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Painting inside the engine block

Practical Engine Building

Relevant Module: Worked Examples > Mitsubishi 4G63 > Step 4: Block Preparation

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Discussion and questions related to the course Practical Engine Building

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I have a cast iron gearbox case with very light surface rust, it was painted inside and out from factory but the paint has since come of. I refered to this lesson to learn about how to I should paint the inside of the gearbox but you didn't paint the inside of the engine. How do I deal with this surface rust inside the cast iron gearbox case?

Thank you.

Hi, It will most likely need to be media blasted and then chemically cleaned to return the metal back to a base level, once this is done then it would need to be prepped, under coated and then finished with a paint or similar finish that can handle the heat and oil found inside a gearbox. This is not likely to be a cheap process to do.

Thank you BlackRex. I imagined that would be the best solution, but hoped for an easier way.

I know there's some paints or primers that stick very well to surface rust, used on outdoor structures. I would think there are products with similar behaviour made for engines and gearboxes, to handle the heat and oil as you mentioned.

If there aren't any products that stick to rusted surfaces, which products should I use in case I remove all the rust?

I will take some pictures and post here later.

Different folks will have different views, and techniques, so scope out as many as you can and make your own call, but that said...

Off hand, the only cases I recall being lightly painted internally have been for heavy vehicles, and that was a light coat of what seemed to be primer to apparently prevent corrosion.

With engines, some do it in the theory that it will fill the casting pores and irregularities and aid oil return - like most such things, some swear by it, some swear at it - your choice which.

My, personal, thinking is that anything more than a very light internal coat, on a well prepared surface, is a potential flake that could cause a problem. If you do go that route, a thorough cleaning wuith a brush and solvent, followed by a hard wire brush to remove and loose paint and rust - if the paint is well adhered, I'd leave it as it's already well bonded - then more solvent and brushing, allow to dry, then use a spray bottle/can to wash it down and allow to air dry - lightly heating will help - to allow the pores to dry out.

Depending on the paint you want to use, metal primer, primer and finishing coat - or I just use a Hammerite paint which can be directly applied.

Don't forget to pay special attention to bearing bores, gasket/seal surfaces, threaded holes, etc, to ensure they're not painted over as that will just cause more problems down the line.

Another item to take into consideration in doing this, is that depending on the paint used, it has the potential to act as a thermal barrier and reduce the heat transfer from the oil, to the metal body and then to the external airflow.

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