Topic: Rust through frame tubes

[traveler]

There is a member "SKTP" in the project section who is building a 750.  He actually found rust holes through the frame tubing, and had to cut it out and weld a patch in.

Is this kind of rust common in these bikes?  I haven't seen any rust holes on my 550 frame, but this is spooky!

~Joe

[KingCustomCycles.com]

I have stripped, cut, welded, and sandblasted 14 CB750 frames.  No rust through ever encountered.  The frame tubing has drain holes from the factory and it seems to have done its job.  I have seen surface rust (scale) chips fall out of a frame that has been stored outside and then lightly tapped with a rubber mallet.  This usually occurs around the rear engine mount/swingarm mounting plates.  For the purist, a syringe and needle can be filled with hot linseed oil and then squirted into the drain holes.  The frame is then rotated to distribute the oil and then is drained out.  Hot to thin the mixture, when it cools it thickens to a very rust resistant surface.  Of course, rotating a frame with an engine and wheels is out of the question, this could be done during restoration though. 

[mycb750k6]

According to the H-man book, "some" of the new factory or old factory I don't recall, frames had their drain holes missed. And yet more had the drain holes painted over. My last 71 had a couple drain holes behind the kick stand on the bottom but did not have the down tube holes in the front of the frame where you need them so I put them in. And you really need them. Power wash your frame and it practically fills with water and you can actually watch it pour out of these drain holes. One of H-mans tests is to squeeze the frame with a pair of pliers. If it collapses - frame's no good. Pretty scary I think. So check for your drain holes and drill them out if not there or painted shut.

[traveler]

Will do....thanks.

~Joe

[SKTP]

I guess I'm just lucky 

A drain hole has been made in the repaired frame. It's off to the powder coater today. He's going to look for any other frame issue when it's blasted. The rust hole most likely occurred due to the PO leaving it on the kick stand under a tree for 3 years. The hole was at the lowppint on the kickstand side. Crossing my fingers....

[MCRider]

Yeah, rust like that is in no way common. But one event really makes you think and its easy to check. I didn't on my project and its got me wondering. My frame hasn't seen outdoors since 1995, but that doesn't mean some condensation may not have been doing its dirty in similar locations all along.

Mines already powdered up, but i may look at what HM has to say and drill some holes anyway.

[gmonkey]

Guy I buy parts from has a pile of old CB frames.  Some of them had the lower part of the frame tubes rusted through.  Not the same as a running bike but it can happen in extreme cases.

[MCRider]

Guy I buy parts from has a pile of old CB frames.  Some of them had the lower part of the frame tubes rusted through.  Not the same as a running bike but it can happen in extreme cases.

Yikes! If some are rusted through, then some are partially rusted and that's bad enough.