BLUEMOMENT

ParaHandy wrote:Since retiring the opportunity to catch up with YM at Paddington WH Smith is no longer available. So, could I beg of you to print herewith the letter?

To save typing, this is a copy of the letter I sent in. The published version is cut down a bit.

Dear Sirs,

October 2013 Question of the Month "Why is my saildrive leaking oil".

I was rather disappointed by Nigel Calder's reply to your "Question of the Month". He identified "only" three places where oil could leak from the gearbox, two of which would not have deposited oil in the engine bay (including his "most likely spot") and a third location unlikely to produce the quantity reported. Could I suggest a more likely fourth possibility? If the O ring seal on the filler cap fails then, with engine running and in gear, oil runs down the gearbox casing quite copiously and accumulates in the corrugations of the sail drive gaiter. I watched this happen when I experienced this problem 2 years ago, and the volume of leak was entirely consistent with Ian Johnson's account. I found when I removed the O ring that it had expanded by about 10mm! Its replacement only lasted a year and then again swelled up, making the filler cap very difficult to undo: I made up a plywood "spanner" to get it off. Curiously, my original O ring had lasted over 10 years without causing a problem. I now change the filler cap O ring as part of my annual service.

Dave Simpson

Perhaps heat and the quality of rubber has something to do with it. Given that Poisson's ratio of rubber is 1, on heating the rubber O ring will try to expand in all directions without any volume change. On cooling it should revert to its previous shape and state of stress which is dependent, inter alia, upon how tight the cap was. The loading, unloading stress strain curves exhibit hysteresis, ie energy is lost, which may contribute to eventual plastic behaviour, ergo permanent expansion.....Over tightening may exacerbate things.............

A good letter to YM. A pity it wasn't acknowleged.

Ghillie wrote:Perhaps heat and the quality of rubber has something to do with it. Given that Poisson's ratio of rubber is 1, on heating the rubber O ring will try to expand in all directions without any volume change. On cooling it should revert to its previous shape and state of stress which is dependent, inter alia, upon how tight the cap was. The loading, unloading stress strain curves exhibit hysteresis, ie energy is lost, which may contribute to eventual plastic behaviour, ergo permanent expansion.....Over tightening may exacerbate things.............

A good letter to YM. A pity it wasn't acknowleged.

Jings! I faintly remember Poisson's ratio, but it was about 40 years ago...

I have never felt the gearbox being noticeably hot, but it's not something I've deliberately checked regularly. I'll maybe start to do so.

DaveS wrote:

Ghillie wrote:Perhaps heat and the quality of rubber has something to do with it. Given that Poisson's ratio of rubber is 1, on heating the rubber O ring will try to expand in all directions without any volume change. On cooling it should revert to its previous shape and state of stress which is dependent, inter alia, upon how tight the cap was. The loading, unloading stress strain curves exhibit hysteresis, ie energy is lost, which may contribute to eventual plastic behaviour, ergo permanent expansion.....Over tightening may exacerbate things.............

A good letter to YM. A pity it wasn't acknowleged.

Jings! I faintly remember Poisson's ratio, but it was about 40 years ago...

I have never felt the gearbox being noticeably hot, but it's not something I've deliberately checked regularly. I'll maybe start to do so.

All sounds a bit fishy if you ask me,

Indeed,

I noted that the fish courses on a restaurant menu I espied once came under the heading
"Poisons"......

Doubtless an O ring had failed on some piece of cookery gizmo, so it was probably apposite.