irish.david wrote:Where the metals are physically joined you'll get very localised galvanic corrosion, but any corrosion elsewhere on the gearbox isn't due to that and the corrosion where the metals meet has nothing to do with your earth connections.
Dave
it's exactly where the steel gearbox mount and aluminium gearbox join that the problem is.
According to Aidan the 5 speed has more contact area and it's a real problem as the case rots through, 4 speed has pillars giving less contact area so has less of a problem but still corrodes causing poor starting
to clean the corrosion means supporting the gearbox and taking the mount off which is a pita.
1988 DG WBX LPG Tin Top
1989 DJ digijet WBX Holdsworth Villa 3 Pop Top itchylinks
irish.david wrote:
The good news for your gearbox, galvanic corrosion isn't an issue as there isn't an electrolyte between the aluminium gearbox and steel body to allow ion migration between the two. The only issues you might have are where there is physical contact where you get the standard reaction between dissimilar metals, but most of these points are fitted with vibration mounts which act as a galvanic barrier and the others (such as the earth cable connections) are easy to take apart and clean from time to time.
Long story short, the corrosion on your gearbox is nothing to do with your earth connections.
Dave
Absolute rubbish!
Aidan can doubtlessly show you examples - he has posted pictures previously of the galvanic corrosion.
The electrolyte is present in the form of salt crystals, left from road salt, lodged in the junction of the mount plate and the end of the gearbox. The electric current flows via this plate! (check where your earth strap is mounted - usually on the mount plate) Salt is by nature hygroscopic - perfect electrolyte.
Long story short, the corrosion on your gearbox is to do with your earth connections.
ghost123uk wrote:Didn't say on the bar Said on the bolt to the chassis rail that holds the bar. I see your point about it being open to the elements though, mind you, there would be easy to get to for routine cleaning and it's a very solid earthing point. A hole drilled into simple bodywork, with a bolt, wouldn't be as good an earth (imho and being a bit nit picky I suppose )
Yeh sorry misread that
Anyway engine bar mounting bolt is no closer to starter than the origional earth point underneath, further away from the wheel spray and like you I seem to think a bolt through thin sheet metal is not a good.
As I said if you only go to engine from body you still have potential for bad connection to gearbox and starter
If I don't see a better suggestion I'm going starter to body with 25mm2 cost about £10
Last edited by itchyfeet on 09 Mar 2015, 21:25, edited 2 times in total.
1988 DG WBX LPG Tin Top
1989 DJ digijet WBX Holdsworth Villa 3 Pop Top itchylinks
I’m not denying that you’ll get some localised corrosion between dissimilar metals when they’re exposed to the atmosphere, but you’ll only get it in the area where the metals are joined and not all over the gearbox and salt crystals when they’re not in a solution aren’t an electrolyte. I’m also not saying that it isn’t a good idea to add extra earthing points to from the engine block to the chassis as you can’t check the condition of the electrical connection between the gearbox and block which will degrade over time.
What i’m saying is that where the engine or gearbox is earthed will have negligible difference to the galvanic process.
irish.david wrote:I’m not denying that you’ll get some localised corrosion between dissimilar metals when they’re exposed to the atmosphere, but you’ll only get it in the area where the metals are joined and not all over the gearbox and salt crystals when they’re not in a solution aren’t an electrolyte. I’m also not saying that it isn’t a good idea to add extra earthing points to from the engine block to the chassis as you can’t check the condition of the electrical connection between the gearbox and block which will degrade over time.
What i’m saying is that where the engine or gearbox is earthed will have negligible difference to the galvanic process.
Dave
disagree, because if you can route the current elsewhere you can insulate the two metals with nylon washers as said.
Also DC current accelerates the corrosion.
1988 DG WBX LPG Tin Top
1989 DJ digijet WBX Holdsworth Villa 3 Pop Top itchylinks
irish.david wrote:
What i’m saying is that where the engine or gearbox is earthed will have negligible difference to the galvanic process.
Dave
Still rubbish!
Factory fit is on the mount plate as already stated - it is not negligible difference.
Sit salt in a slightly damp atmosphere and it will attract water immediately forming an electrolyte. The UK is very rarely dry enough for this to not happen. Humid air is enough.
Insulating the two metals with nylon washers and the both having their own earth is the ideal solution, but it’s not practical as the surfaces of the engine and gearbox are designed to be face to face.
The vast majority of the galvanic corrosion will come from the time when you’ve got two metals with different electrode potentials (engine and gearbox) in contact in the presence of moisture in the atmosphere, which acts as a weak electrolyte. The presence of an additional DC current when the engines running will make negligible difference.
I have a feeling that we’re not going to agree on this one…
irish.david wrote:Insulating the two metals with nylon washers and the both having their own earth is the ideal solution, but it’s not practical as the surfaces of the engine and gearbox are designed to be face to face.
The vast majority of the galvanic corrosion will come from the time when you’ve got two metals with different electrode potentials (engine and gearbox) in contact in the presence of moisture in the atmosphere, which acts as a weak electrolyte. The presence of an additional DC current when the engines running will make negligible difference.
I have a feeling that we’re not going to agree on this one…
Dave
Errrrr............................
Oldiebut goodie wrote:
The electrolyte is present in the form of salt crystals, left from road salt, lodged in the junction of the mount plate and the end of the gearbox. The electric current flows via this plate! (check where your earth strap is mounted - usually on the mount plate) Salt is by nature hygroscopic - perfect electrolyte.
Long story short, the corrosion on your gearbox is to do with your earth connections.
The only reason I'd be fitting one is to match the thick stuff I've used on the positive rail. That weedy little strap on the gearbox looks ridiculous in comparison now. Its been said a few times but a better earth is a good move.
Roller paint your camper at home: http://roller.epizy.com/55554/" onclick="window.open(this.href);return false; for MP4 download.
irish.david wrote:Insulating the two metals with nylon washers and the both having their own earth is the ideal solution, but it’s not practical as the surfaces of the engine and gearbox are designed to be face to face.
The vast majority of the galvanic corrosion will come from the time when you’ve got two metals with different electrode potentials (engine and gearbox) in contact in the presence of moisture in the atmosphere, which acts as a weak electrolyte. The presence of an additional DC current when the engines running will make negligible difference.
I have a feeling that we’re not going to agree on this one…
Dave
not sure if you have ever looked at a wbx engine and gearbox but engine and gearbox are both aluminium so similar metals, it's the gearbox and mounting which are disimmilar metals thats the problem and they can be insulated with nylon washers.
1988 DG WBX LPG Tin Top
1989 DJ digijet WBX Holdsworth Villa 3 Pop Top itchylinks
At no point did i say that there wasn’t going to be localised galvanic corrosion due to normal corrosion that you’d see anywhere where dissimilar metals meet.
“The electrolyte is present in the form of salt crystals”
Salt crystals are not an electrolyte
I think the point that we disagree on is how much the effect of the DC current from the alternator flowing between the metals will have on the corrosion process. I don’t think it will have any noticeable effect and you do.
Said on the bolt to the chassis rail that holds the bar. I see your point about it being open to the elements though, mind you, there would be easy to get to for routine cleaning and it's a very solid earthing point. A hole drilled into simple bodywork, with a bolt, wouldn't be as good an earth (imho and being a bit nit picky I suppose 
)
Just not intuitive is it!
