Differential with paint flaking off

[Warb]

The diff casing/carrier (the rotating bit that does all the work, not the housing) that I am currently working on is painted in a thick orange paint. Unfortunately this paint is starting to flake off in pieces up to the size of a $2 coin, and some (from the corners) are quite thick chunks. I'm not happy about rebuilding it in this condition, as I'm fairly sure that large chunks of paint in the oil wouldn't be good!

When the diff is completely stripped I'm considering sandblasting it to remove the remainder of the paint (having masked the machined surfaces, obviously!). Can anybody think of a reason why this shouldn't be done?

Secondly, having stripped the existing paint is it best to leave it as a cast finish, or repaint before assembly? I'm thinking of a paint like Glyptal 1201 that is used for the inside of engines.

[Recy Mech]

Pictures?

Only say this as the way I read your thread is that the pinnion gear has been painted. If so I'd be removing all traces of paint. That goes for anything inside the diff housing. Remember, there are some angry pieces of metal going round and all they like is lubrication.....no foreign bits ie paint, swarfe etc. You may find the only sorta paint is from a paint pen where a fitter has marked a bolt as torqued and retorqued, that would probably be indicated as a / or X.

Painting is rather subjective. Some do, some don't. The don't camp will quote heat retention, that's where I'm sorta leaning towards. Not that the diff really builds up heat that is detrimental, unless it's being flogged.

[Warb]

Only say this as the way I read your thread is that the pinnion gear has been painted.

The part is known variously as the diff casing or pinion carrier in the old manuals, we always called it the diff carrier as it was the part to which all the mechanical components were mounted.

It is the part to which the crownwheel is bolted, and the pinion shaft (complete with pinions), diff wheels and bearings are attached. These days I doubt that such parts would be painted by the factory (cost reduces profit) but I have found many old vehicles had internal mechanical components painted. Presumably they were painted after casting to stop them rusting, then any machined areas ended up paint free when the unit was assembled. In this case, not only is the diff carrier painted but also the inside of the diff housing. I don't know if this was done by the factory, or during a later overhaul. - it's a '59 diff.

I'm leaning towards not painting it internally, mainly because I can see little point as the components are coated with oil.

Painting is rather subjective. Some do, some don't. The don't camp will quote heat retention, that's where I'm sorta leaning towards. Not that the diff really builds up heat that is detrimental, unless it's being flogged.

I can state quite categorically that unless a really thick layer of paint is applied, it will make no significant difference to heat build up by insulation in the components of a road vehicle.

Black paint will radiate more heat, so potentially reducing the heat of the unit. White paint will radiate less, so potentially increasing the heat. Matt radiates more than gloss. The best option on the outside of a hot component* is therefore a thin coat of matt black paint, but in reality it will make no significant difference to a relatively cool item like the diff. on a road vehicle.

*assuming you want to cool it down - we experimented with white exhaust headers many years ago in an attempt to reduce the engine bay temperatures by reducing the heat radiated from the headers, i.e. to leave the heat in the gases. Again this will make no useful difference to a road going vehicle, but can be beneficial in high performance situations.

[Recy Mech]

Well, every day a school day.

Liked the bit about the painting of parts prior to assembly, when told like that does make sense. Can't ever recall either coming across 'said' parts or discussions - even the 'British' engineering chats. Yes, I've owned a Triumph bike as well as a SIII.

As a slight digression, even up here in the Pilbara some of the lads who do the local speedway meets are cerami-coating the exhausts. Tell of heaps cooler exhaust temps.