Over 40 years of building engines, I have developed opinions about what the effect on cylinder head temperatures would be if they were painted, powder coated, or left bare. I had thought that leaving the cylinder and heads raw might run lower temps than engine paint. Most times, if painted, the cylinders and heads would be black in color. The color black absorbs heat and doesn’t aid in a cooling effect. Even though I own a powder coating business I would not powder cylinders and heads because I thought it would hold in the heat of a running engine. But, were my opinions correct? I decided to run tests to find out. I felt like a high school kid doing a science project and the results were eye opening.

The first coatings I wanted to test were the painted cylinder/head combo vs. the bare, raw aluminum cylinder/head combo. I had a Yamaha RD-350 cylinder and head that were freshly painted and because it was a 2 stroke with an open intake, it was easy to introduce the heat into the cylinder combustion area. As you can see in the picture, I used a heat gun that had an output temperature of 750F. The heat gun was held in place with a square tube welded onto the work bench. This way, the conditions would be similar when I changed to the bare cylinders. I partially restricted the exhaust port with a steel rectangular tube to aid in heat retention into the cylinder. The cylinder was placed on 2 aluminum plates to partially close off the bottom of the cylinder.

First, I turned on the heat gun to start building up some cylinder heat. I wanted no air movement in this area so I blocked it off with some cardboard. The head was placed on top and a spark plug tightened into the head with the thermal couple under the spark plug gasket. The thermal couple gives very accurate temperature readings. Before heating, both cylinders had a room temperature of 68 degrees. Both cylinders were heated for exactly 30 minutes with the heat gun setup. At the end of 30 minutes, the black cylinder had a head temperature of 165F and the bare cylinder had a head temperature of 164F.

To do the next part of the test, I turned on a floor fan on low. The fan was 7’ away and was the same direction the air would hit across the cylinder head if it were on a motorcycle running down the road.

The cylinder head results with the fan on:

Time                           Black Paint Cylinder             Head Bare Cylinder/Head

1. 0 Minutes                          165F                                            164F
2. 1 Minute                            160F                                            158F
3. 3 Minutes                          146F                                             145F
4. 4 Minutes                          141F                                             140F
5. 5 Minutes                          136F                                             136F
6. 7 Minutes                          132F                                             131F
7. 10 Minutes                        126F                                             125F

As you can see, there was really no difference between the black cylinder head temps and the bare aluminum cylinder head temps. This was nice to know, since my next bike will have the cylinders and heads painted black.

The next test was performed to determine if there is a difference in heat dissipation between a painted aluminum piece and a powder-coated one.

For this test, I had an aluminum strip that was half painted black and the other half powder coated blue. The reason blue was chosen was to distinguish it better for you, the reader. On the strip, I also milled out four 5/16” areas to allow temperature readings on the aluminum itself. These areas were the same distance from the center mounting hole.

After placing a 3/8” diameter bolt in the center hole, this assembly was secured in place with a drill press vise. An acetylene/oxygen torch was held in place with another drill press vise and the flame directed at the center mounting point. I will call the painted end “section 1,” and the powder-coated end, “section 2.” The milled areas closest to the center will be called “A” and the outer milled sections will be called “B.” See the photo for clarification. After lighting the torch, I took measurements every minute and, after 5 minutes, placed a fan 8” away and directed towards the center of the strip.

Here are the results.

          Time                                 Paint 1A     Power Coat 2A     Paint 1B      Powder Coat 2B

1. 1 Minute                                      87F            96F                  86F                   95F 
2. 2 Minutes                                    89F           105F                 89F                   98F 
3. 3 Minutes                                    92F           160F                 91F                  149F 
4. 4 Minutes                                  165F           270F                162F                 259F 
5. 5 Minutes                                  169F           430F                167F                 406F 

Fan On

1. 1 Minute                                   147F            279F                127F                 242F
2. 2 Minutes                                  129F            254F                112F                 208F

As you can clearly see, powder coating does act as a heat insulator, trapping heat in the aluminum.

Just think about all that heat trapped in your cylinder and head which could cause major engine damage! Based on my tests, I concluded that it doesn’t really matter if you paint an aluminum cylinder head or leave it bare, but I would definitely not powder coat one.

*This article was originally published in The Horse Magazine Issue #128 March/April 2013