Thursday, February 02, 2012

Tightening Rubber Gaskets

I had an interesting conversation with a customer who was replacing a silicone rubber rocker cover gasket on his Lycoming engine. He mentioned that at every inspection he tightens the rocker cover screws slightly. Knowing that the first rule of tightening gaskets is:
1. Do not over tighten.
RG-530162 Real Gasket
I looked at his old gasket he had brought in and noticed that it had split at the screw holes and had been crushed and distorted.

When one tighens the fastener against a gasket the objective is to "tighten" the joint to prevent leakage. This tightening occurs because the gasket material and joint faying surfaces pushes back against the fastener. 

Rubber gaskets behave differently, rubber does not compress, rather it moves away from stress; it changes shape (extrudes) into any free surface. Extra tighening does not tighen. It just increasingly distorts the gasket. 

Technically, rubber has a poisson's (pronounced "pwa-son") ratio close to the upper limit of 0.5 meaning that it is incompressible. Now compare this with the traditional cork gasket used on Lycoming rocker cover gaskets; cork has a poisson's ratio close to 0.0. This is why it makes a good cork - when you push on it, it doesn't bulge and can still be pushed back into the wine bottle. Rubber bottle corks are always tapered to allow for bulging. Cork crushes over time which loosens the joint causing leakage. Go back and re-establish proper torque on a cork gasket because they take-a-set and a little extra tightening generally doesn't harm them.

Rubber is different. It is to re-establish proper torque on a rubber style gasket  but beware that extra tightening doesn't tighten, it distorts.

What about those hybrid gaskets like Continental's rocker cover gaskets? Follow rule #1 Do not over tighten.

O-Ring Extrusion


  1. Great info John.......always learning from you.

  2. Colin from Germany11:26 AM

    Thanks John - interesting!

    I think Lycoming states 50inlbs screw torque for the cork gaskets. After-market silicone gaskets which we have in stock are recommended at 23inlbs. I don't like them - we've torqued correctly and had the screws loosen themselves after a few hours; if someone's tightening without a torque wrench then they're likely to be too tight, and distort, as you state.

    Cork ones for me!!

  3. Good point on problems retaining the screw at such low torque values.

    How do you like the newer style Continental rocker cover gaskets?

    1. Colin from Germany7:04 AM

      I think you mean the paper gaskets with the silcone bead on them? Seem good to me, although we seldom have rocker cover gasket problems. Normally use RAM PMA (thin) silicon gaskets on the 520/550 motors we maintain (screws torqued to 45inlbs).

      Strange snags we've had more than once are "leaking rocker covers" on TSIO360 motors. Turned out to be oil leaking through the rocker shaft hold-down stud threads! All twelve cylinders on a relatively low-time Seneca!!

  4. Great post, it's important to maintain the gaskets of our appliance and so on to avoid leakages !


  5. Nice information. Learnt many new things about gaskets. Thanks for sharing. Rubber Gaskets

  6. Nice post. Thanks for sharing. Rubber Gaskets

  7. Love your blog! I've been doing a lot of research into rubber gaskets and this has really helped. Thank you!

  8. Lots of silicone gaskets are used in many areas, hope that you will introduce more for us