Marvel Schebler carb screen bowl, retainer and gasket is an incorect correlation, because its is not the "oval" retainer that seals the carburetor, but the bowl. Regardless of what the gasket does, around the retainer, as long as the bowl is without defects and properly sealing the carb.
    But lapping the screen bowl will prevent potential leaks.

  2. Beg-to-differ--The gasket cannot function as intended due to non-parallel surfaces.

  3. Colin from Germany9:13 AM

    Interesting article - thanks.

    Often see "leaking inlet manifold flange gasket" replaced numerous times on Lycomings. It's sometimes the gasket leaking but, more often than not, is the the inlet tube not being clamped correctly onto the gasket by the bolted clamp which attaches the tube to the cylinder (normally due to wear or overtightening). If you look at the tube/clamp from the side before re-fitting you should see the tube end protruding above the clamp.