Fitting exhaust?

[davla]

Fitting a new exhaust and copper gasket as I've done many times but was wondering if there's any merit using exhaust seal as I don't normally bother?

[TS1-200]

I never used to bother, but I now tend to use high temperature silicone to seal it seems to work.

[Warkton Tornado No.1]

Presuming a correctly assembled joint with shake-proof fasteners, common or garden bathroom type silicone (Low Modulus Neutral Silicone Sealants) is ideal as I can vouch. In fact, if I had the choice of either the copper gasket or the sealant, I would choose the sealant.

I'm not alone in advocating the use of common or garden bathroom type silicone. Other forums will usually have some history of discussion about this subject:

bikechatforums.com

locostbuilders.co.uk

triumphrat.net

By all means, seek out a task specific product with “High Temperature” in the description, but if you think about silicone being produced from silica (sand) then you realise that virtually any will cope with the heat & task.However, one famous 'task specific' blue sealant has been proven to fail, despite the incredible endorsements by the likes of Rolls-Royce

[davla]

Thanks for the replies. I've got some Loctite 5990 Copper Silicone Sealant that I use on the slip joint so I'll use some of that.

[Chadley]

I used a copper gasket and Loctite 5920 copper silicon sealant and it seemed okay. However, when I recently did a leakdown test using the inner tube method I found there was a small leak from this area. Removing the U bend shows that although one side of the copper gasket is perfectly flat, the other side isn't due to the way the copper edge folds over. Over time the silicon has probably blown out from this face leading to the leak. I don't see how the non-flat face of the copper gasket will ever seal reliably. I'm thinking of trying one of the fibre type that seem to be available from some suppliers.

However, I'm thinking that a slight leak from the exhaust is not severe as the key thing for the leakdown is that the crankcase and combustion chamber have no leaks. Is that correct?

[EddieStone]

I always thought that the piston could suck in air through a leak in the manifold leading to a lean mixture.

[Warkton Tornado No.1]

Post by EddieStone » Wed Sep 28, 2016 11:38 pm

I always thought that the piston could suck in air through a leak in the manifold leading to a lean mixture.

Of course it can, with disastrous results

It's a struggle to get a point across sometimes, but I would imagine that anybody that has used an oxy-acetylene torch would be able to relate to how the temperature in the flame rises with the introduction of more air. The flame heat changes from that of being able to,say, braze, to being able to cut through steel plate

[coaster]

:!:

Post by EddieStone » Wed Sep 28, 2016 11:38 pm

I always thought that the piston could suck in air through a leak in the manifold leading to a lean mixture.

Of course it can, with disastrous results

It's a struggle to get a point across sometimes, but I would imagine that anybody that has used an oxy-acetylene torch would be able to relate to how the temperature in the flame rises with the introduction of more air. The flame heat changes from that of being able to,say, braze, to being able to cut through steel plate

I don't think anyone is in any doubt as to the consequence of running lean, the poster is asking how big an effect a leaking gasket would have on the mixture i.e. would a slight blow be any thing to worry about?

[Warkton Tornado No.1]

Post by coaster » Thu Sep 29, 2016 4:01 pm

Warkton Tornado No.1 wrote:

Post by EddieStone » Wed Sep 28, 2016 11:38 pm

I always thought that the piston could suck in air through a leak in the manifold leading to a lean mixture.

Of course it can, with disastrous results

It's a struggle to get a point across sometimes, but I would imagine that anybody that has used an oxy-acetylene torch would be able to relate to how the temperature in the flame rises with the introduction of more air. The flame heat changes from that of being able to,say, braze, to being able to cut through steel plate


I don't think anyone is in any doubt as to the consequence of running lean, the poster is asking how big an effect a leaking gasket would have on the mixture i.e. would a slight blow be any thing to worry about?

I take your point, but I can only offer my best intentioned advice & opinion

I think any leak is something to be concerned about, especially on the exhaust side of things.

It may be stating the obvious, but the exhaust manifold joint has to contend with extreme heat & pressure as well as the ‘signals’ that two stroke exhausts exist to supply. So an area where heat & alternating pressure exist cannot really afford any hint of an air leak IMO. My experience is that they get worse, rather than better

I know of an EGT probe that was fitted in a rather baggy weld-on to an exhaust that came loose. I think it’s rather ironic that the read-out warned of the impending doom caused by actual hardware supplying the warning signal....

[Chadley]

Okay. An exhaust leak is bad. Modern exhaust copper gaskets are crap and cause leaks despite a short term bodge solution using silicon sealant. The solution is:?

My guess is to try a different gasket type such as the fibre type?

[rossclark]

The fibre types don't have a very good rep either

[Warkton Tornado No.1]

Post by Chadley » Thu Sep 29, 2016 10:03 pm

Okay. An exhaust leak is bad. Modern exhaust copper gaskets are crap and cause leaks despite a short term bodge solution using silicon sealant. The solution is:?

My guess is to try a different gasket type such as the fibre type?

The only reason that I tend to use a solid, copper gasket is that it's initial role is in enabling port/manifold matching. Once matched, it could be discarded, but there is little point.

If the surfaces are scrupulously clean & flat prior to application & assembly, the RTV silicone on its own will seal & stay put as long as the fasteners ensure the joint remains sound.

[Marty ULC]

I'm wondering if chadley is using the cheaper copper covered more compressible gasket as opposed to the solid type that the tornado refers to.
I've had loads of the cheaper type and a couple of fibre ones fail but never had a problem with the solid ones.