Why paint the inside of engine cases

[stan]

I know there is a special paint for the inside of motors. What is the purpose of this paint? Since some of oil cooling is from the oil making contact with the motor cases, it seams like the painted surfaces will act as a insolution between the oil and the cooler bare aluminum case. What is your opinion? Stan

[Bigincher]

From what I understand, the older cast aluminum was of rather poor quality, and oil could leak through it. The paint--- epoxy, I presume--- helps to prevent the seepage of oil though the cases. Same with transmission cases. If I am wrong, there will be no doubt of being corrected!

[Paps]

You be correct ! Paps

[20scout]

Simple oil base red primer works for me. Yes it does cut down on the oil leakage.

[MeanGene]

Back in the 70's, the big "cool" thing to do was to paint the inside of the block of your hot rod with Glyptal armature paint- in the cars it wasn't so much for leakage, as to make the oil drain back to the pan faster. The problem being, if the engine made any kind of RPM, the oil coming off the crank tended to blast the paint off the block, and chunks of it ended up clogging the oil pump screen. Nowadays, most good builder just grind the inside of the block to get rid of casting flash, stress risers, and smooth it up. If you have porous aluminum, if you can locate the leak, a little gentle peening should fix it up- as it will with a leaky aluminum rim

[panman62851]

The reason is pourous aluminum and glyptal is the stuff. Can't remember when but I did see the stuff mentioned in a Harley publication. I've always used it . It is permanent and oil proof. Clean cases real good leaving zero residue and yer golden.
Kenny

[donie]

Yes, porous metal was part of the problem. The important reason is, early cases were sand cast.
What happened was, as the cases heated up, casting sand would dislodge and get into the bearings.
This could be most seen on the right main shaft and case bushing, as it is oiled by non filtered oil, when looked at, the roller bearing surfaces will have a gray appearance instead of shiny. That is, the surfaces will appear as they were lapped.
I was told, the old paint was Fuller Obrien metal primer.

[phrakworld]

Is painting with Red Glyptal still a suggested practice? Heard there may be a heat issue with painting.

[flat-happy]

the original stuff was called gasoila and is still available. originally i think it was used because of porous metal but i use it in case there is one or more glass beads that didn't get cleaned off the surface of the metal. it locks every last bit of dirt or debris in place so it won't get into the bearings. no room there even for microscopic particles. i think the factory used it for that reason too

[DaveSwanson]

Originally Posted by phrakworld

Is painting with Red Glyptal still a suggested practice? Heard there may be a heat issue with painting.

Yes Glyptal is still used by many. I use it on Flatheads but refrain on the OHV's.

Modern corrosive fuels have been known to slough off this paint. This can happen on overhead valve Harleys parked on the sidestand, if the petcock leaks and fuel seeps into the intake and down the cylinders.