classic gasket alternatives

[vamr]

Perhaps a silly question but here it is anyway...

The process of applying a gasket, these days, pretty much always involves a small amount of silicone rubber applied to both sides. So you end up with two seals with small layer of fibrous material between them.

So my question is, why include a conventional gasket at all and instead have a thicker layer of high temperature resistant silicone rubber sealing a given engine case?

Am thinking in theory it'd offer the same if not a higher level of performance (as long as you don't tighten the hell out of the case bolts prior to curing) then the conventional method, or am I missing something?

[Motu]

When silicone gasket sealer first came out that was the theory - throw away your gaskets and just use ****** instead.But we still use gaskets....for a good reason.

[vamr]

And what reason would that be?

As far as I am aware, mechanical grade silicone can withstand substantial pressure, heat, shock etc. so what is it that I'm missing?

[Ixion]

Experience. Tried it. Didn't work. Did sometimes, but not often enough to risk, unless a gasket can't be had. Dunno why, just doesn't.

[Motu]

For a perfect mating surface like crankcase halves a thin smear of high quality silicone is fine....for other joints not so.

However you seem determind to answer your own question.Try it and see....

[rwh]

I had it pointed out to me that if you use a bit too much silicone, it squeezes lumps out the sides. On the outside, you can clean it up, no problem. On the inside, you can't see it or get at it - and if those lumps fall off, they can wander round in the oil stream and block up the galleries ... not good.

Richard

[vamr]

However you seem determind to answer your own question.Try it and see....

Not so, was just wondering why gaskets are used as opposed to silicone rubber on its own. As Ixion stated, I don't have much experience in mechanics and am not attempting to sound haughty, am genuinely interested (repairs on a tight budget).

[pete376403]

silicone is not really that strong - look how easy it peels off when you pull the motor apart again. Paper or composite gaskets (but not cork) have a finite compressibility - once they have compressed then they are effectively solid, which gives the fasteners something to tension (ie stretch) against. Silicone will just squeeze out, unless you want to leave the fasteners a bit loose.

[TLDV8]

Not so, was just wondering why gaskets are used as opposed to silicone rubber on its own.

They are two totally different things.A gasket as a rule is a solid object which will withstand compression.
No matter how thin the silicone bead once a cover is tightened you risk that shit being on the inside.
There is is also its ability to deal with vibration compared to a gasket with torqued fasteners.

Silicone is like builders and No More Gaps...Not to be confused with Three Bond (Fuel resistant so good for both 2 and 4 stroke cases) or quality Loctite products.

[scumdog]

To be the odd man out - I've built motors with only head gaskets, crank seals, intake gaskets and silicone, on some I used the water-pump gaskets as well.

Never had a problem, the big secret is wait at least a day for the silicone to 'set' and go hard.

The problems I've seen on engines where the silicone got into the oil was a combination of too much silicone and not letting it 'set' for long enough before running the engine.

[Motig]

Dredging my memory- Gaskets need to be used where a certain tolerance is required, silicone/locktite where it does'n't matter (something like that anyway) That said,centuries ago I used thick silver paint as a head gasket on my RD350. Still went OK too !

[Disco Dan]

silicone is not really that strong - look how easy it peels off when you pull the motor apart again. Paper or composite gaskets (but not cork) have a finite compressibility - once they have compressed then they are effectively solid, which gives the fasteners something to tension (ie stretch) against. Silicone will just squeeze out, unless you want to leave the fasteners a bit loose.

I disagree.

Silicone has incredible strength! My 8ft long aquarium for example weighs close to a tonne when full of water. You wanna take a guess at what is holding the glass together???

[Motu]

That's most likely glazing grade urethane,that stuff is super strong - I use it sometime to really stick things to gether...but not really oil resistant.Silicone won't hold a gap,it'll just blow out.That's why we need a gasket,to take up any small inperfections,and as Pete said,to supply tension.

[Ixion]

Ah. Thank you Mr Motu. That's the key to what was niggling my memory. The problem with using a polymer as a gasket is it has no structural integrity. The stuff just bends and stretches.

So, if you have a joint with a slight warp - I'm talking slight here, fraction of a mm, as bike casings often have after a few years, whereas a gasket will compress to fill up the gap (or, strictly, compress either side of the gap) , silicone/loctite will simply "fill in" the gap. Which is fine, until theres a bit of pressure , as occurs when an engine runs. Then the "fill in" bit of silicone just stretches and blows out. Cos it's stretchy, it doesn't have a fixed shape like a gasket does.

[imdying]

Hell, I go the other way. I always use a new gasket, and don't put any other 'adhesives' silicon or otherwise down. Occasionally on awkward cars I'll use a few dabs of grease to hold a difficult gasket in place.

[Flyingpony]

To be the odd man out - I've built motors with only head gaskets, crank seals, intake gaskets and silicone, on some I used the water-pump gaskets as well.
I must be even more of an odd man. Sometimes (depending on application), I've used the thick cardboard you find on the back of notepads :innocent:

[stanko]

Is it really common practice to use silicon / RTV when using gaskets? Maybe if you are re-using them. On my RD350LC I dont use a head gasket, just RTV although the surfaces were lapped first and the rest of the gaskets go on dry.

[Ixion]

I cannot let this trhead go past without mentioning shellac. Wonderful stuff.

[Motu]

Not on your balls it isn't!

[Ixion]

My balls don't leak , so I never needed it there. Let me guess, there was this skinny little 'prentice boy, right ? And a bunch of traditionalists in the workshop . And you didn't come back with the striped paint they sent you for.

[Motu]

Not my balls! But I know guys who had it done to them.Shellac is good stuff,pity you can't get the real stuff now....although you can get shellac flakes and mix it in meths to whatever consistency you like.It's still used to seat pads into saxophones and clarinets....just a little heat under the valve and you can move the pad to get a good seating.

When I worked on compressors the boss made us use shellac - "I know it's horrible shit,but I don't want any of these things coming back with oil leaks''.When I fitted a new top end on my TM400 it kept blowing out the base gasket - a shit load of crankcase compression.I was using that silly silicone shit....so I glued it down with shellac and that was that.

[Ixion]

I've always mixed my own. But I did get a little bottle premixed of the proper stuff about a year ago.

[Big Dog]

Pays to read the manufacturer instructions some have compounds between the sheets of material designed to soften and expand under use.
My last head gasked on a 4.1 recomended that it be put in place clean and driven at low revs until maximum temperature is achieved for 1/2 an hour or so before allowing to cool and draining the water and replacing the coolant as the compounds do not work properly when in the prescence of Glycol.

[Motu]

Yep - most modern gaskets have a heat setting sealer impregnated...that's why you can't get the bloody things off when you didn't even use gasket goop.Actualy it's one reason why I use silicone.so I can remove a gasket easily.Head gaskets are the same,and glycol can attack the sealer - so we put them through a couple of heat cycles with plain water before adding inhibitor.