Silicone for bikes.

November 17, 2009 by mike.cox

Silicone sealant as a replacement for paper gaskets.
If you do some research into this remarkable product you will find there are literally 100 plus different types, all specifically manufactured for specific purposes.
 
So is the product sold at hardware stores for general use, suitable for our bike applications?
 

I don’t think so actually, and I say this because I’ve come across people who have used the general purpose products and ended up with oil leaks, peeling, dissolving in oil etc. But they saved a few Rands by purchasing the cheapest they could find, or they didn’t know any better?
And of course they will have hassles because the manufacturer never intended the general purpose product or any of their products to be a ‘be all and end all’ product suitable for any application.
 
Silicone sealant is a powerful, flexible product that can be used in many different applications. Although silicone sealant is not as strong as some sealants or adhesives, silicone sealant remains very flexible, even once it has fully dried or cured.
Silicone sealant can also withstand very high temperatures, making it ideal for applications that suffer high heat exposure, such as on engine gaskets.
 
The most common type of single-product silicone sealant is called Room Temperature Vulcanizing (RTV). This form of sealant starts to cure as soon as it is exposed to the air – or, more precisely, the moisture in the air.
Therefore, it is necessary that you may have to work quickly when using RTV silicone sealant.
 
Many bikes don’t have paper type gaskets between mating surfaces. They rely on the perfect flatness (is that a word?) of both mating surfaces.
You should flatten the surfaces prior to assembly, or check they are flat. Then silicone will provide a perfect sealing material.
All surfaces where the silicone must adhere to cannot have any oily residue, unless you are deliberately making one surface removable without damaging the silicone previously applied.
Only the thinnest smear of sealant is required because what you see squeezing out is all the excess when the parts are tightened up, and the same excess will be identical to the amount on the inside of the casings which you cannot wipe off.
 
You don’t want bits coming off and possibly blocking oil-ways?
 
I have found Loctite’s grey RTV to be a very good product. The Ducati doesn’t have any major gaskets and RTV is the only method I have used for many years. It have also kept leaks away on the numerous Triumph timing covers, primary cases, crankcase halves and gearboxes I’ve assembled also.
 
Some users recommend laying a thin film on one mating surface and letting it dry. Only then assembling the two halves.
Others say ‘assemble straight away’. As to who is right I don’t know. Both seem to work.
But, if you let it dry and there’s different thicknesses all around the mating surfaces, I think problems might occur?
 
RTV can also be used for other uses around the bike.
Excellent for water-proofing HT leads, distributors, moulding a special seal for switchgear etc. After making off an electrical connection, no reason why a coating of RTV to protect the join cannot be used?
 
Paper and RTV.
I like using RTV on the paper gasket, one side only. The other side I use grease. This allows me to remove covers many times without damaging the paper gasket. The alternative in place of RTV is contact adhesive!
 
Now if your machine has paper or thickish card type gaskets and you want to replace with RTV, is this a good idea?
Make sure the thick card type gasket isn’t used to give a clearance between two items.
For instance, in the Ducati bevel drive assemblies, if you change the gasket thickness, you will have to re-shim the bevels to compensate.
 
Where not to use silicone.
·         Well, barrel to head mating surfaces cannot withstand the heat or pressure and the proper copper or other special gaskets specifically made for this is the only and correct one to use.
·         Where petrol will come into contact with silicone. Silicone cannot ‘fix’ a leaking joint on petrol-taps or provide thread sealing between tank and tap.
·         As a replacement for ‘O’ rings, just not enough tensile strength.
·         As a physical barrier where items rub together. Again no shear strength.
 
Getting the most out of a tube.
If you use the screw-on plastic extender on the tube, it always is set solid when you come to use it again. Wasteful, time consuming to clean out, only for it to happen again next time.
 
Also you still smear the silicone out with your finger anyway. So what use is the extender?
 
I do it slightly different. I don’t use the extender but put a small amount onto my finger and wipe it where it’s needed. Then when finished I use a small piece of plastic bag and stick it over the open hole.
Now I peel off the plastic next time and the silicone is still soft and usable.