Alloy Corrosion in engine cover

[RE5Guy]

Hi all,

the small cover that holds the dip stick/supports the gear change shaft on the engine left side of the '79 was leaking so I thought I would replace the gasket. Like what else could it be right?

Well after I got it off and cleaned it up it is obvious that the alloy has corroded and allowed the oil to leak out. The attached pictures are what it looked like after I had cleaned it up. The holes grew somewhat after I attacked them with a wire brush, so obviously the alloy is very weak and brittle. After I had given it a good clean I dried it with a gas torch and oil bubbled out of the holes you can see on the outside of the cover.

The alloy on the other side of the mating surface (ie the gearbox) is perfect still.

Never seen this before and thought some people out there may be interested.

cheers

Tony

[NobleHops]

Hi all,

the small cover that holds the dip stick/supports the gear change shaft on the engine left side of the '79 was leaking so I thought I would replace the gasket. Like what else could it be right?...

Wow, that is gruesome Tony. Looks like something contaminated or otherwise fouled the casting when it was manufactured, and the fact that it is on the lower side of the bike while on the sidestand makes me think that old or contaminated oil might have contributed to the corrosion. Louis will no doubt have a spare for you.

Interesting bit of errata.

N.

[EMS]

There are still brand new covers available and they show up on ebay ever so often. I bought one years ago, just so I had one. I always wondered when you would actually need to replace one of these. Now I know.

Used ones run around $30.- At least four of them on ebay right now

http://cgi.ebay.com/ebaymotors/HONDA-CB ... ccessories

http://cgi.ebay.com/ebaymotors/HONDA-CB ... ccessories

http://cgi.ebay.com/ebaymotors/81-Honda ... ccessories

http://cgi.ebay.com/ebaymotors/1981-Hon ... ccessories

[Don]

. . . . the fact that it is on the lower side of the bike while on the sidestand makes me think that old or contaminated oil might have contributed to the corrosion

I think you probably hit the nail on the head there Nils - That area is exactly where all of the heavier than oil contaminants (water and acids) would collect and sit for years and years and that probably has much more to do with this corrosion than any metallurgical or casting problems

Don

[Larry Zimmer]

Does anyone happen to know if that piece is magnesium? Somewhere/how, I'm thinking that this cover and one or two other pieces are mag instead of aluminum. (Wouldn't be the first time my memory has failed, though.) Mag just doesn't get along with many things when it comes to corrosion.

[EMS]

I don't think this cover was magnesium, Larry. The 79 and 80 part have the same part-number 11341-422-000. And I have not seen the typical oxydation that is present on the 79's magnesium alternator cover, for example.

[Larry Zimmer]

Thanks, Mike. And, I couldn't remember. Just that I remembered a few 'subversions' into mag in the automotive world where corrosion ate some people's dinner. Particularly bad along the coast with a little extra salt in the air.

[EMS]

I am not 100% sure, Larry, either. Unprotected magnesium doesn't like moisture. I remember Chemistry classes where the teacher threw a magnesium bit into water and it started to burn.

[spencer]

I remember Chemistry classes where the teacher threw a magnesium bit into water and it started to burn.

Are you sure it was magnesium? I don't think magnesium has a strong reaction to room temperature water. Sodium does though.

[RE5Guy]

You guys are right about contaminates in old oil probably attacking the alloy. This bike originally came from Illinois and was a "Barn Bike". Literally sitting in a shed for 20 years.

When I took this cover off there was a good build up of sludge there that needed cleaning out. Some of it was white which I thought curious.

I think this cover is made of something different than the engine casings though because they are spotless.

I have sourced a replacement thanks Gents, takes a few days to get down here to New Zealand, so I am patiently waiting for it.

[Larry Zimmer]

Tony,
If you want to test for mag, scrape some fillings off the old part. Put a torch to them. If they burst into flame, it's mag. In which case, I suggest you do a good clear coat of the replacement part to prevent erosion.

[Don]

I don't think Honda used any magnesium on any CBX other than the alternator covers on the '79's

Don

[EMS]

I don't think Honda used any magnesium on any CBX other than the alternator covers on the '79's

Don

Only on the '79 magnesium covers were used. And indeed this cover was magnesium (you were right,Larry!) also the alternator housing and cover and the countersprocket cover were magnesium. And possibly also the oil line cover on the left