How to make a custom fan shroud
This is my improved instructions on making a custom fiberglass fan shroud based on the Jalopy Journals posting. The basic principle is to make a hardboard template of the fan and rad core and stretch material between the two. Fiberglass the material, cut out the hardboard templates and what's left over is your fan shroud - it is actually very easy!
I ended up making 3 fan shrouds before I found a method that worked to my satisfaction. The first two didn't fit as my measurements were off and I didn't have my car (it was out getting the paint wet sanded and polished) to verify that the first one didn't fit. The first one had the fiberglass matte added to the outside of the fan shroud and required way too much work to smooth out (bondo city). The second one uses the method described here. And the third and final one, used the second method with proper dimensions.
Working with fiberglass irritates the skin of a lot of people, I've found that wearing painter's throw-away overalls helps a lot. They are inexpensive ($10-20), light, durable and last a long time if you are a casual wearer. They have elastics on the leg and arm openings plus some have a hoody.
Painter's overalls for protection
- Take measurements:
You will need 4 basic measurements:
- The diameter of your fan - mine was 19 inches
- The dimensions of your radiator core - mine was 18 inches high x 22 inches wide
- Position of fan center in relationship to radiator core - my fan was centered horizontally and 8 1/2 inches from the top of the rad core
- Depth of fan shroud. How far from the radiator should the fan shroud extend over 1/2 of the fan blade. A very common error is to completely enclose the fan blade within the shroud. The fan shroud should only enclose 1/2 the fan blade. Note: most engines sit at an angle to the radiator so expect that the depth of the fan shroud will be longer at the top than the bottom.
The first fan shroud I made completely covered the fan blades. At a stop light, I could see the engine temperature slowly creeping up. I happened to meet an aeronautical engineer at a local cruise in and he explained that 1/3 of the fan's air flow comes off the tips in a vortex. If you completely cover the tips, the air is trapped, forced back in and counter-acts the air flow. I took his advice, cut off about 1" all around and now the temperature is rock steady even at stop and go traffic in 90 degreeF weather.
- The diameter of your fan - mine was 19 inches
- Mark and cut-out templates:
I used 1/8" masonite (sometimes called hardboard) to make the templates.
- Make a circle template 1" wider in diameter than your fan - mine was 20 inches in diameter.
- Make a template to the dimensions of your radiator core. In my case, the fan extended above the core by 1.5" and below by 1" so I added a circular area to the rad template (see following picture).
- Make a block of wood the thickness of the depth of the fan shroud. I used a 4x4 post and cut it to 4" length. At this point I didn't bother with worrying about the engine tilt. I'll cut the fan shroud to fit later.
- Drill a centering hole:
You will need a long bolt to align the three template pieces together. I used a handy 5" long bolt that I had in my misc bolt container.
- Drill a hole in the center of the fan template
- Drill a hole in the rad core template that corresponds to the location of the fan's center.
- Drill a hole through the center of the block of wood corresponding to the fan shroud depth.
- Assembly the three pieces together:
- Bolt the three pieces together using the long bolt - this aligns the three pieces
- Screw the fan template to the block of wood using four #10 x 1" wood screws - these stop the template from rotating
- Screw the rad core template to the block of wood using four #10 x 1" wood screws - these stop the template from rotating
- Remove long bolt as it's no longer needed.
Three pieces screwed together
- Stretch and staple fabric over the template:
I went to the local Walmart's fabric section and purchased 2 metres of stretchy fabric for $1.97/m which is way more than I need. It comes out to about 7 feet x 4 feet. I'm glad I did because I ended up making 3 fan shrouds - even then I still have enough for one or two more! The pattern doesn't matter as it will be fiberglassed over and painted.
Stretch the fabric over the fan template side and staple using 1/4" staples to the front of the rad core template. Don't stretch too tight as you will distort the template - typically the corners of the rad shroud will turn up.
Fabric stretched and stapled over template
- Fiberglass the fabric:
You will need two coatings of fiberglass resin mixed with hardner on the fabric. Don't apply too much hardner as it will cure too fast before you can use it. It normally cures in about 3 to 5 hours. I used margarine containers to mix the fiberglass resin as fiberglass doesn't stick to it and you can peel it off after it cures.
Wear plastic throw-away gloves and use cheap 1" wide paint brushes to paint on the fiberglass. Use acetone for clean-up.
You must let the first coat dry before applying the second. The first coat will give a carbon fiber finish but will not be strong enough to hold the shape. The second coat will give it a smooth finish and hold the shape. I found that looking at the shiny reflection of a trouble light allowed me to easily see the areas where I missed. You only need to fiberglass the area where the fan shroud will be and a little bit of the templates.
2 coats of fiberglass resin mixed with hardner coating the fabric
- Cut out the rad core template:
When the second coat of fiberglass is cured, turn it over and cut out the rad core template leaving about an 1/8" of edging all around.
Backside of template
Rad core template trimmed with dremel tool and removing template screws
Rad core template removed
- Fiberglass matte inside of fan shroud:
I applied two coats of fiberglass matte to the inside of the fan shroud. I used small pieces typically 4" x 2" rectangles and smaller. The first coat of matte was applied horizontally and the second coat applied vertically. I waited until the first coat dried before applying the second but you can probably apply the second immediately over the first.
You first apply a coat of fiberglass resin with hardner to a small area, cover it with matte then apply a coat of resin to the matte until it soaks through. The matte will turn transparent from it's normal white color. You can't bend the matte much and only after it's soaked with resin.
Two layers of matte fiberglassed on the inside
- Cut out the fan template:
Using the dremel tool, I cut out the fan template. What remained was the fan shroud. For ease of installation, I cut the fan shroud vertically down the middle. I used two small angle iron pieces to mate the each of the two sections together. The fan shroud mounts were made using some flat 20 gauge sheet metal. I pop rivetted the angle iron and fan shroud mounts to the fan shroud with washers on the back side to pull against. The pictures show it painted and just a little finishing putty applied to smooth out some sections.
Fan template cut-out, fan shroud painted, cut down the middle and some finishing putty applied
Close up of two piece angle iron clamp to keep the two pieces together.
Simple mounting bracket made from sheet metal - two per side.
- Install and admire:
Here's the finished fan shroud. The picture doesn't do it justice, it looks way better. The flash seems to show up every little thing that the naked eye can't see including a mysterious orange reflection. Notice that there is plenty of clearance between the fan blades and the fan shroud.
Fan shroud mounted and working perfectly - just as planned!